title: 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.
  
  
  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.
authors.affiliation: 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
authors.forename: Emilio
authors.initials: E
authors.lastname: Ortega
authors.forename: Nicola
authors.initials: N
authors.lastname: Pannacciulli
authors.forename: Clifton
authors.initials: C
authors.lastname: Bogardus
authors.forename: Jonathan
authors.initials: J
authors.lastname: Krakoff
mesh_terms: 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*
chemical_list: D014284:Triiodothyronine; D013974:Thyroxine