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
Anita Boelen, Marianne C Platvoet-ter Schiphorst, Nico van Rooijen, and Wilmar M Wiersinga
Boelen A, Platvoet-ter Schiphorst MC, van Rooijen N, Wiersinga WM. Selective macrophage depletion in the liver does not prevent the development of the sick euthyroid syndrome in the mouse. Eur J Endocrinol 1996;134:513–8. ISSN 0804–4643
A decreased serum triiodothyronine (T3) level is one of the main characteristics of the sick euthyroid syndrome, caused mainly by a decreased 5′-deiodination of thyroxine (T4) in the liver. Cytokines have been implicated in the pathogenesis of the changes in thyroid hormone metabolism during illness. We therefore investigated the role of cytokines produced by the liver macrophages (Kupffer cells) in the development of the sick euthyroid syndrome, which was induced in mice by a single injection of bacterial endotoxin (lipopolysaccharide) or by 24-h starvation. Experiments were carried out with or without previous selective depletion of liver macrophages by intravenous administration of liposome-encapsulated dichloromethylene diphosphonate. Relative to saline-injected pair-fed controls, the administration of lipopolysaccharide caused a decrease of serum T3 and T4 and liver 5′-deiodinase mRNA. Selective depletion of liver macrophages did not affect these changes. Starvation for 24 h decreased serum T3 and T4, associated with a slight decrease of liver 5′-deiodinase mRNA. There were no differences between macrophage-depleted and non-depleted animals in this respect. In summary, selective depletion of liver macrophages did not affect the decrease in serum T3, T4 or liver 5′-deiodinase mRNA induced by lipopolysaccharide or 24-h starvation in mice. We conclude that cytokines produced by Kupffer cells are not involved in the pathogenesis of the sick euthyroid syndrome in this experimental model.
A Boelen, Department of Endocrinology, F5-171 Academic Medical Center, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands
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.
Charlotte A Heinen, Zhi Zhang, Lars P Klieverik, Tim C de Wit, Edwin Poel, Maqsood Yaqub, Anita Boelen, Andries Kalsbeek, Peter H Bisschop, A S Paul van Trotsenburg, Hein J Verberne, Jan Booij, and Eric Fliers
Brown adipose tissue (BAT) activity in humans is stimulated by cold and by a limited number of pharmacological agents, including β3-adrenergic agonists and bile acids. Although thyrotropin-releasing hormone (TRH) is known to activate BAT in several mammals, this has not been reported in humans.
A randomized, placebo-controlled, double-blind, cross-over trial.
We investigated the effects of intravenous bolus administration of 400 µg TRH or 2 mL saline on BAT activity in healthy, lean men. BAT activity was measured as standardized 18F-fluorodeoxyglucose (18F-FDG) uptake and glucose metabolic rate (MRglu) using dynamic PET/CT imaging. The first six individuals were studied at room temperature, while subsequently nine were exposed to mild cold (17°C ± 1°C) for 60 min before imaging. During the dynamic scan, blood was withdrawn for measurement of thyroid hormone and catecholamine concentrations. This trial is registered with The Netherlands National Trial Register (number NTR5512).
Sixteen participants were recruited. Six men studied at room temperature showed no visible BAT activity during either session. After exposure to mild cold, four of nine men (44.4%) showed clear increase of 18F-FDG uptake after TRH administration compared to placebo. Maximal standardized 18F-FDG uptake showed a trend toward increase after TRH compared to placebo (P = 0.066). MRglu showed a significant increase after TRH administration (P = 0.014). The increase in 18F-FDG uptake was not paralleled by changes in plasma thyroid hormone or catecholamine concentrations.
Systemic TRH administration can increase the activity of cold-stimulated BAT in adult men. These findings may assist developing pharmacological strategies for modulating BAT activity in the management of obesity.
Kevin Stroek, Annemieke C Heijboer, Marelle J Bouva, Catharina P B van der Ploeg, Marie-Louise A Heijnen, Gert Weijman, Annet M Bosch, Robert de Jonge, Peter C J I Schielen, A S Paul van Trotsenburg, and Anita Boelen
Congenital hypothyroidism (CH) is defined as thyroid hormone deficiency at birth due to disorders of the thyroid gland (thyroidal CH, CH-T), or the hypothalamus or pituitary (central CH, CH-C). The Dutch Newborn Screening (NBS) strategy is primarily based on determination of thyroxine (T4) concentrations in dried blood spots followed, if necessary, by thyroid-stimulating hormone (TSH) and thyroxine-binding globulin (TBG) measurement enabling detection of both CH-T and CH-C. A calculated T4/TBG ratio serves as an indirect measure for free T4. A T4/TBG ratio ≤ 17 in a second heel puncture is suggestive of CH-C.
Design and methods:
In the present study, we evaluated 11 years of Dutch CH NBS using a database of referred cases by assessing the contribution of each criterion in the unique stepwise T4-TSH-TBG NBS algorithm.
Between 2007 and the end of 2017, 1 963 465 newborns were screened in the Netherlands. Use of the stepwise algorithm led to 3044 referrals and the identification of 612 CH cases, consisting of 496 CH-T, 86 CH-C, and 30 CH of unknown origin diagnoses. We detected 62.8% of CH-C cases by the T4/TBG ratio in the second heel puncture. The positive predictive value (PPV) of the stepwise T4-TSH-TBG NBS algorithm was 21.0%.
This evaluation shows that the Dutch stepwise T4-TSH-TBG NBS algorithm with a calculated T4/TBG ratio is of great value for the detection of both CH-T and CH-C in the Netherlands, at the cost of a lower PPV compared to TSH-based NBS strategies.