Influence of D-Thyroxine on Plasma Thyroid Hormone Levels and TSH Secretion

Abstract

Triiodothyronine (T₃), thyroxine (T₄), basal TSH and TSH after stimulation with TRH were determined in healthy subjects and patients treated with D-thyroxine (DT₄). After a dosage of 6 mg DT₄ the D/L T₄ plasma concentration rose about 4-fold 4 hours after application and was only moderately elevated 14 hours later. To achieve constantly elevated T₄ levels 3 mg DT₄ were applied in the further experiment every 12 hours. The D/L T₄ plasma concentration rose 2.5-4-fold and there was a small but significant increase of the D/L T₃ plasma concentration. 74 hours after onset of treatment basal TSH was below detectable limits and the increase of TSH 30 min after injection of 200 µg TRH (TRH test) was only about 15% compared to zero time. The time course of TSH suppression was investigated after treatment with DT₄ and LT₄ (single dosage of 3 mg). TRH-tests were performed before, 10, 26, 50 and 74 hours after the first dosage of D or LT₄. There was no difference in the time course of basal TSH and TSH stimulated by TRH. In 10 patients on DT₄ longterm therapy, basal and stimulated TSH were found to be below the detectable limits of 0.4 µg/ml. Our results show that (1) plasma half-life of DT₄ is less than 1 day, (2) TSH suppression after D and LT₄ treatment is very similar, and (3) in patients on long-term DT₄ treatment, TSH plasma concentration is below detectable limits even after stimulation with TRH.

DT₄ - T₄ - T₃ - TSH - TRH