A retrospective study of the natural history of endogenous subclinical hyperthyroidism.

OBJECTIVE The treatment of subclinical hyperthyroidism is controversial because the natural history is uncertain. We undertook a retrospective study to examine the natural history of endogenous subclinical hyperthyroidism. METHODS Between 2002 and 2006, we identified 116 patients with thyroid-stimulating hormone (TSH) concentrations <0.4 microIU/mL but normal free thyroxin and triiodothyronine or free triiodothyronine levels and >6 months of follow-up. The medical records of these subclinical hyperthyroid patients were reviewed for demographic data, clinical outcomes, and thyroid function test results. Because the etiology of hyperthyroidism could not be identified in 57 (49%) patients, we compared patients with normal-sized or diffusely enlarged thyroid glands with patients with nodular thyroid glands. We also compared the results of patients with initial TSH levels <0.1 microIU/mL to patients with TSH levels between 0.1 and 0.39 microIU/mL, and patients age <65 years to older patients. RESULTS Of 116 patients with subclinical hyperthyroidism, 88 (76%) were women and 28 (24%) were men. They ranged in age from 19 to 98 years (mean = 55 years). Ninety-eight patients did not have thyroid nodules, and 18 had thyroid nodules. Follow-up ranged from 6 months to 6.5 years (median, 3.2 years). TSH reverted to normal in 58 (59%) patients without nodules; we treated only 4 (4%) of these patients for hyperthyroidism. In contrast, TSH levels in only 3 (17%) patients with nodules reverted to normal and 7 (39%) received antithyroid treatment. Atrial fibrillation was present in 8 (8%) patients without thyroid nodules and in 3 patients (17%) with thyroid nodules (P=.373). There were no significant outcome differences based on initial TSH levels or age. CONCLUSION We conclude that most patients with subclinical hyperthyroidism who do not have thyroid nodules rarely require antithyroid therapy but should be followed carefully.