The effects of chronic administration of excess iodine on thyroidal hormone synthesis in the chick.

Abstract

The response of thyroid weight and iodine metabolism to excess ingestion of iodide was studied in the chick. Treatment of chicks with 1mg of iodide for 4 weeks induced a marked increase in thyroid weight. The radiochromatography of the 131I labelled thyroid hydrolysate showed that a single injection of carrier iodide resulted in a significant but transient reduction of a relative proportion of organic 131I as well as thyronine 131I and an elevation of the MIT/DIT ratio. After chronic treatment of chicks with excess iodide, the thyronine 131I proportion was still decreased notwithstanding the fact that the organic 131I proportion showed a return to normal and MIT/DIT was rather low. The increased thyroid weight and decreased thyronine proportion returned to normal 9 weeks after termination of the iodide feeding. In the goitrous chicks, the relative proportion of intrathyroidal 127I compounds were virtually identical with that of 131I, although the absolute amount of thyronine 127I was increased depending on the increase in total 127I content. There was no significant difference in both turnover rate of radiothyroxine and plasma thyronine 127I concentration between the goitrous and control chicks. It is suggested that these alterations of iodine metabolism seem to be closely related to the mechanism of goitrogenesis by excess iodide in chicks and may represent an inhibitory effect of iodide on intrathyroidal hormogenesis.