PROLONGED INTRAVENOUS INFUSION OF LABELLED IODOCOMPOUNDS IN THE RAT: [125I]THYROXINE AND [125I]TRI-IODOTHYRONINE METABOLISM AND EXTRATHYROIDAL CONVERSION OF THYROXINE TO TRI-IODOTHYRONINE

Abstract

SUMMARY: Extrathyroidal tissues of the rat were labelled to steady state by prolonged continuous intravenous infusion of ¹²⁵I-labelled thyroxine (T₄) or tri-iodothyronine (T₃). Labelled iodocompounds extracted from various tissues were analysed by thin-layer chromatography. Significant amounts of labelled T₃ were found in all tissues examined after infusion of [¹²⁵I]T₄, confirming that conversion of T₄ to T₃ occurs in extrathyroidal tissues of the rat. Faecal excretion of labelled T₃ after [¹²⁵I]T₄ infusion provided an assessment of the extent of extrathyroidal conversion: about a third of the T₄ was metabolized by this pathway. Extrathyroidal conversion was independently estimated to account for about a third of the total production of T₃. The site of extrathyroidal conversion was established by comparing the distributions of labelled T₃ after the two types of infusion: kidney and liver were both prominent sites of conversion of T₄ to T₃.