Plasma concentrations of free triiodothyronine predict weight change in euthyroid persons.
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.


METHODS
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 (T(4)), free triiodothyronine (T(3)), and leptin concentrations were measured in fasting plasma samples.


RESULTS
TSH, but neither free T(3) nor free T(4), was associated with %BF and leptin concentrations (r = 0.27 and 0.29, respectively; both: P <or= 0.01). In multiple regression analyses adjusted for age, sex, fat mass, and fat-free mass, free T(3) was a positive predictor of SMR (P = 0.02). After adjustment for age, sex, %BF, and energy balance, free T(3) was 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 T(3) concentrations 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 T(3) but not free T(4) concentrations were an independent predictor of SMR and lipid oxidation and a predictor of weight gain. This finding indicates that control of T(4)-to-T(3) conversion may play a role in body weight regulation.
Ortega
Emilio
E
Phoenix Epidemiological Clinical and Research Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Department of Health and Human Services, Phoenix, AZ 85016, USA. emilioo@mail.nih.gov
Pannacciulli
Nicola
N
Bogardus
Clifton
C
Krakoff
Jonathan
J
D050154:Adiposity / Q000502:physiology; D000293:Adolescent; D000328:Adult; D004734:Energy Metabolism / Q000502:physiology; D005260:Female; D006801:Humans; D007198:Indians, North American; D015227:Lipid Peroxidation; D008297:Male; D013961:Thyroid Gland / Q000502:physiology*; D013974:Thyroxine / Q000097:blood; D014284:Triiodothyronine / Q000097:blood*; D015430:Weight Gain / Q000502:physiology*; D015431:Weight Loss / Q000502:physiology*
D014284:Triiodothyronine; D013974:Thyroxine