Oral low-dose testosterone administration induces whole-body protein anabolism in postmenopausal women: a novel liver-targeted therapy

in European Journal of Endocrinology
Authors:
Vita BirznieceDepartment of Endocrinology, University of New South Wales, School of Medicine, Diabetes and Metabolic Medicine, Bioanalytical Mass Spectrometry Facility and School of Medical Sciences, ANZAC Research Institute, Princess Alexandra Hospital, Garvan Institute of Medical Research, St Vincent's Hospital, Sydney, New South Wales, Australia
Department of Endocrinology, University of New South Wales, School of Medicine, Diabetes and Metabolic Medicine, Bioanalytical Mass Spectrometry Facility and School of Medical Sciences, ANZAC Research Institute, Princess Alexandra Hospital, Garvan Institute of Medical Research, St Vincent's Hospital, Sydney, New South Wales, Australia
Department of Endocrinology, University of New South Wales, School of Medicine, Diabetes and Metabolic Medicine, Bioanalytical Mass Spectrometry Facility and School of Medical Sciences, ANZAC Research Institute, Princess Alexandra Hospital, Garvan Institute of Medical Research, St Vincent's Hospital, Sydney, New South Wales, Australia

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Margot A UmplebyDepartment of Endocrinology, University of New South Wales, School of Medicine, Diabetes and Metabolic Medicine, Bioanalytical Mass Spectrometry Facility and School of Medical Sciences, ANZAC Research Institute, Princess Alexandra Hospital, Garvan Institute of Medical Research, St Vincent's Hospital, Sydney, New South Wales, Australia

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Anne PoljakDepartment of Endocrinology, University of New South Wales, School of Medicine, Diabetes and Metabolic Medicine, Bioanalytical Mass Spectrometry Facility and School of Medical Sciences, ANZAC Research Institute, Princess Alexandra Hospital, Garvan Institute of Medical Research, St Vincent's Hospital, Sydney, New South Wales, Australia

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David J HandelsmanDepartment of Endocrinology, University of New South Wales, School of Medicine, Diabetes and Metabolic Medicine, Bioanalytical Mass Spectrometry Facility and School of Medical Sciences, ANZAC Research Institute, Princess Alexandra Hospital, Garvan Institute of Medical Research, St Vincent's Hospital, Sydney, New South Wales, Australia

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Ken K Y HoDepartment of Endocrinology, University of New South Wales, School of Medicine, Diabetes and Metabolic Medicine, Bioanalytical Mass Spectrometry Facility and School of Medical Sciences, ANZAC Research Institute, Princess Alexandra Hospital, Garvan Institute of Medical Research, St Vincent's Hospital, Sydney, New South Wales, Australia
Department of Endocrinology, University of New South Wales, School of Medicine, Diabetes and Metabolic Medicine, Bioanalytical Mass Spectrometry Facility and School of Medical Sciences, ANZAC Research Institute, Princess Alexandra Hospital, Garvan Institute of Medical Research, St Vincent's Hospital, Sydney, New South Wales, Australia
Department of Endocrinology, University of New South Wales, School of Medicine, Diabetes and Metabolic Medicine, Bioanalytical Mass Spectrometry Facility and School of Medical Sciences, ANZAC Research Institute, Princess Alexandra Hospital, Garvan Institute of Medical Research, St Vincent's Hospital, Sydney, New South Wales, Australia

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Objective

In hypopituitary men, oral delivery of unesterified testosterone in doses that result in a solely hepatic androgen effect enhances protein anabolism during GH treatment. In this study, we aimed to determine whether liver-targeted androgen supplementation induces protein anabolism in GH-replete normal women.

Design

Eight healthy postmenopausal women received 2-week treatment with oral testosterone at a dose of 40 mg/day (crystalline testosterone USP). This dose increases portal concentrations of testosterone, exerting androgenic effects on the liver without a spillover into the systemic circulation.

Outcome measures

The outcome measures were whole-body leucine turnover, from which leucine rate of appearance (LRa, an index of protein breakdown) and leucine oxidation (Lox, a measure of irreversible protein loss) were estimated, energy expenditure and substrate utilization. We measured the concentration of liver transaminases as well as of testosterone, SHBG and IGF1.

Results

Testosterone treatment significantly reduced LRa by 7.1±2.5% and Lox by 14.6±4.5% (P<0.05). The concentration of liver transaminases did not change significantly, while that of serum SHBG fell within the normal range by 16.8±4.0% and that of IGF1 increased by 18.4±7.7% (P<0.05). The concentration of peripheral testosterone increased from 0.4±0.1 to 1.1±0.2 nmol/l (P<0.05), without exceeding the upper normal limit. There was no change in energy expenditure and fat and carbohydrate utilization.

Conclusions

Hepatic exposure to unesterified testosterone by oral delivery stimulates protein anabolism by reducing protein breakdown and oxidation without inducing systemic androgen excess in women. We conclude that a small oral dose of unesterified testosterone holds promise as a simple novel treatment of protein catabolism and muscle wasting.

Abstract

Objective

In hypopituitary men, oral delivery of unesterified testosterone in doses that result in a solely hepatic androgen effect enhances protein anabolism during GH treatment. In this study, we aimed to determine whether liver-targeted androgen supplementation induces protein anabolism in GH-replete normal women.

Design

Eight healthy postmenopausal women received 2-week treatment with oral testosterone at a dose of 40 mg/day (crystalline testosterone USP). This dose increases portal concentrations of testosterone, exerting androgenic effects on the liver without a spillover into the systemic circulation.

Outcome measures

The outcome measures were whole-body leucine turnover, from which leucine rate of appearance (LRa, an index of protein breakdown) and leucine oxidation (Lox, a measure of irreversible protein loss) were estimated, energy expenditure and substrate utilization. We measured the concentration of liver transaminases as well as of testosterone, SHBG and IGF1.

Results

Testosterone treatment significantly reduced LRa by 7.1±2.5% and Lox by 14.6±4.5% (P<0.05). The concentration of liver transaminases did not change significantly, while that of serum SHBG fell within the normal range by 16.8±4.0% and that of IGF1 increased by 18.4±7.7% (P<0.05). The concentration of peripheral testosterone increased from 0.4±0.1 to 1.1±0.2 nmol/l (P<0.05), without exceeding the upper normal limit. There was no change in energy expenditure and fat and carbohydrate utilization.

