The hypothalamus–pituitary–thyroid (HPT) axis represents a classical example of an endocrine feedback loop. This review discusses dynamic changes in HPT axis setpoint regulation, identifying their molecular and cellular determinants, and speculates about their functional role. Hypothalamic thyrotropin-releasing hormone neurons were identified as key components of thyroid hormone (TH) setpoint regulation already in the 1980s, and this was followed by the demonstration of a pivotal role for the thyroid hormone receptor beta in negative feedback of TH on the hypothalamic and pituitary level. Gradually, the concept emerged of the HPT axis setpoint as a fixed entity, aiming at a particular TH serum concentration. However, TH serum concentrations appear to be variable and highly responsive to physiological and pathophysiological environmental factors, including the availability or absence of food, inflammation and clock time. During food deprivation and inflammation, TH serum concentrations decrease without a concomitant rise in serum TSH, reflecting a deviation from negative feedback regulation in the HPT axis. Surprisingly, TH action in peripheral organs in these conditions cannot be simply predicted by decreased serum TH concentrations. Instead, diverse environmental stimuli have differential effects on local TH metabolism, e.g. in liver and muscle, occurring quite independently from decreased TH serum concentrations. The net effect of these differential local changes is probably a major determinant of TH action at the tissue level. In sum, hypothalamic HPT axis setpoint regulation as well as TH metabolism at the peripheral organ level is flexible and dynamic, and may adapt the organism in an optimal way to a range of environmental challenges.
Eric Fliers, Andries Kalsbeek and Anita Boelen
Anke J Borgers, Anneke Alkemade, Henk W Venema, Eric Fliers and Peter H Bisschop
Endocrine deficiencies, like GH and estrogen deficiencies, are likely candidates to explain increased visceral to subcutaneous fat ratio in patients with pituitary insufficiency. However, recent reports pointed to cranial radiotherapy (CRT) as an additional determinant of an unfavorable fat distribution. Therefore, we determined the effect of CRT on abdominal fat distribution in men with treated pituitary insufficiency.
Thirty-five consecutive male subjects (16 men with and 19 men without CRT aged 62±12 and 56±14 years respectively, P=0.175) visiting our Endocrine Outpatient Clinic for pituitary insufficiency were invited to participate in this study. A standardized single-slice abdominal CT scan at the level of fourth lumbar vertebra was performed to determine visceral fat area, subcutaneous fat area, and visceral to subcutaneous fat ratio. In addition, we assessed body mass index, total fat percentage with bioelectrical impedance analysis, resting energy expenditure with indirect calorimetry, calorie intake using a diary, and serum hormone concentrations.
Subjects with CRT had a smaller subcutaneous fat area (225.1 (71.1–480.7) vs 269.0 (133.2–59.9) cm2, P=0.022) and a higher visceral to subcutaneous fat ratio (0.79 (0.39–1.55) vs 0.63 (0.23–0.88), P=0.001) than subjects without CRT. Both the groups were comparable for body mass index, waist–hip ratio, resting energy expenditure, and calorie intake. Importantly, serum hormone concentrations were similar.
In men treated for pituitary insufficiency, previous CRT is associated with a higher visceral to subcutaneous fat ratio.
Jacqueline E Siljee, Unga A Unmehopa, Andries Kalsbeek, Dick F Swaab, Eric Fliers and Anneke Alkemade
The melanocortin 4 receptor (MC4R) is an essential regulator of energy homeostasis and metabolism, and MC4R mutations represent the most prevalent monogenetic cause of obesity in humans known to date. Hypothalamic MC4Rs in rodents are well characterized in neuroanatomical and functional terms, but their expression pattern in the human hypothalamus is unknown.
Design and methods
To determine the topographic distribution and identity of cells expressing MC4R mRNA in the human hypothalamus, locked nucleic acid in situ hybridization was performed on nine human postmortem hypothalami. In addition, co-expression of MC4R with glial fibrillary acidic protein (GFAP), vasopressin/oxytocin (AVP/OXT), corticotropin-releasing hormone (CRH), neuropeptide Y (NPY), agouti-related protein (AgRP), and α-melanocyte stimulating hormone (α-MSH) was examined.
