Hypothyroidism induced by xenobiotic treatment was analyzed for possible underlying mechanism(s) on the basis of different responses of the thyroid gland and the liver, using a newly-created database of repeated-dose toxicity of 500 chemicals. Two mechanisms are proposed: direct inhibition of thyroid hormone biosynthesis in the thyroid gland, and stimulated degradation of thyroid hormone by induction of hepatic drug-metabolizing enzymes. In the database there were 10 chemicals inducing hypertrophy/ hyperplasia of follicular cells in the thyroid gland and having data on thyroid glands. On the basis of the chemical structure and information available in the literature, we judged three chemicals to be typical thioamide derivatives that act directly on the thyroid gland, and the others as non-thioamide derivatives that were unlikely to have any direct action on the thyroid gland. All these chemicals were classified into two groups using the ratios of relative weight increase rate of thyroid gland versus that of the liver. These values were at least 1.7, but 3.2 or more in the most of the cases for thioamide derivatives, and 1.2 or less for non-thioamide derivatives. This background analysis suggests the feasibility of parametersupported speculation on the possible underlying mechanism when new repeated-dose toxicity data on hypothyroidism becomes available.
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