Conclusions

Hepatic exposure to unesterified testosterone by oral delivery stimulates protein anabolism by reducing protein breakdown and oxidation without inducing systemic androgen excess in women. We conclude that a small oral dose of unesterified testosterone holds promise as a simple novel treatment of protein catabolism and muscle wasting.

Introduction

Testosterone is a potent anabolic agent that increases muscle mass and physical performance (1, 2, 3). A reduction in the concentration of testosterone is linked to the development of sarcopenia and frailty in men (4). It has emerged as a potential therapeutic agent for muscle loss associated with ageing and systemic disease. A meta-analysis of 11 randomized controlled trials has provided strong evidence that testosterone supplementation increases muscle strength in elderly men (5). Bhasin et al. (1) have demonstrated that the increase in muscle mass and strength from testosterone supplementation is dose dependent in men. The low levels of circulating testosterone in women are also associated with a reduction in muscle mass, which can be reversed by testosterone supplementation (6, 7). The exploitation of systemic testosterone supplementation has been limited by systemic side effects, in both men and women (1, 8, 9, 10, 11, 12, 13). These include gynaecomastia, sleep apnoea, mood changes, polycythemia and acceleration of prostatic disease in elderly men and virilization in women.

Testosterone and GH exert independent and additive regulatory effects on protein metabolism (14, 15, 16, 17). The impact of testosterone may be related to the initial levels of circulating testosterone. It is significantly anabolic in hypogonadal men (18), whereas this is not observed always in healthy elderly men (19). Importantly, both testosterone and GH are required for a maximal anabolic effect (16, 17, 19). We have recently investigated whether the site of GH and testosterone interaction during protein metabolism is hepatic or extrahepatic. We compared the impact of testosterone administered via the oral and transdermal routes on whole-body protein metabolism in hypopituitary men during GH replacement therapy (17). We employed a 40 mg daily oral dose of unesterified testosterone that increases portal concentrations of testosterone without inducing a pharmacological hepatic response and without a spillover into the systemic circulation (20). The effects were compared with those of transdermal delivery that elevated the concentration of testosterone in peripheral blood to the normal range. The protein anabolic impact of testosterone supplementation was similar during systemic and solely hepatic administrations, indicating the liver as the primary site of interaction between these two hormones (17). Therefore, in the GH-sufficient state, oral delivery that achieves solely hepatic exposure to testosterone via the portal vein may induce a whole-body anabolic effect without systemic androgenic effects.

Our previous studies involved hypogonadal men in whom oral testosterone significantly stimulated protein anabolism (17). In the present study, we determined whether the delivery of a similar dose of oral testosterone induces a significant protein anabolic effect in healthy postmenopausal women.

Subjects and methods

Subjects

Eight healthy postmenopausal women were recruited from the community through an advertisement. The mean age of the participants was 64.2±1.6 years and mean BMI was 26.8±1.7 kg/m2. The study participants were in good health, and all had normal haematological test results and renal and hepatic function. Exclusion criteria included hypothalamic or pituitary disorders, diabetes mellitus and chronic renal or hepatic illnesses. The subjects were not taking hormone replacement therapy or any medications known to interfere with the endocrine system. All the participants were instructed to follow their usual diet and physical activity as well as to continue their usual medications or supplements throughout the study period.

The Human Research Ethics Committee of St Vincent's Hospital approved the study. The study was conducted in accordance with the principles of the Declaration of Helsinki. All participants gave written informed consent. The study was registered with the Australian and New Zealand Clinical Trials Registry (ACTRN12605000482662).

Study design

This was an open-label study. The participants were studied at baseline and after 2 weeks of testosterone administration at a dose of 40 mg/day orally. Testosterone was prepared by Fresh Therapeutics (Sydney, NSW, Australia) as capsules filled with crystalline testosterone USP without excipients. The daily 40 mg dose of crystalline testosterone was divided into three doses taken every 8 h to achieve steady hepatic exposure. The dose was based on our previous work on hypogonadal men in whom 40 mg/day of oral crystalline testosterone was the highest dose that did not increase systemic levels of testosterone and did not reduce the blood levels of SHBG, indicative of an absence of pharmacological androgen effects on the liver (20). No significant increase in the concentration of testosterone occurred in peripheral blood, indicative of the absence of a systemic spillover (20).

We investigated the impact in postmenopausal women of oral testosterone delivery on i) the concentrations of testosterone, SHBG and markers of hepatic function in peripheral blood; ii) resting energy expenditure and fat oxidation; and iii) whole-body leucine turnover from which the rates of leucine appearance (LRa, an index of protein breakdown), leucine oxidation (Lox, an index of oxidative loss of protein) and leucine incorporation into protein (LIP, an index of protein synthesis).

The participants were studied after an overnight fast in the early morning in the Clinical Research Facility, Garvan Institute of Medical Research. Studies were undertaken at baseline and at the end of the treatment period. Blood samples for the measurement of the concentrations of testosterone and SHBG and liver function were collected around 0900 h before the start of the leucine turnover study. At each visit, study blood samples were collected and placed on ice; plasma and serum were separated and stored at −80 °C until analysis.

Methods

Protein turnover

Whole-body protein metabolism was measured using the leucine turnover technique over a 3-h period. The method is based on the principle of steady-state kinetics in which the rate of appearance of a substrate equals its rate of disposal. For leucine, there are two pathways of disposal: oxidation and re-incorporation into protein. The fractional partitioning between these two pathways of disposal is determined from the fraction of infused isotope that appears in breath. The LRa and Lox were calculated as described previously (21). α-Ketoisocaproic acid (KIC) is formed when leucine undergoes transamination and is used as a surrogate marker of leucine as it more accurately reflects the intracellular environment (22).

After an overnight fast, a 0.104 mg/kg priming dose of NaH13CO3 was administered to the participants, followed by a primed constant 3-h infusion of 1-[13C]leucine (prime 0.5 mg/kg, infusion 0.5 mg/kg per h), as described previously (23, 24). NaH13CO3 and 1-[13C]leucine were obtained from Cambridge Isotope Laboratories, Woburn, MA, USA. On each visit, blood and breath samples were collected before (−10 and 0 min) and during (140, 160 and 180 min) leucine infusion. Blood was placed on ice and plasma was separated and stored at −80 °C. KIC was extracted from plasma as described by Nissen et al. (25). Plasma KIC enrichment with 13C was measured by gas chromatography–mass spectrometry (MSD 5971A, model 5890, Hewlett-Packard Co., Palo Alto, CA, USA). CO2 enrichment with 13C in breath samples was measured at the University of Surrey, United Kingdom, on a Delta Plus XP isotope ratio mass spectrometer fitted with a Gas Bench II inlet system (Thermo Fisher Scientific, Hemel Hempstead, UK). The CV for LRa and Lox in our study based on seven subjects studied on a second occasion after a median of 34 days were 3.5 and 6.1% respectively (26).

