Weight gain in patients after therapy for hyperthyroidism.

OBJECTIVE: To determine the prevalence of obesity following therapy for hyperthyroidism and to assess the contributing factors associated with an undesirable weight gain.
DESIGN: A retrospective analysis was undertaken of clinical records for 160 hyperthyroid patients attending an endocrine clinic in Bloemfontein (1994-2001).
RESULTS: Of the 160 patients, 143 had Graves' disease and 17 patients had multinodular goitre. Most of our patients (N = 147) were treated with radioiodine, 10 patients with carbimazole and 3 patients had thyroidectomy. The median weight gain 6 months after therapy was 5.0 kg, after 12 months 9.0 kg, and after 24 months 12 kg, whereafter body mass stabilised. Before therapy 27.5% of patients had a body mass index (BMI) of < 22 kg/m2, 29.4% were overweight (BMI > 25 kg/m2) and 19.3% were obese (BMI > 30 kg/m2). Two years after treatment only 8.7% of patients had a BMI of < 22 kg/m2, 27.5% had a BMI > 25 kg/m2, and 51.3% had become obese. The main factors associated with weight gain 24 months after therapy were poor control of thyroid function on replacement therapy, diagnosis of Graves' disease and need for thyroxine replacement.
CONCLUSION: This study has shown a large increase (32%) in the prevalence of obesity following treatment for hyperthyroidism. The main weight gain was during the first 2 years after therapy. The main factors contributing to excessive weight gain were need for replacement therapy and poor control of thyroid function.