Mechanism of thiazopyr-induced effects on thyroid hormone homeostasis in male Sprague-Dawley rats.

Abstract Chronic administration of thiazopyr in the diet at dose levels of 1000 and 3000 ppm, but not 100 ppm, has demonstrated an increase in thyroid follicular cell tumors in male Sprague–Dawley rats. In the studies reported here we have evaluated the mechanism of thiazopyr-induced thyroid tumors by studying the effect of thiazopyr on a number of endpoints that indicate hypothalamic–pituitary–thyroid homeostasis. At a dose level of 3000 ppm, thiazopyr caused a marked depression in circulating levels of T 4 as soon as 7 days after commencement of treatment. Concurrent with this decrease in T 4 was an increase in TSH levels, an increase in thyroid and liver weights, a three- to sixfold increase in hepatic T 4 -uridine diphosphate glucuronosyl transferase (UDPGT) activity, and increases in thyroid follicular cell hypertrophy and hyperplasia. Dose-related changes associated with thiazopyr treatment were significant increases in liver weight, thyroid weight, and hepatic T 4 -UDPGT activity at high doses. Increased levels of serum TSH, T 3 , and rT 3 , decreased levels of T 4 , and an increased incidence of thyroid follicular cell hypertrophy and hyperplasia were observed 56 days after the initiation of 3000 ppm thiazopyr. All the changes, except thyroid weight, were partially or completely reversible upon removal of thiazopyr from the diet. Increased thyroid T 4 elimination, primarily via increased hepatic conjugation by T 4 -UDPGT, resulting in decreased serum T 4 , appeared to be responsible for the increased TSH levels. The sustained increase in TSH by thiazopyr appears responsible for the stimulation of the thyroid follicular cells resulting in follicular cell hypertrophy, hyperplasia, and ultimately neoplasia. In summary, evidence is presented for a hormonally mediated, threshold-dependent process for the development of thyroid follicular cell tumors from high-dose thiazopyr administration in male rats. This mechanism is not considered to be relevant to humans, since the thyroid of humans is much less sensitive to this pathogenic phenomenon than rodents.