Peripheral Autoregulation of Thyromimetic Activity in Man

Summary

This study describes an extrathyroidal mechanism for regulating serum concentration of triiodothyronine (T₃) in thyroxine (T₄) deficiency or excess. Serum levels of T₃, T₄, reverse T₃ (rT₃), and thyrotropin were determined in two series of athyreotic patients (n=22 and n=16, respectively) during various doses of T₄ substitution therapy. The patients were followed from the severe hypothyroid state up to the modest hyperthyroid state, induced by stepwise increasing doses of administered T₄. The periods of constant T₄ replacement doses were 1 week (group I) and 5 weeks (group II), respectively. As serum T₄ levels rose from less than 0.5 μg/dl to 18.9 μg/dl, serum T₃ levels showed a curvilinear increase, which was pronounced in the very low T₄ range while it was flattened at the higher end of the spectrum of T₄ levels. As to rT₃, an analogous course did not occur. The different character of the relationships between T₄ and its conversion products was further elucidated by calculating the T₃/T₄ and rT₃/T₄ ratios. T₃/T₄ ratio declined fourfold from 43.3 · 10^-3 in the severe hypothyroid to 11.7 · 10^-3 in the hyperthyroid range. The results support the hypothesis of a peripheral autoregulation of T₄ to T₃ conversion which appears to be operative at both ends of the T₄ spectrum and which serves to maintain or defend serum T₃ levels.

Key-Words

Thyroxine - Triiodothyronine - Reverse Triiodothyronine - Thyroxine Conversion - Autoregulation