Triiodothyronine Production in Graves' Hyperthyroidism

Abstract

To quantify the relative contributions of thyroid secretion and peripheral generation to triiodothyronine (T(3)) production in untreated Graves' hyperthyroidism. PATIENTS, DESIGN, AND MEASUREMENTS: Thirty-one patients with hyperthyroidism, of whom 6 had T(3) toxicosis, and 21 surgically and radioiodine ablated patients with thyroid cancer on thyroid stimulating hormone-suppressive therapy in whom serum T(3) should reflect peripheral generation alone were compared with respect to serum free thyroxine (T(4)) and serum free T(3) concentrations. Serum free T(4)/free T(3) molar ratios were virtually identical in the patients with T(4)+T(3) toxicosis (2.7 +/- 0.4) and those with T(3) toxicosis (2.6 +/- 0.4) and were significantly lower than in the patients with thyroid cancer (4.0 +/- 0.4) (p < 0.001). In the hyperthyroid patients, peripherally generated T(3) was calculated as the quotient of the individual serum free T(4) concentration and the free T(4)/free T(3) molar ratio in thyroid cancer; this value was subtracted from the individual measured free T(3) concentration to derive the value for secreted T3. Secreted T(3) accounted for 33 +/- 6% of T(3) production in T(4)+T(3) toxicosis and 34 +/- 10% in T(3) toxicosis. This study indicates that about one third of T3 production in untreated Graves' hyperthyroidism, irrespective of whether presenting as T4+T3 toxicosis or T3 toxicosis, arises from thyroid secretion as compared to about 20% in normal individuals.