Background: Hyperthyroidism is known to induce a hypercoagulable state. It stimulates plasma levels of procoagulative factors and reduces fibrinolytic activity. So far most of the data have been derived from patients with endogenous hyperthyroidism with a wide variability in the underlying pathogenesis and severity of the disease. Objectives: In this study we experimentally induced thyrotoxicosis in healthy volunteers to explore the effects of thyroxine excess on the plasma proteome. Using a shotgun proteomics approach, the abundance of plasma proteins was monitored before, during and after thyrotoxicosis. Methods: Sixteen healthy male subjects were sampled at baseline, 4 and 8 weeks under 250 µg/day thyroxine p.o., as well as 4 and 8 weeks after stopping the application. Plasma proteins were analyzed after depletion of 6 high-abundance proteins (MARS6) by LC-ESI-MS/MS mass spectrometry. Mass spectrometric raw data were processed using a label-free, intensity-based workflow. Subsequently, the linear dependence between protein abundances and fT₄ levels were calculated using a Pearson correlation. Results: All subjects developed biochemical thyrotoxicosis, and this effect was reversed within the first 4 weeks of follow-up. None of the volunteers noticed any subjective symptoms. Levels of 10 proteins involved in the coagulation cascade specifically correlated with fT₄, supporting an influence of thyroid hormone levels on blood coagulation even at nonpathological levels. Conclusions: The results suggest that experimental thyrotoxicosis exerts selective and specific thyroxine-induced effects on coagulation markers. Our study design allows assessment of thyroid hormone effects on plasma protein levels without secondary effects of other diseases or therapies.

Background: Clinical features of thyroid dysfunction vary with age, and an oligosymptomatic presentation of hyperthyroidism is frequently observed in the elderly. This suggests age modulation of thyroid hormone (TH) action, which may occur, for example, by alterations in TH production, metabolism and/or TH action in target organs. Objectives: In this paper, we address possible changes in TH transporter expression in liver tissues as a mechanism of age-dependent variation in TH action. Methods: Chronic hyperthyroidism was induced in 4- and 20-month-old C57BL6/NTac male mice (n = 8-10) by intraperitoneal injections of 1 µg/g body weight <smlcap>L</smlcap>-thyroxine (T₄) every 48 h over 7 weeks. Control animals were injected with PBS. Total RNA was isolated from liver samples for analysis of the TH transporter and TH-responsive gene expression. TH concentrations were determined in mice sera. Results: Baseline serum free T₄ (fT₄) concentrations were significantly higher in euthyroid young compared to old mice. T₄ treatment increased total T₄, fT₄ and free triiodothyronine to comparable concentrations in young and old mice. In the euthyroid state, TH transporter expression was significantly higher in old than in young mice, except for Mct8 and Oatp1a1 expression levels. Hyperthyroidism resulted in upregulation of Mct10, Lat1 and Lat2 in liver tissue, while Oatp1a1, Oatp1b2 and Oatp1a4 expression was downregulated. This effect was preserved in old animals. Conclusion: Here, we show age-dependent differences in TH transporter mRNA expression in the euthyroid and hyperthyroid state of mice focusing on the liver as a classical TH target organ.

Context: 3,5-Diiodo-<smlcap>L</smlcap>-thyronine (3,5-T₂) is a thyroid hormone metabolite which exhibited versatile effects in rodent models, including the prevention of insulin resistance or hepatic steatosis typically forced by a high-fat diet. With respect to euthyroid humans, we recently observed a putative link between serum 3,5-T₂ and glucose but not lipid metabolism. Objective: The aim of the present study was to widely screen the urine metabolome for associations with serum 3,5-T₂ concentrations in healthy individuals. Study Design and Methods: Urine metabolites of 715 euthyroid participants of the population-based Study of Health in Pomerania (SHIP-TREND) were analyzed by ¹H-NMR spectroscopy. Multinomial logistic and multivariate linear regression models were used to detect associations between urine metabolites and serum 3,5-T₂ concentrations. Results: Serum 3,5-T₂ concentrations were positively associated with urinary levels of trigonelline, pyroglutamate, acetone and hippurate. In detail, the odds for intermediate or suppressed serum 3,5-T₂ concentrations doubled owing to a 1-standard deviation (SD) decrease in urine trigonelline levels, or increased by 29-50% in relation to a 1-SD decrease in urine pyroglutamate, acetone and hippurate levels. Conclusion: Our findings in humans confirmed the metabolic effects of circulating 3,5-T₂ on glucose and lipid metabolism, oxidative stress and enhanced drug metabolism as postulated before based on interventional pharmacological studies in rodents. Of note, 3,5-T₂ exhibited a unique urinary metabolic profile distinct from previously published results for the classical thyroid hormones.