"In Vivo" and "In Vitro" Regulation by Iodide of Thyroglobulin lodination and Thyroxine Synthesis

Abstract

Thyroglobulin iodination and thyroid hormone formation were studied as a function of iodide concentration both "in vivo" and "in vitro". The studies were performed "in vivo" using rats equilibrated with iodide (¹²⁷I labelled with ¹²⁵1). Double labelling experiments were also performed using tracer ¹³¹I injected with the last dose of ¹²⁷I at equilibrium. "In vitro" experiments were performed using purified thyroid peroxidase, non iodinated goiter thyroglobulin, an H₂O₂ generating system and increasing iodide concentrations.

"In vitro" as "in vivo" iodide, present in high concentration inhibited thyroglobulin iodination. Both "in vivo" and "in vitro" biphasic responses to iodide were observed i.e. increase in iodination as the size of the carrier iodide increased was followed by severe inhibition by higher iodide concentrations. Both "in vitro" and "in vivo" inhibition was only transient, and was no longer observed when the iodide concentration decreased. In these respects results from "in vivo" and "in vitro" experiments agreed with the classical Wolff-Chaikoff effect.

Hormone formation was also severely inhibited as iodide concentration was increased. Both "in vivo" and "in vitro" experiments showed a clear inhibition of the coupling reaction.

Thus additional evidence is provided which demonstrates the existence of at least one intrinsic thyroidal mechanism, at the level of thyroid peroxidase, which responds to variations in the iodide concentration both for total iodination of thyroglobulin and for coupling of iodotyrosine residues to thyroid hormones.

Key words

Iodide - Thyroglobulin - Iodination - Thyroxine - Regulation