Indirect calorimetry

Whole-body energy expenditure and substrate oxidation were measured by indirect calorimetry. This involved using an open-circuit ventilated hood system (ParvoMedics, Sandy, UT, USA), calibrated against standard gases before each study. The participants rested on a bed for at least 30 min. A clear plastic hood was placed over their head for a 20-min period. The measurements were taken during two 20-min periods and averaged.

Assays

All samples for each individual were measured in the same assay run for each analyte. The concentration of serum IGF1 was measured using RIA after acid ethanol extraction as described previously (18, 24, 27). The CV for IGF1 were 8.3% at 14.7 nmol/l and 7.4% at 28.6 nmol/l. The concentration of serum SHBG was measured by RIA using a commercial assay kit (Immulite 2000, Siemens Medical Solution Diagnostics, Los Angeles, CA, USA). In postmenopausal women, the normal range for SHBG concentration is 21–149 nmol/l (RPAH, Australia). Testosterone, 17β-oestradiol (E2), and DHT and its 3α-diol and 3β-diol metabolites were measured using LC–MS/MS, as described previously (28). The limit of detection for testosterone was 0.04 nmol/l, for E2 3.4 pmol/l and for DHT 0.01 nmol/l. In cases where the level of E2 was below the detection limit, the value was imputed as 0. The inter-assay CV for testosterone was 3.9–6.5%, for E2 4.8–8.6% and for DHT 6.7–13.4%. The CV for SHBG at 5.3 and 86.2 nmol/l were 5.0 and 7.5% respectively.

Statistical analysis

Treatment effects were assessed using the paired t-tests. The results are expressed as means with s.e.m., and a P value of <0.05 was considered significant. Statistical analysis was undertaken using the statistical software package SPSS statistics v20 (IBM corporation).

Results

The clinical, hormonal and metabolic indices obtained at baseline and during testosterone treatment are given in Table 1. Oral testosterone administration increased the levels of circulating testosterone from 0.4±0.1 to 1.1±0.2 nmol/l (P<0.05; Fig. 1). The mean level of DHT along with that of its 3α-diol and 3β-diol metabolites (data not shown) did not change significantly during testosterone administration. The level of E2 was below the detection limit in most of the samples and did not change during testosterone administration. The mean level of SHBG decreased slightly but significantly during oral testosterone treatment by 8.8±2.5 nmol/l (P<0.01). All the changes in hormone levels occurred within the normal range for women of reproductive age.

Table 1

Clinical, hormonal and metabolic measures at baseline and during oral testosterone treatment. Data are presented as means±s.e.m.

BaselineTestosteroneP value
Weight (kg)69.5±4.569.6±4.60.75
Testosterone (nmol/l)0.4±0.11.1±0.2<0.05
SHBG (nmol/l)47.7±8.138.9±6.4<0.01
DHT (nmol/l)0.3±0.10.4±0.10.59
Oestradiol (pmol/l)1.5±1.56.5±3.60.11
IGF1 (nmol/l)12.6±1.114.8±1.3<0.05
REE (kcal/day)1182±57.71105±40.80.11
Fox (mg/min)51.3±5.444.3±4.50.10
Cox (mg/min)43.5±9.047.0±11.40.67
Urea (mmol/l)5.2±0.55.3±0.50.90
Creatinine (μmol/l)65.1±3.065.7±3.70.75
Albumin (g/l)39.9±0.641.1±0.50.13
ALT (IU/l)16.1±2.017.6±1.70.51
AST (IU/l)21.9±0.721.9±1.71.00
GGT (IU/l)22.5±5.518.2±3.70.06

REE, resting energy expenditure; Fox, fat oxidation; Cox, carbohydrate oxidation.

Figure 1
Figure 1

Circulating testosterone levels at baseline and at the end of the 2-week treatment period with oral unesterified testosterone (40 mg/day). Dotted line represents the upper normal limit of circulating testosterone levels measured by LC–MS/MS in women (36, 37, 38). Data are expressed as means±s.e.m.

Citation: European Journal of Endocrinology 169, 3; 10.1530/EJE-13-0406

Oral testosterone treatment significantly increased the levels of IGF1 by 18.4±7.7% (P<0.05). The change in the levels of IGF1 did not significantly correlate with any of the parameters measured. Neither serum urea, creatinine and albumin levels nor ALT, AST and GGT levels changed significantly with oral testosterone administration.

Energy expenditure and substrate metabolism

In the post-absorptive state, oral testosterone administration did not significantly affect resting energy expenditure and carbohydrate and fatty acid oxidation in healthy postmenopausal women.

Protein metabolism

Oral testosterone administration significantly reduced the LRa, a measure of protein degradation, by 7.1±2.5% compared with baseline (P<0.05; Fig. 2A). Net Lox, a measure of irreversible loss of protein, was significantly reduced by 14.7±4.5% (P<0.05; Fig. 2B).

Figure 2
Figure 2

Effects of testosterone on whole-body protein turnover. (A) Leucine rate of appearance (LRa), a measure of protein breakdown, and (B) net leucine oxidation (Lox), a measure of irreversible loss of protein. Data are expressed as means±s.e.m.

Citation: European Journal of Endocrinology 169, 3; 10.1530/EJE-13-0406

Next, we next analysed the data to determine whether the intervention induced a significant reduction in leucine oxidized as a proportion of LRa, representing an inverse measure of protein synthesis. This proportion was reduced by testosterone treatment, by 8.1±4.0%, with the difference approaching significance (P=0.07).

Discussion

We studied for the first time whether hepatic exposure to a low dosage of testosterone by oral delivery increases whole-body protein anabolism in healthy women. Oral testosterone administration resulted in a significant reduction in the rate of leucine appearance, an index of protein breakdown, and the rate of Lox, an index of irreversible loss of protein. The concentration of serum SHBG decreased, while those of testosterone and IGF1 increased, all within the physiological range. There was no significant change in energy expenditure and fat and carbohydrate utilization. The concentrations of circulating liver transaminases were unaffected by testosterone treatment. Thus, physiological androgen exposure of the liver to orally administered unesterified testosterone induced net protein anabolism by reducing protein breakdown and oxidation without inducing peripheral androgen excess in women.

