Prolonged intravenous infusion of labelled iodocompounds in the rat: [125I]thyroxine and [125I]tri-iodothyronine metabolism and extrathyroidal conversion of thyroxine to tri-iodothyronine.

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. Signifficant 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 distribution of labelled T3 after the two types of infusion: kidney and liver were both prominent sites of conversion of T4 to T3.