Objective: Males have a larger thyroid gland than females, and this has been related to the difference in body weight. In view of the different body composition of men and women, we hypothesized that lean body mass 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 distribution 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/m2) were studied. None used medication and all had normal thyroid function tests.
Measurements: Thyroid volume was measured by ultrasonography, and lean body mass with a body impedance analyser.
Results: The thyroid volume 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, thyroid volume was no longer related to body weight (r = 0.23, NS) but was still correlated with lean body mass (r = 0.54, P = 0.01). Taking both groups together, the correlation between thyroid volume and lean body mass (r = 0.64, P < 0.001) was stronger than between thyroid volume and body weight (r = 0.50, P < 0.001). Thyroid volume was also related to body length (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 associated with slightly but significantly higher TSH and lower free T4 serum concentrations as compared to the non-obese subjects.
Conclusions: In healthy adults, lean body mass rather than body weight explains the differences in thyroid volume between males and females and between obese and non-obese subjects. Lean body mass appears to be a major determinant of thyroid size.