Serum TSH related to measures of body mass: longitudinal data from the HUNT Study, Norway

OBJECTIVE: Thyroid function and body mass are related, but the causal relationship remains unclear. Our objective was to investigate the longitudinal relationship between thyroid stimulating hormone (TSH) and body mass measures [body weight, body mass index (BMI), waist circumference (WC) and waist-hip-ratio (WHR)]. DESIGN: We used data from two waves of a population-based study: HUNT 2 (1995-1997) and 3 (2006-2008). Average follow-up time was 10·5 years. Multivariable general linear and logistic regression models were used to assess the relation between TSH and the body mass measures. PARTICIPANTS: In total 9954 women and 5066 men without self-reported thyroid disease and TSH within the reference range (0·5-3·5 mU/l) at baseline and <10 mU/l at follow-up. RESULTS: For each mU/l increase in TSH among women, weight increased 0·9 kg (95% CI 0·8, 1·1), BMI 0·3 kg/m(2) (95% CI 0·3, 0·4) and WC 0·6 cm (95% CI 0·3, 0·8). In men, the corresponding figures were 0·8 kg (95% CI 0·5, 1·0), 0·2 kg/m(2) (95% CI 0·2, 0·3) and 0·5 cm (95% CI 0·2, 0·8). In line with this, a weight gain of more than 5 kg was associated with a TSH increase of 0·08 mU/l (95% CI 0·06, 0·11) in women and 0·15 mU/l (95% CI 0·12, 0·18) in men. Women who lost more than 5 kg decreased their TSH by 0·12 mU/l (95% CI 0·09, 0·16) and men by 0·03 mU/l (95% CI -0.02, 0·09). CONCLUSION: Weight gain is accompanied by increasing TSH, and weight loss in women is related to decreasing TSH.