Triiodothyronine Production in Graves' Hyperthyroidism

Abstract

Objective: To quantify the relative contributions of thyroid secretion and peripheral generation to triiodothyronine(T₃) production in untreated Graves' hyperthyroidism. Patients, Design, and Measurements: Thirty-one patients with hyperthyroidism, of whom 6 had T₃ toxicosis, and 21 surgically and radioiodine ablated patients with thyroid cancer on thyroid stimulating hormone–suppressive therapy in whom serum T₃ should reflect peripheral generation alone were compared with respect to serum free thyroxine (T₄) and serum free T₃ concentrations.Main Outcomes: Serum free T₄/free T₃ molar ratios were virtually identical in the patients with T₄+T₃ toxicosis (2.7 ± 0.4) and those with T₃ 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 T₃ was calculated as the quotient of the individual serum free T₄ concentration and the free T₄/free T₃ molar ratio in thyroid cancer; this value was subtracted from the individual measured free T₃ concentration to derive the value for secreted T3. Secreted T₃ accounted for 33 ± 6% of T₃ production in T₄+T₃ toxicosis and 34 ± 10% in T₃ toxicosis.Conclusions: 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.