Over a 2-year period, thyroid function was studied in 102 patients infected with the human immunodeficiency virus (HIV) and in 102 age- and sex-matched controls with various infectious diseases. Biochemical abnormalities were observed in 1-20% of the patients, depending on thyroid indices, but thyroid disease (hypothyroidism) was diagnosed in only 1. Compared to controls, patients, especially those with the acquired immunodeficiency syndrome (AIDS), had a significant increase in serum thyroxine-binding globulin, a lower T3 and free thyroxine index, and a higher frequency of thyroglobulin antibodies.