Most intense MC4R mRNA expression was present in the paraventricular nucleus (PVN), the supraoptic nucleus (SON), and the nucleus basalis of Meynert. Most MC4R-positive cells in the SON also expressed AVP/OXT. Co-expression with AVP/OXT in the PVN was less abundant. We did not observe co-expression of MC4R mRNA and GFAP, CRH, NPY, AgRP, or α-MSH. However, fiber-like staining of NPY, AgRP, and α-MSH was found adjacent to MC4R-positive cells in the PVN.
Expression of MC4R mRNA in the human hypothalamus is widespread and in close approximation to endogenous MC4R binding partners AgRP and α-MSH.
Peter H Bisschop, Arno W Toorians, Erik Endert, Wilmar M Wiersinga, Louis J Gooren and Eric Fliers
Objective: Estrogen and androgen administration modulate the pituitary–thyroid axis through alterations in thyroid hormone-binding globulin (TBG) metabolism, but the effects of sex steroids on extrathyroidal thyroxine (T4) to triiodothyronine (T3) conversion in humans are unknown.
Design and methods: We studied 36 male-to-female and 14 female-to-male euthyroid transsexuals at baseline and after 4 months of hormonal treatment. Male-to-female transsexuals were treated with cyproterone acetate (CA) 100 mg/day alone (n=10) or in combination with either oral ethinyl estradiol (or-EE) 100 μg/day (n=14) or transdermal 17β-estradiol (td-E) 100 μg twice a week (n=12). Female-to-male transsexuals were treated with i.m. testosterone 250 mg twice a week. A t-test was used to test for differences within groups and ANOVAwith post hoc analysis to test for differences between the groups.
Results: Or-EE increased TBG (100 ± 12%, P<.001) and testosterone decreased TBG (−14 ± 4%, P =0.01), but free T4 did not change. Td-E and CA did not affect TBG concentrations. TSH was not different between groups at baseline or after treatment. CA decreased T3/T4 ratios (−9 ± 3%, P=0.04), suggesting that T4 to T3 conversion was lower. Testosterone increased T3/T4 ratios (30 ± 9%, P=0.02), which probably reflects higher T4 to T3 conversion.
Conclusion: Oral but not transdermal estradiol increases TBG, whereas testosterone lowers TBG. Testosterone increases T3/T4 ratios. Estradiol does not affect T3/T4 ratios, irrespective of the route of administration.
Anneke Alkemade, Edith C Friesema, George G Kuiper, Wilmar M Wiersinga, Dick F Swaab, Theo J Visser and Eric Fliers
Objective: An increasing number of proteins appear to be involved in thyroid hormone feedback action at the level of the anterior pituitary, but the cell types expressing these proteins are largely unknown. The aim of the present study was to identify cell types in the human anterior pituitary that express type II and type III deiodinase (D2 and D3), the recently described thyroid hormone transporter (MCT8) and thyroid hormone receptor (TR) isoforms by means of double-labeling immunocytochemistry.
Results: We found TR isoforms to be expressed most prominently in gonadotropes and – although to a lesser extent – in thyrotropes, corticotropes, lactotropes and somatotropes. D3 staining showed a distribution pattern that was remarkably similar. By contrast, D2 immunoreactivity was observed exclusively in folliculostellate (FS) cells showing coexpression with human leukocyte antigen (HLA), a marker of major histocompatibility complex (MHC)-class II. MCT8 immunostaining was present in FS cells without HLA coexpression.
Conclusions: From these results, we propose a novel neuroanatomical model for thyroid hormone feedback on the human pituitary, with a central role for FS cells in thyroid hormone activation, which thus play an important role in the suppression of TSH secretion by circulating thyroxine (T4).
Edith C H Friesema, Theo J Visser, Anke J Borgers, Andries Kalsbeek, Dick F Swaab, Eric Fliers and Anneke Alkemade
Thyroid hormone (TH) signaling in brain cells is dependent on transport of TH across the plasma membrane followed by intracellular deiodination and binding to the nuclear TH receptors. The aim of this study is to investigate the expression of the specific TH transporters monocarboxylate transporter 8 (MCT8 (SLC16A2)), MCT10, organic anion transporting polypeptide 1C1 (OATP1C1 (SLCO1C1)), and the types 2 and 3 deiodinases (D2 and D3) in the developing human hypothalamus.
