The development of specific radioimmunoassays has allowed measurements of 3,3',5'-triiodothyronine (reverse T3), 3,3'-diiodothyronine (3,3'T2), and thyroglobulin to be performed in serum and in various biological fluids both in normal and in altered states of thyroidal economy. The physiology, kinetics, and metabolic actions of reverse T3, 3,3'T2 and thyroglobulin are reviewed. Presently, it appears that reverse T3 and 3,3'T2 measurements in amniotic fluid and cord serum may potentially be useful in diagnosing fetal or neonatal thyroid dysfunction, and serum thyroglobulin measurements appear to be important as a measure of the efficacy of treatment of patients with differentiated thyroid carcinoma.