To determine the frequency of thyroxine (T4) toxicosis, we calculated a free triiodothyronine index (FT3I) on 124 patients who displayed elevations of the free T4 index (FT4I) on initial screening. If the clinical presentation was not characteristic of hyperthyroidism or if the FT3I was not elevated, a thyrotropin-releasing hormone (TRH) test was performed. Of 83 hyperthryoid patients, 70 displayed elevations of both the FT4 and FT3 indices. Thirteen patients, however, had elevations of the FT4I alone. This frequency of T4 toxicosis is higher than generally appreciated. Forty-one patients, most of whom were ill with nonthyroidal diseases, had normal TRH test results and were classified euthyroid. We conclude that FT4I elevations owing to T4 toxicosis and nonthyroidal illnesses are common and that the laboratory differentiation between the syndromes may be difficult.