Danazol is a synthetic steroid with antigonadotropic properties useful in treating endometriosis, especially in young infertile women. Prior to its availability for clinical use in September 1976, thyroid function studies other than thyroid-stimulating hormone (TSH), which was normal, had not been reported. Soon after danazol began to be used in the infertility clinic to treat documented endometriosis, it was observed that changes occurred in thyroid function studies. While no patients manifested clinical evidence of hypothyroidism, all 8 patients receiving 800 mg of danazol daily for 1 to 5 months had laboratory evidence of decreased thyroid function. The triiodothyronine (T3) uptake was elevated and the total serum thyroxine (T4) was decreased. The finding that TSH and the free thyroid index (FTI) were normal confirmed that these patients were euthyroid during danazol therapy. The abnormality of thyroid function tests is believed to reflect an androgen-like reduction in thyroxine-binding protein rather than a true decrease in thyroid function or interference with the pituitary-thyroid axis.