The liver is a major site of protein metabolism and a principal site where testosterone interacts with GH to promote anabolism (17). The mechanism for this is unknown, but it may involve nitrogen recycling, which is regulated by the liver. It has been demonstrated that in humans a reduction in urea synthesis is accompanied by an increase in amino acid uptake in muscle (29). Hepatic urea formation is suppressed by GH administration in healthy men and in GH-deficient patients, providing at least partly the mechanism for the anabolic effect of GH (30, 31, 32). We propose that the reduction in oxidative metabolism of protein estimated from whole-body leucine turnover in this study represents the reduction in urea synthesis, partitioning nitrogen away from disposal. testosterone reduced Lox; therefore, reduction in hepatic urea production may be a determining factor underlying the whole-body anabolic effect. This is supported by the observation that testosterone supplementation reduces blood urea levels (33). In this study, we did not observe a significant decrease in the concentration of urea; however, this may be due to the small sample size. Taken together, we propose that the liver is a critical site for the regulation of whole-body protein metabolism and that hepatic exposure to testosterone may shift the partitioning of nitrogen towards protein synthesis, which merits further investigation.

To gauge the significance of changes in the indices of whole-body leucine metabolism, we compared the magnitude of change with those observed during androgen deprivation and replacement. In a study of normal men in whom hypogonadism was induced by a GNRH antagonist, protein synthesis decreased by ∼13% and was accompanied by a reduction in fat-free mass of 2 kg (34). In a study of hypopituitary men, testosterone replacement at standard doses by i.m. injection reduced Lox by 28%, when compared with GH replacement, which reduced it by 15% (18). In our recent study of GH-replaced hypopituitary men, oral testosterone administration reduced Lox by 16% (17). In the present study, the reduction in protein oxidation in postmenopausal women by oral testosterone of ∼15% is of similar magnitude. As oxidation represents the irreversible loss of protein, a reduction equates the net anabolism of protein. We have previously observed that a reduction in protein oxidation of 14% was accompanied by a 2.2 kg gain of lean body mass (LBM) after 12 weeks of GH replacement (26). Considering that leucine represents 8% of whole-body protein or 590 μmol/g protein, the reduction in protein oxidation ∼5.7±1.6 mg/min. If this reduction in protein loss is maintained over the long term, oral testosterone supplementation theoretically translates into a gain of 3 kg protein mass over 1 year.

Testosterone treatment is usually delivered parenterally by injection of testosterone esters or transdermal gel or patch delivery of unesterified testosterone and not by the oral route apart from oral testosterone undecanoate. When testosterone or its esters are administered orally, they are rapidly metabolized in the gut before reaching the liver via the portal circulation. Testosterone undecanoate is absorbed mainly through the lymphatic system with low bioavailability. Thus, a substantial fraction bypasses the liver, elevating the levels of testosterone in peripheral blood. Large doses (over 250 mg vs endogenous production rate of 0.25 mg/day in women) are required to overcome hepatic metabolism to achieve physiological levels in peripheral blood required for androgen replacement therapy (35). Our testosterone formulation does not bypass the liver and hepatic exposure is achieved without peripheral androgen excess, thereby potentially extending anabolic testosterone therapy to women in whom systemic effects of testosterone are undesirable.

We used an oral testosterone dose that was similar to that used in an earlier study in hypogonadal men, in whom the levels of circulating testosterone did not increase after administration of a 40 mg daily dose orally (17, 20). In the present study carried out in postmenopausal women, we observed a small but significant increase in the levels of circulating testosterone, reaching 1.1±0.2 nmol/l. Studies assessing the levels of testosterone in women of reproductive age by LC–MS/MS have reported the levels of circulating testosterone to be in the range of 0.1–2.8 nmol/l, with median levels of 0.9 nmol/l (36, 37, 38). Thus, this low-dose oral regimen did not cause androgen excess in peripheral blood. There was a small but significant reduction in the levels of SHBG within the normal range for postmenopausal women, consistent with a local hepatic effect of mildly elevated levels of testosterone in the portal circulation. The significance of these changes in androgen indices is unknown. Testosterone replacement in women with low levels of androgen has been studied, showing benefits on mood, sexual function, body composition and general health (7, 39). Thus, it is conceivable that the modest increase in the levels of circulating testosterone may provide some benefits to postmenopausal women during therapy with this oral formulation of low-dose unesterified testosterone.

The reduction in Lox in postmenopausal women by oral testosterone delivery was accompanied by a small increase in the mean levels of circulating testosterone of 0.65 nmol/l. In hypogonadal men, systemic testosterone replacement that elevated the levels of circulating testosterone by more than 15 nmol/l did not exert an additional protein anabolic effect over and above that achieved by oral testosterone administration with solely hepatic exposure (17). Thus, the liver plays a crucial role in the modulation of protein turnover, and the small increase in the concentration of testosterone in peripheral blood in this study is unlikely to have reduced leucine turnover and oxidation rates by 15%. This study provides evidence that sole hepatic exposure to testosterone can be developed as a novel, safe and cost-effective treatment for frailty in both men and women.

We observed an 18% increase in the levels of IGF1, which may have contributed to the anabolic effect of the liver-targeted testosterone administration. This finding is in agreement with those of our study in hypopituitary men, in whom during GH replacement administration of oral testosterone significantly increased the levels of circulating IGF1 by 16% (17). In oophorectomized women, systemic testosterone administration results in a significant increase in the levels of IGF1, at the same time reducing the secretion of GH (40). This observation suggests that there is a direct effect of testosterone on IGF1 production, which is not centrally mediated. The liver is the principal source of circulating IGF1; however, there is also a substantial local production in other tissues, such as muscle. In muscle, testosterone locally potentiates the effect of GH on muscle IGF1 gene expression (41, 42). However, this mechanism is probably not responsible in this study as the increase in the levels of circulating testosterone was minimal. Therefore, we speculate that orally administered testosterone may potentiate GH receptor signalling in the liver, in this way increasing hepatic IGF1 production. To our knowledge, studies investigating the direct effects of testosterone on hepatic IGF1 production have not been published. Thus, there is emerging evidence that the liver may be a central site of interaction between testosterone and GH signalling.

This study has some limitations. A weakness is the small sample size and that it was not blinded or placebo controlled. Nevertheless, this study is a proof of principle that in healthy women small doses of liver-targeted testosterone administration significantly reduce protein loss after only 2 weeks of administration. The results are unlikely to occur by chance since changes in the components of protein turnover (LRa −7.1% and Lox −14.7%) exceed those of the reproducibility of the method (CV for LRa and Lox 3.5 and 6.1% respectively).

