Subclinical thyrotoxicosis is defined as low serum thyrotropin (TSH) and normal serum thyroid hormone concentrations. It must be distinguished from nonthyroidal illness and secondary hypothyroidism. The most common causes are excessive thyroid hormone therapy, autonomously functioning thyroid adenoma, multinodular goiter, and Graves' disease, but many patients have no evident thyroid disease. A few patients have minor symptoms and signs of hyperthyroidism. The likelihood of progression to overt thyrotoxicosis is low, and many patients have normal serum TSH concentrations weeks or months later. Treatment should be based on consideration of the cause of the subclinical thyrotoxicosis, and whether the patient has any clinical manifestations of thyroid hormone excess or underlying problems likely to be aggravated by small increases in thyroid secretion.