Direct measurement of free thyroxine in patients with T3-toxicosis.

Circulating levels of free thyroxine were directly measured using a new kinetic radioimmunoassay system. Free thyroxine was abnormally elevated in: 1) each of eleven hyperthyroid individuals having elevated concentrations of thyroxine and triiodothyronine; and 2) all five hyperthyroid subjects who had normal circulating levels of thyroxine but elevated concentrations of triiodothyronine (T3-toxicosis). Using this assay system, free thyroxine appears to be elevated in all hyperthyroid individuals.