In summary, LRa and oxidative loss were reduced by testosterone administration in healthy postmenopausal women. The levels of circulating testosterone stayed within the normal range for postmenopausal women. We conclude that oral testosterone administration of a physiological dose stimulates protein anabolism by reducing protein breakdown and oxidation without inducing peripheral androgen excess in women. Testosterone administration via the oral route may open new treatment strategies for frail elderly men and women.

Declaration of interest

The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.

Funding

This research did not receive any specific grant from any funding agency in the public, commercial or not-for-profit sector.

Acknowledgements

The authors thank research nurse Vanessa Travers for clinical assistance; Nicola Jackson from Diabetes and Metabolic Medicine, Faculty of Health and Medical Sciences, University of Surrey, UK, Reena Desai from ANZAC Research Institute, Concord Hospital, Sydney, Australia, the Bioanalytical Mass Spectrometry Facility, University of New South Wales, Australia, and the Endocrinology Laboratory, Royal Prince Alfred Hospital, Sydney, Australia, for providing technical and laboratory assistance.

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    • Export Citation
  • 16

    Mauras N, Rini A, Welch S, Sager B, Murphy SP. Synergistic effects of testosterone and growth hormone on protein metabolism and body composition in prepubertal boys. Metabolism 2003 52 964969. (doi:10.1016/S0026-0495(03)00163-X).

    • Search Google Scholar
    • Export Citation
  • 17

    Birzniece V, Meinhardt UJ, Umpleby MA, Handelsman DJ, Ho KK. Interaction between testosterone and growth hormone on whole-body protein anabolism occurs in the liver. Journal of Clinical Endocrinology and Metabolism 2011 96 10601067. (doi:10.1210/jc.2010-2521).

    • Search Google Scholar
    • Export Citation
  • 18

    Gibney J, Wolthers T, Johannsson G, Umpleby AM, Ho KK. Growth hormone and testosterone interact positively to enhance protein and energy metabolism in hypopituitary men. American Journal of Physiology. Endocrinology and Metabolism 2005 289 E266E271. (doi:10.1152/ajpendo.00483.2004).

    • Search Google Scholar
    • Export Citation
  • 19

    Giannoulis MG, Jackson N, Shojaee-Moradie F, Nair KS, Sonksen PH, Martin FC, Umpleby AM. The effects of growth hormone and/or testosterone on whole body protein kinetics and skeletal muscle gene expression in healthy elderly men: a randomized controlled trial. Journal of Clinical Endocrinology and Metabolism 2008 93 30663074. (doi:10.1210/jc.2007-2695).

    • Search Google Scholar
    • Export Citation
  • 20

    Birzniece V, Meinhardt UJ, Handelsman DJ, Ho KK. Testosterone stimulates extra-hepatic but not hepatic fat oxidation (Fox): comparison of oral and transdermal testosterone administration in hypopituitary men. Clinical Endocrinology 2009 71 715721. (doi:10.1111/j.1365-2265.2009.03524.x).

    • Search Google Scholar
    • Export Citation
  • 21

    Matthews DE, Motil KJ, Rohrbaugh DK, Burke JF, Young VR, Bier DM. Measurement of leucine metabolism in man from a primed, continuous infusion of L-[1-3C]leucine. American Journal of Physiology 1980 238 E473E479.

    • Search Google Scholar
    • Export Citation
  • 22

    Horber FF, Horber-Feyder CM, Krayer S, Schwenk WF, Haymond MW. Plasma reciprocal pool specific activity predicts that of intracellular free leucine for protein synthesis. American Journal of Physiology 1989 257 E385E399.

    • Search Google Scholar
    • Export Citation
  • 23

    Hoffman DM, Pallasser R, Duncan M, Nguyen TV, Ho KK. How is whole body protein turnover perturbed in growth hormone-deficient adults? Journal of Clinical Endocrinology and Metabolism 1998 83 43444349. (doi:10.1210/jc.83.12.4344).

    • Search Google Scholar
    • Export Citation
  • 24

    Wolthers T, Hoffman DM, Nugent AG, Duncan MW, Umpleby M, Ho KK. Oral estrogen antagonizes the metabolic actions of growth hormone in growth hormone-deficient women. American Journal of Physiology. Endocrinology and Metabolism 2001 281 E1191E1196.

    • Search Google Scholar
    • Export Citation
  • 25

    Nissen SL, Miles JM, Gerich JE, Haymond MW. Regulation of α-ketoisocaproate binding to albumin in vivo by free fatty acids. American Journal of Physiology 1982 242 E67E71.

    • Search Google Scholar
    • Export Citation
  • 26

    Burt MG, Gibney J, Hoffman DM, Umpleby AM, Ho KK. Relationship between GH-induced metabolic changes and changes in body composition: a dose and time course study in GH-deficient adults. Growth Hormone & IGF Research 2008 18 5564. (doi:10.1016/j.ghir.2007.07.005).

    • Search Google Scholar
    • Export Citation
  • 27

    Gibney J, Johannsson G, Leung KC, Ho KK. Comparison of the metabolic effects of raloxifene and oral estrogen in postmenopausal and growth hormone-deficient women. Journal of Clinical Endocrinology and Metabolism 2005 90 38973903. (doi:10.1210/jc.2005-0173).

    • Search Google Scholar
    • Export Citation
  • 28

    Harwood DT, Handelsman DJ. Development and validation of a sensitive liquid chromatography–tandem mass spectrometry assay to simultaneously measure androgens and estrogens in serum without derivatization. Clinica Chimica Acta 2009 409 7884. (doi:10.1016/j.cca.2009.09.003).

    • Search Google Scholar
    • Export Citation
  • 29

    Grofte T, Wolthers T, Jorgensen JO, Poulsen PL, Vilstrup H, Moller N. Hepatic amino- to urea-N clearance and forearm amino-N exchange during hypoglycemic and euglycemic hyperinsulinemia in normal man. Journal of Hepatology 1999 30 819825. (doi:10.1016/S0168-8278(99)80134-2).

    • Search Google Scholar
    • Export Citation
  • 30

    Wolthers T, Grofte T, Jorgensen JO, Vilstrup H. Growth hormone prevents prednisolone-induced increase in functional hepatic nitrogen clearance in normal man. Journal of Hepatology 1997 27 789795. (doi:10.1016/S0168-8278(97)80314-5).

