Steady-State Serum T3 Concentrations for 48 Hours Following the Oral Administration of a Single Dose of 3,5,3'-Triiodothyronine Sulfate (T3S).

Objective: Sulfate conjugation of thyroid hormones is an alternate metabolic pathway that facilitates the biliary and urinary excretion of iodothyronines and enhances their deiodination rate, leading to the generation of inactive metabolites. A desulfating pathway reverses this process, and thyromimetic effects have been observed following the parenteral administration of 3,5,3'-triiodothyronine (T3) sulfate (T3S) in rats. The present study investigated whether T3S is absorbed after oral administration in humans and if it represents a source of T3.Methods: Twenty-eight hypothyroid patients (7 men and 21 women; mean age, 44 ± 11 years) who had a thyroidectomy for thyroid carcinoma were enrolled. Replacement thyroid hormone therapy was withdrawn (42 days for thyroxine, 14 days for T3) prior to 131I remnant ablation. A single oral dose of 20, 40, 80 (4 patients/group), or 160 μg (16 patients/group) of T3S was administered 3 days before the planned administration of 131I. Blood samples for serum T3S and total...