Free triiodothyronine toxicosis in two adolescents.

Two male adolescents had subtle symptoms and signs of thyrotoxicosis but normal levels of total and free thyroxine and total triiodothyronine. Serum concentrations of thyrotropin were undetectable in basal specimens and after administration of thyrotropin-releasing hormone; only free triiodothyronine values were elevated. An increase in serum levels of free triiodothyronine may be the earliest secretory abnormality of an overactive thyroid gland.