    • Search Google Scholar
    • Export Citation
  • 31

    Norrelund H, Moller N, Nair KS, Christiansen JS, Jorgensen JO. Continuation of growth hormone (GH) substitution during fasting in GH-deficient patients decreases urea excretion and conserves protein synthesis. Journal of Clinical Endocrinology and Metabolism 2001 86 31203129. (doi:10.1210/jc.86.7.3120).

    • Search Google Scholar
    • Export Citation
  • 32

    Wolthers T, Grofte T, Moller N, Vilstrup H, Jorgensen JO. Effects of long-term growth hormone (GH) and triiodothyronine (T3) administration on functional hepatic nitrogen clearance in normal man. Journal of Hepatology 1996 24 313319. (doi:10.1016/S0168-8278(96)80010-9).

    • Search Google Scholar
    • Export Citation
  • 33

    Conway AJ, Boylan LM, Howe C, Ross G, Handelsman DJ. Randomized clinical trial of testosterone replacement therapy in hypogonadal men. International Journal of Andrology 1988 11 247264. (doi:10.1111/j.1365-2605.1988.tb00999.x).

    • Search Google Scholar
    • Export Citation
  • 34

    Hayes VY, Urban RJ, Jiang J, Marcell TJ, Helgeson K, Mauras N. Recombinant human growth hormone and recombinant human insulin-like growth factor I diminish the catabolic effects of hypogonadism in man: metabolic and molecular effects. Journal of Clinical Endocrinology and Metabolism 2001 86 22112219. (doi:10.1210/jc.86.5.2211).

    • Search Google Scholar
    • Export Citation
  • 35

    Johnsen SG, Bennett EP, Jensen VG. Therapeutic effectiveness of oral testosterone. Lancet 1974 2 14731475. (doi:10.1016/S0140-6736(74)90216-5).

    • Search Google Scholar
    • Export Citation
  • 36

    Salameh WA, Redor-Goldman MM, Clarke NJ, Reitz RE, Caulfield MP. Validation of a total testosterone assay using high-turbulence liquid chromatography tandem mass spectrometry: total and free testosterone reference ranges. Steroids 2010 75 169175. (doi:10.1016/j.steroids.2009.11.004).

    • Search Google Scholar
    • Export Citation
  • 37

    Janse F, Eijkemans MJ, Goverde AJ, Lentjes EG, Hoek A, Lambalk CB, Hickey TE, Fauser BC, Norman RJ. Assessment of androgen concentration in women: liquid chromatography–tandem mass spectrometry and extraction RIA show comparable results. European Journal of Endocrinology 2011 165 925933. (doi:10.1530/EJE-11-0482).

    • Search Google Scholar
    • Export Citation
  • 38

    Kushnir MM, Blamires T, Rockwood AL, Roberts WL, Yue B, Erdogan E, Bunker AM, Meikle AW. Liquid chromatography–tandem mass spectrometry assay for androstenedione, dehydroepiandrosterone, and testosterone with pediatric and adult reference intervals. Clinical Chemistry 2010 56 11381147. (doi:10.1373/clinchem.2010.143222).

    • Search Google Scholar
    • Export Citation
  • 39

    Panzer C, Guay A. Testosterone replacement therapy in naturally and surgically menopausal women. Journal of Sexual Medicine 2009 6 818.(quiz 19–20) (doi:10.1111/j.1743-6109.2008.01128.x).

    • Search Google Scholar
    • Export Citation
  • 40

    Azziz R, Deal CL, Potter HD, Gargosky SE, Rosenfeld RG. Regulation of extragonadal insulin-like growth factor-binding protein-3 by testosterone in oophorectomized women. Journal of Clinical Endocrinology and Metabolism 1994 79 17471751. (doi:10.1210/jc.79.6.1747).

    • Search Google Scholar
    • Export Citation
  • 41

    Sculthorpe N, Solomon AM, Sinanan AC, Bouloux PM, Grace F, Lewis MP. Androgens affect myogenesis in vitro and increase local IGF-1 expression. Medicine and Science in Sports and Exercise 2012 44 610615. (doi:10.1249/MSS.0b013e318237c5c0).

    • Search Google Scholar
    • Export Citation
  • 42

    Brill KT, Weltman AL, Gentili A, Patrie JT, Fryburg DA, Hanks JB, Urban RJ, Veldhuis JD. Single and combined effects of growth hormone and testosterone administration on measures of body composition, physical performance, mood, sexual function, bone turnover, and muscle gene expression in healthy older men. Journal of Clinical Endocrinology and Metabolism 2002 87 56495657. (doi:10.1210/jc.2002-020098).

    • Search Google Scholar
    • Export Citation

 

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    Circulating testosterone levels at baseline and at the end of the 2-week treatment period with oral unesterified testosterone (40 mg/day). Dotted line represents the upper normal limit of circulating testosterone levels measured by LC–MS/MS in women (36, 37, 38). Data are expressed as means±s.e.m.

  • View in gallery

    Effects of testosterone on whole-body protein turnover. (A) Leucine rate of appearance (LRa), a measure of protein breakdown, and (B) net leucine oxidation (Lox), a measure of irreversible loss of protein. Data are expressed as means±s.e.m.

  • 1

    Bhasin S, Woodhouse L, Casaburi R, Singh AB, Mac RP, Lee M, Yarasheski KE, Sinha-Hikim I, Dzekov C, Dzekov J et al.. Older men are as responsive as young men to the anabolic effects of graded doses of testosterone on the skeletal muscle. Journal of Clinical Endocrinology and Metabolism 2005 90 678688. (doi:10.1210/jc.2004-1184).

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    • Export Citation
  • 2

    Brodsky IG, Balagopal P, Nair KS. Effects of testosterone replacement on muscle mass and muscle protein synthesis in hypogonadal men – a clinical research center study. Journal of Clinical Endocrinology and Metabolism 1996 81 34693475. (doi:10.1210/jc.81.10.3469).

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    • Export Citation
  • 3

    Mauras N, Hayes V, Welch S, Rini A, Helgeson K, Dokler M, Veldhuis JD, Urban RJ. Testosterone deficiency in young men: marked alterations in whole body protein kinetics, strength, and adiposity. Journal of Clinical Endocrinology and Metabolism 1998 83 18861892. (doi:10.1210/jc.83.6.1886).

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  • 4

    Travison TG, Nguyen AH, Naganathan V, Stanaway FF, Blyth FM, Cumming RG, Le Couteur DG, Sambrook PN, Handelsman DJ. Changes in reproductive hormone concentrations predict the prevalence and progression of the frailty syndrome in older men: the concord health and ageing in men project. Journal of Clinical Endocrinology and Metabolism 2011 96 24642474. (doi:10.1210/jc.2011-0143).

