PROLONGED INTRAVENOUS INFUSION OF LABELLED IODOCOMPOUNDS IN THE RAT: [125I]IODIDE METABOLISM AND EFFECTS OF MODERATE DIETARY IODINE DEFICIENCY

Abstract

SUMMARY: Radio-iodide was administered by prolonged continuous intravenous infusion to rats maintained under iodine-replete conditions and in moderate iodine deficiency. A close approximation to equilibrium labelling was thereby achieved. Labelled iodocompounds extracted from various tissues were analysed by thin-layer chromatography. Moderate iodine deficiency resulted in a slight increase in the ratio of mono-iodotyrosine to di-iodotyrosine in the thyroid. No change in the ratio of tri-iodothyronine (T₃) to thyroxine (T₄) was found in thyroid, plasma or skeletal muscle. Faecal excretion of T₃ declined appreciably relative to that of T₄. Under iodine-replete conditions the ratio of thyroidal secretion rates of T₃ and T₄ was estimated to be more than three times higher than the ratio of these iodocompounds within the thyroid. Heterogeneity of thyroglobulin structure and function may explain these observations.