Familial dysalbuminemic hyperthyroxinemia, a syndrome involving an abnormal affinity of albumin for thyroxine, results in elevated total thyroxine and free thyroxine index levels but normal triiodothyronine resin uptake and thyroid-stimulating hormone levels. Danazol is a synthetic androgen that increases triiodothyronine resin uptake and decreases total thyroxine, secondary to a decrease in thyroid-binding globulin levels. A 35-year-old woman with familial dysalbuminemic hyperthyroxinemia who was given danazol, in whom nervousness, insomnia, and weight gain developed, is described. Thyroid tests performed after initiation of danazol therapy revealed an increase in triiodothyronine resin uptake, with persistently elevated total thyroxine and free thyroxine index levels, and normal thyroid-stimulating hormone levels. Once the danazol was withdrawn, the symptoms resolved, the triiodothyronine resin uptake returned to normal, and the thyroid-stimulating hormone remained normal. The effects of danazol on a patient with familial dysalbuminemic hyperthyroxinemia correlate well with the effects on normals, and the ultrasensitive thyroid-stimulating hormone was the most useful test in separating hyperthyroxinemia from hyperthyroidism.