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  • 5

    Ottenbacher KJ, Ottenbacher ME, Ottenbacher AJ, Acha AA, Ostir GV. Androgen treatment and muscle strength in elderly men: a meta-analysis. Journal of the American Geriatrics Society 2006 54 16661673. (doi:10.1111/j.1532-5415.2006.00938.x).

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  • 6

    Burd ID, Bachmann GA. Androgen replacement in menopause. Current Women's Health Reports 2001 1 202205.

  • 7

    Miller KK, Biller BM, Beauregard C, Lipman JG, Jones J, Schoenfeld D, Sherman JC, Swearingen B, Loeffler J, Klibanski A. Effects of testosterone replacement in androgen-deficient women with hypopituitarism: a randomized, double-blind, placebo-controlled study. Journal of Clinical Endocrinology and Metabolism 2006 91 16831690. (doi:10.1210/jc.2005-2596).

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  • 8

    Basaria S, Dobs AS. Clinical review: controversies regarding transdermal androgen therapy in postmenopausal women. Journal of Clinical Endocrinology and Metabolism 2006 91 47434752. (doi:10.1210/jc.2006-0740).

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  • 9

    Basaria S, Coviello AD, Travison TG, Storer TW, Farwell WR, Jette AM, Eder R, Tennstedt S, Ulloor J, Zhang A et al.. Adverse events associated with testosterone administration. New England Journal of Medicine 2010 363 109122. (doi:10.1056/NEJMoa1000485).

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  • 10

    Basaria S, Dobs AS. Safety and adverse effects of androgens: how to counsel patients. Mayo Clinic Proceedings 2004 79 S25S32.

  • 11

    Somboonporn W, Davis SR. Testosterone effects on the breast: implications for testosterone therapy for women. Endocrine Reviews 2004 25 374388. (doi:10.1210/er.2003-0016).

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  • 12

    Sih R, Morley JE, Kaiser FE, Perry HM III, Patrick P, Ross C. Testosterone replacement in older hypogonadal men: a 12-month randomized controlled trial. Journal of Clinical Endocrinology and Metabolism 1997 82 16611667. (doi:10.1210/jc.82.6.1661).

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  • 13

    Nigro N, Christ-Crain M. Testosterone treatment in the aging male: myth or reality? Swiss Medical Weekly 2012 142 w13539. (doi:10.4414/smw.2012.13539).

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  • 14

    Albanese A, Stanhope R. Treatment of growth delay in boys with isolated growth hormone deficiency. European Journal of Endocrinology 1994 130 6569. (doi:10.1530/eje.0.1300065).

    • Search Google Scholar
    • Export Citation
  • 15

    Bourguignon JP. Linear growth as a function of age at onset of puberty and sex steroid dosage: therapeutic implications. Endocrine Reviews 1988 9 467488. (doi:10.1210/edrv-9-4-467).

    • Search Google Scholar
    • Export Citation
  • 16

    Mauras N, Rini A, Welch S, Sager B, Murphy SP. Synergistic effects of testosterone and growth hormone on protein metabolism and body composition in prepubertal boys. Metabolism 2003 52 964969. (doi:10.1016/S0026-0495(03)00163-X).

    • Search Google Scholar
    • Export Citation
  • 17

    Birzniece V, Meinhardt UJ, Umpleby MA, Handelsman DJ, Ho KK. Interaction between testosterone and growth hormone on whole-body protein anabolism occurs in the liver. Journal of Clinical Endocrinology and Metabolism 2011 96 10601067. (doi:10.1210/jc.2010-2521).

    • Search Google Scholar
    • Export Citation
  • 18

    Gibney J, Wolthers T, Johannsson G, Umpleby AM, Ho KK. Growth hormone and testosterone interact positively to enhance protein and energy metabolism in hypopituitary men. American Journal of Physiology. Endocrinology and Metabolism 2005 289 E266E271. (doi:10.1152/ajpendo.00483.2004).

    • Search Google Scholar
    • Export Citation
  • 19

    Giannoulis MG, Jackson N, Shojaee-Moradie F, Nair KS, Sonksen PH, Martin FC, Umpleby AM. The effects of growth hormone and/or testosterone on whole body protein kinetics and skeletal muscle gene expression in healthy elderly men: a randomized controlled trial. Journal of Clinical Endocrinology and Metabolism 2008 93 30663074. (doi:10.1210/jc.2007-2695).

    • Search Google Scholar
    • Export Citation
  • 20

    Birzniece V, Meinhardt UJ, Handelsman DJ, Ho KK. Testosterone stimulates extra-hepatic but not hepatic fat oxidation (Fox): comparison of oral and transdermal testosterone administration in hypopituitary men. Clinical Endocrinology 2009 71 715721. (doi:10.1111/j.1365-2265.2009.03524.x).

    • Search Google Scholar
    • Export Citation
  • 21

    Matthews DE, Motil KJ, Rohrbaugh DK, Burke JF, Young VR, Bier DM. Measurement of leucine metabolism in man from a primed, continuous infusion of L-[1-3C]leucine. American Journal of Physiology 1980 238 E473E479.

    • Search Google Scholar
    • Export Citation
  • 22

    Horber FF, Horber-Feyder CM, Krayer S, Schwenk WF, Haymond MW. Plasma reciprocal pool specific activity predicts that of intracellular free leucine for protein synthesis. American Journal of Physiology 1989 257 E385E399.

    • Search Google Scholar
    • Export Citation
  • 23

    Hoffman DM, Pallasser R, Duncan M, Nguyen TV, Ho KK. How is whole body protein turnover perturbed in growth hormone-deficient adults? Journal of Clinical Endocrinology and Metabolism 1998 83 43444349. (doi:10.1210/jc.83.12.4344).

    • Search Google Scholar
    • Export Citation
  • 24

    Wolthers T, Hoffman DM, Nugent AG, Duncan MW, Umpleby M, Ho KK. Oral estrogen antagonizes the metabolic actions of growth hormone in growth hormone-deficient women. American Journal of Physiology. Endocrinology and Metabolism 2001 281 E1191E1196.

    • Search Google Scholar
    • Export Citation
  • 25

    Nissen SL, Miles JM, Gerich JE, Haymond MW. Regulation of α-ketoisocaproate binding to albumin in vivo by free fatty acids. American Journal of Physiology 1982 242 E67E71.

