Abstract Six patients with hyperthyroxinemia (five men and one woman) were evaluated for possible hyperthyroidism. All were taking large daily doses of propranolol—480 ± 155 (± SE) mg—for treatment of angina pectoris. The patients had no clinical evidence of hyperthy-roidism, although three had small goiters. Further evaluation of the patients revealed elevated serum free thyroxine levels and/or free thyroxine index, low-normal serum triiodothyronine levels, and elevated serum reverse triiodothyronine levels in all six. The thyroid-stimulating hormone response to thyrotropin-releasing hormone was normal in two patients, subnormal in three patients, and absent in one patient. One patient was restudied while receiving low-dose propranolol (80 mg a day), with normalization of all thyroid functional parameters. The data suggest that the abnormalities seen in patients taking high doses of propranolol were due to drug-induced blockade of iodothyronine deiodination. Signs and symptoms of hyperthy-roidism are lacking in patients taking large doses of propranolol. If such a patient is discovered to have an elevated serum thyroxine level, a more complete evaluation of thyroid function is necessary before the diagnosis of thyrotoxicosis can be made. The thyrotropin-releasing hormone test may be of particular value in this circumstance.