Effects of TCDD on thyroid hormone homeostasis in the rat

A physiological dosimetric model was constructed to describe the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on circulating thyroid hormones in the rat and to test the hypothesis that these hormonal changes cause chronically elevated serum thyrotropin (thyroid stimulating hormone, TSH), which mediates growth promotion and may lead to thyroid tumors in TCDD-treated rats. The model included diffusion restricted distribution of TCDD among compartments for liver, kidney, white fat, slowly and rapidly perfused tissues, and the thyroxine-sensitive tissues brown fat, pituitary, and thyroid. Blood was distributed among major vessels and the capillary beds of the tissues. Metabolism of TCDD was limited to the liver. Secretion of 3,5,3'-triiodothyronine (T3) and thyroxine (3,5,3',5'-tetraiodothyronine, T4) from the thyroid was modeled as stimulated by circulating TSH, whose release from the pituitary was regulated by the hypothalamic peptides thyrotropin releasing hormone (activating) and somatostatin (inhibiting). Release of these peptides was represented as inhibited and activated, respectively, by circulating T4. Binding proteins for T3 and T4 and metabolism of the hormones by deiodination were included in thyroxine-sensitive tissues. Induction of hepatic UDP-glucuronosyltransferase-1*6 (UGT), the enzyme which glucuronidates T4, was modeled as induced by the complex formed between TCDD and the aryl hydrocarbon receptor. The computed extent of deiodination, primacy of the thyroid in generating T3 from T4, dependence of liver and kidney on locally produced T3, and export of T3 formed in the pituitary agreed with experimental observations. The model reproduced the observed decrease in circulating T4 and elevated serum TSH following chronic administration of TCDD. The altered levels were attributed to the increased clearance of T4 by the induced UGT and the consequent modification of feedback control of hormone releases. These results are consistent with the hypothesis of growth stimulation by elevated TSH, but measured values of this hormone in blood of rats vary over a large range, and the change induced by TCDD is often small. Measured UGT levels are less variable and the increase in this protein is much greater, suggesting that this response may be a more reliable biomarker for effects of TCDD on the thyroid.