Study on experimental goiter induced by the mass administration of iodine (decrease of synthetic rate of thyronine in the thyroid gland

Abstract

The effects of large doses of iodine on thyroid weight and the hormone synthesis was studied in chick and rat with the aid of 131I and chromatographic analysis.Male and female Whiet-Rock chicks weighing 900 to 1500 g. were fed on a standord diet throughout the experiments. In order to determine the effect of a single dose of excessive iodide, chicks were given an intraperitoneal injection of 100-200 μc 131I with carrier 1.0 mg. iodide. Thyroids were removed and analysed at an appropriate time after the injection. Control chicks were injected 10-20 μc 131I alone and sacrificed at the corresponding time. Long-term treatment with iodide was carried out as follows : Chicks were given about 1.0 mg. iodide every day for the last 4 weeks. and each one was injected 1.0 mg. iodide together with 200 μc 131I. The thyoids were removed at 4 and 24 hours 1 after 131I.For the comparison among different species, male Wistar rats weighing 100-150 g. were also fed on a low iodine diet throughout the experiments. The method was the same as used in chicks, but the quantity of iodide was 500 sg. and the duration of iodide treatment was 3 weeks.Other experiments were studied with chicks at 1, 3 and 9 weeks after withdrawal of a long-term treatment of iodide for the study of residual effect of the treatment. In these experiments, chicks were sacrificed at 4 hours after the injection of carrier free 311I.Radiochromatographic analysis of the thyroid tissue after enzymatic hydrolysis was performed either by paperchromatography or columu chromatography using Pileggi's modification.Long-term treatment of iodide for 4 weeks induced a significant increment of the thyroid weight in chick; about 1.8 times greater than the controls. Histology of the goiter showed flat epithels and marked colloid storage within large follicles. While the 3-week treatment of iodide failed to produce goiter in rats.In both animals, it was shown on chromatogram of the thyroid that a single of carrier iodide resulted in a significant but transient reduction of organic 131I and thyronine 131I proportion as well as an elevation of 131I MIT/DIT ratio. After a long-term treatment of rat with iodide, an “escape” occured from this inhibitory effect of iodide; formation of thyronines was resumed with recovery of organification of iodide and MIT/DIT ratio, as previously reported by Pitt-Rivers et al. On the contrary, in the goitrous chicks continuo usly treated with iodide, synthesis of thyronine 131I was significantly depressed notwithstanding the presence of normal proportion of organic 131I and rather lower MIT/DIT ratio than controls.After a 3-week withdrawal of iodide feeding, MIT/DIT ratio recovered first. The depressed thyronine 131I synthesis as well as increased thyroid weight had not returned to control levels until 9 weeks after stopping the iodide treatment.It appears that the depressed rate of thyronine synthesis would be a cause of the goiter through the negative feed-back mechanism in iodide-fed chicks.