Iodide and T4 kinetics in plasma, thyroid gland and skin of 10-day-old rats: effects of iodine deficiency
Résumé
The effects of iodine deficiency on the peripheral metabolism of thyroid hormone in immature rat were evaluated by measuring the kinetics of iodide and thyroxine (T 4 ) in control and iodine-deficient 10-day-old rats. Iodine-deficient pups were obtained by giving the mother drinking water containing perchlorate; this anion is not transferred but prevents iodine transfer in the mother's milk. Labelled iodocompounds were measured in plasma, thyroid and skin for 48 h following intravenous injection of Na 131 I plus [ 125 I]T 4 . Data were interpreted by compartmental analysis. The iodide plasma clearance rate, plasma equivalent distribution volume, plasma concentration, production and iodine thyroid content of iodine-deficient rats were significantly lower (−29%, −31%, −84%, −89% and −87% respectively) than in control 10-day-old rats. The iodide thyroid uptake was reduced (−47%) but remained higher than the release of iodine as T 4 . Cutaneous iodine was lost much more quickly by iodine-deficient pups than by control pups, explaining the decreased iodide distribution volume. The parameters of T 4 metabolism were not changed by iodine deficiency, except for a slight but significant reduction of the thyroxinemia and T 4 pools (−13%). Thence, T 4 production was not significantly changed (about 8 pmol/h in control and iodine-deficient rats). The labelled T 4 curves in iodine-deficient and control skin were superimposed and the patterns of labelled T 3 derived from [ 125 I]T 4 were identical. Thus, the skin of immature rats converts T 4 to T 3 ; this process was not disturbed by iodine deficiency. The thyroid function of immature rats is particularly resistant to iodine deficiency, but the mechanisms remain unknown.