Background: Thyroid disease is associated with BMI with hyperthyroidism causing weight loss and hypothyroidism leading to weight gain. However, the relation of body weight distribution to thyroid function in euthyroid individuals is unclear. This study assessed body composition in euthyroid women.
Methods: Euthyroid women (n=1072) from the Osteoporosis and Ultrasound Study (OPUS), a population based cohort study recruited participants from 5 European cities, had their thyroid function measured in 2001/02. Individuals with history of thyroid illness, those on any drugs influencing thyroid metabolism and FT3 concentrations <2.1 pmol/l, as a marker of non-thyroidal illness were excluded. Whole body adiposity and lean mass was measured in 2007/08 by dual energy X-ray absorptiometry (DXA) scan. The association between body composition and thyroid function was analysed using correlation analyses (Pearsons r for FT4 and FT3 and Spearmans rho for TSH) and linear regression analyses after correction for other baseline confounding variables such as age, BMI, smoking and diabetes.
Results: The mean age (S.D.) was 61 (13) years. Median (range) TSH levels were 0.86 (0.253.48) mIU/l, mean FT4 and FT3 levels were 12.8 (1.8) pmol/l and 3.7 (0.84) pmol/l respectively. Whole body and truncal lean mass were negatively correlated with FT4 (r=−0.16 and −0.19), FT3 (r=−0.18 and −0.21) and positively correlated with TSH (ρ=0.09 and 0.11). Whole body and truncal fat mass were positively correlated with FT3 (r=0.09 and 0.12) but not with FT4 and TSH levels. In multiple linear regression analysis, FT3 and TSH independently predicted whole body and truncal lean mass whereas whole body and truncal fat mass were predicted by FT3 alone. FT3 to FT4 ratio was also significantly correlated with truncal fat levels (r=0.09, P=0.002).
Conclusions: Thyroid hormones levels, even in the euthyroid range, independently predict whole body lean and fat mass as well as central adiposity. This suggests that thyroid hormones play an important role in the pathogenesis of obesity.