Fifteen postmortem brain samples of fetuses and young children ranging between 17 weeks of gestation and 29 months of postnatal age including one child (28 months) with central congenital hypothyroidism were studied.
Sections of the different hypothalami were stained with polyclonal rabbit antisera against MCT8, MCT10, OATP1C1, D2, and D3.
We found MCT8 and D3 but not D2 protein expression to be present in our earliest sample of 17 weeks of gestation, indicating triiodothyronine degradation, but not production at this time of development. At term, expression of TH transporters and D2 decreased and D3 expression increased, suggesting decreased TH signaling just before birth. The child with central congenital hypothyroidism showed higher MCT8 and D2 expression compared with the other children of similar age.
This study reports the developmental timing of expression of components crucial for central TH signaling in the human hypothalamus. In general, during fetal hypothalamic development, the coordinated expression of D2 and D3 in combination with the different TH transporters suggests that proper TH concentrations are regulated to prevent untimely maturation of brain cells.
Ellie M Wekking, Bente C Appelhof, Eric Fliers, Aart H Schene, Jochanan Huyser, Jan G P Tijssen and Wilmar M Wiersinga
Objective: Hypothyroidism is associated with neurocog.nitive impairment. Sparse data suggest that treatment of hypothyroidism, resulting in a return to euthyroidism, may be associated with only partial recovery of overall neurocognitive functioning. The aim of this study was to assess neurocognitive functioning and well-being in euthyroid patients with primary hypothyroidism on adequate thyroxine (T4) treatment. We also investigated whether serum TSH and thyroid antibodies are determinants of neurocognitive functioning and well-being.
Design: We assessed neurocognitive functioning and well-being in 141 patients with primary hypothyroidism.
Methods: Neurocognitive test results and scores on questionnaires measuring well-being of 141 patients were compared with the reference values for these tests as published and used in Dutch clinical neuropsychological practice. Assessment of neurocognitive functioning included tests for cognitive or psychomotor speed, attention, working memory as well as learning and memory. Well-being was measured with the Symptom Check List-90 total score and the Rand 36-item Health Survey subscales for ‘mental health’ and ‘vitality’.
Results: Patients showed poor performance on various domains of neurocognitive functioning compared with mean standard reference values, especially on a complex attention task and on verbal memory tests. Levels of well-being were significantly lower for patients compared with those of the general population. Neither serum TSH nor thyroid antibodies were determinants of neurocognitive functioning and well-being.
Conclusion: The results of this study suggest that neurocognitive functioning as well as psychological well-being may not be completely restored in patients with hypothyroidism, despite T4 treatment.
Laura P B Elbers, Carla Moran, Victor E A Gerdes, Bregje van Zaane, Joost C M Meijers, Erik Endert, Greta Lyons, Krishna Chatterjee, Peter H Bisschop and Eric Fliers
Hyperthyroidism is associated with a hypercoagulable state, but the underlying mechanism is unknown. Patients with resistance to thyroid hormone (RTH) due to defective thyroid hormone receptor β (THRB or THRB) exhibit elevated circulating thyroid hormones (TH) with refractoriness to TH action in THRB-expressing tissues. We tested the hypothesis that the hypercoagulable state in hyperthyroidism is mediated via the THRB.
We conducted a cross-sectional study from November 2013 to January 2015 in 3 hospitals in the Netherlands and the United Kingdom.
Patients with RTH due to defective THRB (n=18), patients with hyperthyroidism (n=16) and euthyroid subjects (n=18) were included. TH concentrations and markers of coagulation and fibrinolysis were measured. Data are expressed as median (interquartile range).
Free thyroxine (FT4) levels were slightly higher in hyperthyroid patients than in RTH patients (53.9 (30.5–70.0) and 34.9 (28.4–42.2)pmol/L, respectively, P=0.042). Both groups had raised FT4 levels compared with euthyroid subjects (14.0 (13.0–15.8)pmol/L, P≤0.001). Levels of von Willebrand factor (VWF), factor (F) VIII, fibrinogen and d-dimer were significantly higher in hyperthyroid patients than in RTH patients (VWF 231 (195–296) vs 111 (82–140)%, FVIII 215 (192–228) vs 145 (97–158)%, fibrinogen 3.6 (3.0–4.4) vs 2.8 (2.5–3.2)g/L, d-dimer 0.41 (0.31–0.88) vs 0.20 (0.17–0.26)mg/L, respectively, P≤0.001), while there were no differences between RTH patients and euthyroid controls.
