LEAN BODY-MASS AS A DETERMINANT OF THYROID SIZE

OBJECTIVE Males have a larger thyroid gland than females, and this has been related to the difference in body weight. In view of the differe nt body composition of men and women, we hypothesized that lean body m ass is a better determinant of thyroid volume than body weight. DESIGN A cross-sectional study in an area not deficient in iodine. SUBJECTS 44 non-obese healthy adults (group I, 21 men, 23 women with equal dist ribution of sexes in age groups between 21 and 70 years) and 20 adults with marked obesity (group II, 8 men, 12 women, BMI> 30 kg/m(2)) were studied. None used medication and all had normal thyroid function tes ts. MEASUREMENTS Thyroid volume was measured by ultrasonography, and l ean body mass with a body impedance analyser. RESULTS The thyroid volu me in men was larger than in women in both groups; it was also larger in the obese than in the non-obese subjects. In the nonobese subjects, thyroid volume was related both to body weight (r = 0.42, P < 0.005) and to lean body mass (r=0.55, P=0.0001). In the obese subjects, thyro id volume was no longer related to body weight (r= 0.23, NS) but was s till correlated with lean body mass (r=0.54, P=0.01). Taking both grou ps together, the correlation between thyroid volume and lean body mass (r=0.64, P<0.001) was stronger than between thyroid volume and body w eight (r= 0.50, P< 0.001). Thyroid Volume was also related to body len gth (group I, r=0.42, P<0.005; group II, r=0.54, P=0.01), but to body- surface area only in the non-obese subjects (group I, r=0.45, P<0.01; group II, r=0.38, NS). The larger thyroid size in the obese was associ ated with slightly but significantly higher TSH and lower free T4 seru m concentrations as compared to the non-obese subjects. CONCLUSIONS In healthy adults, lean body mass rather than body weight explains the d ifferences in thyroid volume between males and females and between obe se and non-obese subjects. Lean body mass appears to be a major determ inant of thyroid size.