A longitudinal study of changes in body mass index and total body composition after radioiodine treatment for thyrotoxicosis.

Patients treated for thyrotoxicosis often complain of increases in body weight after treatment of their thyroid disorder. The objective of this study was to define the extent of changes in body mass and composition following treatment of thyrotoxicosis with radioiodine 131I. We prospectively measured body mass index (BMI) in 75 patients (18 males, 57 females), ranging in age from 15-88 years treated for thyrotoxicosis with 131I (doses ranging from 5.2 to 25.7 mCi) between 1978 and 1994. BMI pre- and post-radioiodine treatment were recorded for up to 10 years after treatment. Body composition studies were performed on a subgroup of 9 patients (1 male, 8 females), aged 24 to 74 years treated with 6.0 to 12.2 mCi 131I. Bone mineral content, lean mass, and fat content were determined prior to and following 131I treatment through whole body scanning with a Hologic QDR-1000/W dual energy x-ray densitometer. Sustained increases in BMI averaging 2.33 kg/m2 above baseline (p < .01) were observed from the initial 6 months through the first 5 years after 131I therapy. BMI was no different from pretreatment levels over the remainder of the 10-year follow-up period. Lean mass increased significantly (p = .0004) by an average of 7.2 kg. Although both fat and mineral content appeared to increase, these changes were not statistically significant. Weight gain occurs within the first year after 131I treatment of patients with thyrotoxicosis and is predominantly due to increased lean mass (20.2%, p < .0005) although bone mineral content increased only marginally (4.62%, p = .10).