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).