Parameters of coagulation and fibrinolysis were elevated in hyperthyroid patients compared with patients with RTH due to defective THRB, whereas these parameters were not different between euthyroid controls and RTH patients, despite elevated FT4 concentrations in RTH patients. This indicates that the procoagulant effects observed in hyperthyroidism are mediated via the THRB.
Marloes Louwerens, Bente C Appelhof, Herman Verloop, Marco Medici, Robin P Peeters, Theo J Visser, Anita Boelen, Eric Fliers, Johannes W A Smit and Olaf M Dekkers
Research on determinants of well-being in patients on thyroid hormone replacement therapy is warranted, as persistent fatigue-related complaints are common in this population. In this study, we evaluated the impact of different states of hypothyroidism on fatigue and fatigue-related symptoms. Furthermore, the relationship between fatigue and the TSH receptor (TSHR)-Asp727Glu polymorphism, a common genetic variant of the TSHR, was analyzed.
A cross-sectional study was performed in 278 patients (140 patients treated for differentiated thyroid carcinoma (DTC) and 138 with autoimmune hypothyroidism (AIH)) genotyped for the TSHR-Asp727Glu polymorphism.
The multidimensional fatigue inventory (MFI-20) was used to assess fatigue, with higher MFI-20 scores indicating more fatigue-related complaints. MFI-20 scores were related to disease status and Asp727Glu polymorphism status.
AIH patients scored significantly higher than DTC patients on all five MFI-20 subscales (P<0.001), independent of clinical and thyroid hormone parameters. The frequency of the TSHR-Glu727 allele was 7.2%. Heterozygous DTC patients had more favorable MFI-20 scores than wild-type DTC patients on four of five subscales. The modest effect of the TSHR-Asp727Glu polymorphism on fatigue was found in DTC patients only.
AIH patients had significantly higher levels of fatigue compared with DTC patients, which could not be attributed to clinical or thyroid hormone parameters. The modest effect of the TSHR-Asp727Glu polymorphism on fatigue in DTC patients should be confirmed in other cohorts.
Jantien P Brouwer, Bente C Appelhof, Witte J G Hoogendijk, Jochanan Huyser, Erik Endert, Cassandra Zuketto, Aart H Schene, Jan G P Tijssen, Richard Van Dyck, Wilmar M Wiersinga and Eric Fliers
Objective: Major depressive disorder has been associated with changes in the hypothalamus–pituitary–thyroid (HPT) axis and with hypercortisolism. However, the changes reported have been at variance, probably related to in- or outpatient status, the use of antidepressant medication and the heterogeneity of depression. We therefore conducted a controlled study in unipolar depressed outpatients who had been free of antidepressants for at least 3 months.
Design: We assessed endocrine parameters in 113 depressed outpatients and in 113 sex- and age-matched controls.
Methods: Patients were included if they had a major depression according to a Structural Clinical Interview for the Diagnostic and Statistical Manual of Mental Disorders (DSM), fourth edition (SCID-IV) and if they had a 17-item Hamilton rating scale for depression (HRSD) score of ≥16. Endocrine parameters contained serum concentrations of TSH, (free) thyroxine, tri-iodothyronine, cortisol, thyroid peroxidase (TPO) antibody titre and 24-h urinary excretion of cortisol.
Results: The serum concentration of TSH was slightly higher in depressed patients as compared with controls (P < 0.001), independent of the presence of subclinical hypothyroidism and/or TPO antibodies (n = 28). All other HPT axis parameters were similar in both groups. The 24-h urinary cortisol excretion was similar in patients and controls. In atypical depression, serum cortisol was lower than in non-atypical depression (P = 0.01). Patients with neither melancholic depression nor severe depression (HRSD ≥23) had altered endocrine parameters. Finally, serum TSH values could not be related to cortisol values.
Conclusion: When compared with matched control subjects, outpatients with major depression had slightly higher serum TSH, while urinary cortisol levels were similar. Furthermore, we observed lower serum cortisol in atypical depression than in non-atypical depression.