    • Search Google Scholar
    • Export Citation
  • 26

    Burt MG, Gibney J, Hoffman DM, Umpleby AM, Ho KK. Relationship between GH-induced metabolic changes and changes in body composition: a dose and time course study in GH-deficient adults. Growth Hormone & IGF Research 2008 18 5564. (doi:10.1016/j.ghir.2007.07.005).

    • Search Google Scholar
    • Export Citation
  • 27

    Gibney J, Johannsson G, Leung KC, Ho KK. Comparison of the metabolic effects of raloxifene and oral estrogen in postmenopausal and growth hormone-deficient women. Journal of Clinical Endocrinology and Metabolism 2005 90 38973903. (doi:10.1210/jc.2005-0173).

    • Search Google Scholar
    • Export Citation
  • 28

    Harwood DT, Handelsman DJ. Development and validation of a sensitive liquid chromatography–tandem mass spectrometry assay to simultaneously measure androgens and estrogens in serum without derivatization. Clinica Chimica Acta 2009 409 7884. (doi:10.1016/j.cca.2009.09.003).

    • Search Google Scholar
    • Export Citation
  • 29

    Grofte T, Wolthers T, Jorgensen JO, Poulsen PL, Vilstrup H, Moller N. Hepatic amino- to urea-N clearance and forearm amino-N exchange during hypoglycemic and euglycemic hyperinsulinemia in normal man. Journal of Hepatology 1999 30 819825. (doi:10.1016/S0168-8278(99)80134-2).

    • Search Google Scholar
    • Export Citation
  • 30

    Wolthers T, Grofte T, Jorgensen JO, Vilstrup H. Growth hormone prevents prednisolone-induced increase in functional hepatic nitrogen clearance in normal man. Journal of Hepatology 1997 27 789795. (doi:10.1016/S0168-8278(97)80314-5).

    • Search Google Scholar
    • Export Citation
  • 31

    Norrelund H, Moller N, Nair KS, Christiansen JS, Jorgensen JO. Continuation of growth hormone (GH) substitution during fasting in GH-deficient patients decreases urea excretion and conserves protein synthesis. Journal of Clinical Endocrinology and Metabolism 2001 86 31203129. (doi:10.1210/jc.86.7.3120).

    • Search Google Scholar
    • Export Citation
  • 32

    Wolthers T, Grofte T, Moller N, Vilstrup H, Jorgensen JO. Effects of long-term growth hormone (GH) and triiodothyronine (T3) administration on functional hepatic nitrogen clearance in normal man. Journal of Hepatology 1996 24 313319. (doi:10.1016/S0168-8278(96)80010-9).

    • Search Google Scholar
    • Export Citation
  • 33

    Conway AJ, Boylan LM, Howe C, Ross G, Handelsman DJ. Randomized clinical trial of testosterone replacement therapy in hypogonadal men. International Journal of Andrology 1988 11 247264. (doi:10.1111/j.1365-2605.1988.tb00999.x).

    • Search Google Scholar
    • Export Citation
  • 34

    Hayes VY, Urban RJ, Jiang J, Marcell TJ, Helgeson K, Mauras N. Recombinant human growth hormone and recombinant human insulin-like growth factor I diminish the catabolic effects of hypogonadism in man: metabolic and molecular effects. Journal of Clinical Endocrinology and Metabolism 2001 86 22112219. (doi:10.1210/jc.86.5.2211).

    • Search Google Scholar
    • Export Citation
  • 35

    Johnsen SG, Bennett EP, Jensen VG. Therapeutic effectiveness of oral testosterone. Lancet 1974 2 14731475. (doi:10.1016/S0140-6736(74)90216-5).

    • Search Google Scholar
    • Export Citation
  • 36

    Salameh WA, Redor-Goldman MM, Clarke NJ, Reitz RE, Caulfield MP. Validation of a total testosterone assay using high-turbulence liquid chromatography tandem mass spectrometry: total and free testosterone reference ranges. Steroids 2010 75 169175. (doi:10.1016/j.steroids.2009.11.004).

    • Search Google Scholar
    • Export Citation
  • 37

    Janse F, Eijkemans MJ, Goverde AJ, Lentjes EG, Hoek A, Lambalk CB, Hickey TE, Fauser BC, Norman RJ. Assessment of androgen concentration in women: liquid chromatography–tandem mass spectrometry and extraction RIA show comparable results. European Journal of Endocrinology 2011 165 925933. (doi:10.1530/EJE-11-0482).

    • Search Google Scholar
    • Export Citation
  • 38

    Kushnir MM, Blamires T, Rockwood AL, Roberts WL, Yue B, Erdogan E, Bunker AM, Meikle AW. Liquid chromatography–tandem mass spectrometry assay for androstenedione, dehydroepiandrosterone, and testosterone with pediatric and adult reference intervals. Clinical Chemistry 2010 56 11381147. (doi:10.1373/clinchem.2010.143222).

    • Search Google Scholar
    • Export Citation
  • 39

    Panzer C, Guay A. Testosterone replacement therapy in naturally and surgically menopausal women. Journal of Sexual Medicine 2009 6 818.(quiz 19–20) (doi:10.1111/j.1743-6109.2008.01128.x).

    • Search Google Scholar
    • Export Citation
  • 40

    Azziz R, Deal CL, Potter HD, Gargosky SE, Rosenfeld RG. Regulation of extragonadal insulin-like growth factor-binding protein-3 by testosterone in oophorectomized women. Journal of Clinical Endocrinology and Metabolism 1994 79 17471751. (doi:10.1210/jc.79.6.1747).

    • Search Google Scholar
    • Export Citation
  • 41

    Sculthorpe N, Solomon AM, Sinanan AC, Bouloux PM, Grace F, Lewis MP. Androgens affect myogenesis in vitro and increase local IGF-1 expression. Medicine and Science in Sports and Exercise 2012 44 610615. (doi:10.1249/MSS.0b013e318237c5c0).

    • Search Google Scholar
    • Export Citation
  • 42

    Brill KT, Weltman AL, Gentili A, Patrie JT, Fryburg DA, Hanks JB, Urban RJ, Veldhuis JD. Single and combined effects of growth hormone and testosterone administration on measures of body composition, physical performance, mood, sexual function, bone turnover, and muscle gene expression in healthy older men. Journal of Clinical Endocrinology and Metabolism 2002 87 56495657. (doi:10.1210/jc.2002-020098).

    • Search Google Scholar
    • Export Citation