Changes in body composition in women following treatment of overt and subclinical hyperthyroidism

Objective: To determine changes in weight, body composition, and bone density after treatment of overt hyperthyroidism (OH) and subclinical hyperthyroidism (SCH) in women. Methods: Women with OH and SCH referred to the Mayo Clinic Thyroid Clinic were recruited. Hyperthyroid patients and euthyroid control women were matched for age (within decade) and body mass index. Patients with OH and SCH were treated to normalize thyroid function test results and were restudied after 6 months of normal thyroid function. Baseline and posttreatment studies included measurement of height, weight, bone density, lean mass, fat mass, and thigh muscle cross-sectional area. All participants had normal thyroid function test results for at least 6 months before completion of the posttreatment studies. Results: Twenty-four patients with OH, 21 patients with SCH, and 36 control patients were studied. In the OH group, fat-free mass increased from a mean of 36.8 kg (95% confidence interval [CI], 34.8-38.8) to 40.4 kg (95% CI, 38.5-42.3); in the SCH group, fat-free mass increased from a mean of 40.3 kg (95% CI, 38.1-42.5) to 42.2 kg (95% CI, 39.7-44.7). In both groups, fat mass increased to approximately the same extent, and both groups experienced significant weight gain with no change in percent body fat. Thigh muscle cross-sectional area increased in both groups - from 100.6 cm² (95% CI, 92.7-108.5) to 113.3 cm2 (95% CI, 105.5-121.1) in the OH group and from 106.1 cm2 (95% CI, 96.7-115.5) to 112.2 cm2 (95% CI, 102.0-122.4) in the SCH group. Bone density increased in patients with OH (P<.01) and in patients with SCH (P<.05). Conclusions: Treatment of OH and SCH leads to increases in muscle area and bone density. Weight gain reflects increases in both fat and fat-free mass. While these results provide some support for actively treating SCH in women, further prospective studies are needed to determine whether the changes documented translate into real patient benefit.