Plasma concentrations of free triiodothyronine predict weight change in euthyroid persons

Abstract

Background:Factors that influence energy metabolism and substrate oxidation, such as thyroid hormones (THs), may be important regulators of body weight.Objective:We investigated associations of THs cross-sectionally with obesity, energy expenditure, and substrate oxidation and prospectively with weight change.Design:Euthyroid, nondiabetic, healthy, adult Pima Indians (n= 89; 47 M, 42 F) were studied. Percentage body fat (%BF) was measured by using dual-energy X-ray absorptiometry; sleeping metabolic rate (SMR), respiratory quotient, and substrate oxidation rates were measured in a respiratory chamber. Thyroid-stimulating hormone (TSH), free thyroxine (T4), free triiodothyronine (T3), and leptin concentrations were measured in fasting plasma samples.Results:TSH, but neither free T3nor free T4, was associated with %BF and leptin concentrations (r= 0.27 and 0.29, respectively; both: P≤ 0.01). In multiple regression analyses adjusted for age, sex, fat mass, and fat-free mass, free T3was a positive predictor of SMR (P= 0.02). After adjustment for age, sex, %BF, and energy balance, free T3was a negative predictor of 24-h respiratory quotient (P< 0.05) and a positive predictor of 24-h lipid oxidation rate (P= 0.006). Prospectively, after an average follow-up of 4 ± 2 y, the mean increase in weight was 3 ± 9 kg. Baseline T3concentrations were associated with absolute and annual percentage of changes in weight (r= −0.27, P= 0.02, and r= −0.28, P= 0.009, for the age- and sex-adjusted associations, respectively).Conclusions:In euthyroid Pima Indians, lower free T3but not free T4concentrations were an independent predictor of SMR and lipid oxidation and a predictor of weight gain. This finding indicates that control of T4-to-T3conversion may play a role in body weight regulation.