Quantitative Interrelationships Between the Effects of Iodine and Thiouracil on Thyroid Function

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Abstract
In quantitative studies of the effect of goitrogens at different iodine levels it was found that the synthetic goitrogen, thiouracil, exerted a maximum effect on the thyroid glands of rats when the iodine level in the diet was near the minimum level required to prevent thyroid enlargement in the normal rat. High dietary iodine decreased, but did not prevent, the effects of thiouracil on thyroid size, histology, and iodine content. It was found that the mathematical relationship between thyroid size and the dose of thiouracil changed with alterations in the dietary iodine level. Increasing the iodine intake completely counteracted the effect of thiouracil on growth only at the lower levels of thiouracil. Thiouracil appeared to exert its growth-reducing effect by way of the thyroid gland, at least at the lower levels used. The iodine level in the diet prior to the feeding of thiouracil had a pronounced effect on the response to thiouracil. When thiouracil was fed for 6 weeks, adrenal enlargement was observed. This is in contrast to previous reports, and may have been due to differences in the iodine level of the diet.