N-(4-Fluorophenyl)-N-(1-methylethyl)-2-[[5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl]oxy]acetamide (FOE 5043) is a new acetanilide-type herbicide undergoing regulatory testing. Previous work in this laboratory suggested that FOE 5043-induced reductions in serum thyroxine (T4) levels were mediated via an extrathyroidal site of action. The possibility that the alterations in circulating T4levels were due to chemical induction of hepatic thyroid hormone metabolism was investigated. Treatment with FOE 5043 at a rate of 1000 ppm as a dietary admixture was found to significantly increase the clearance of [125I]T4from the serum, suggesting an enhanced excretion of the hormone. In the liver, the activity of hepatic uridine glucuronosyl transferase, a major pathway of thyroid hormone biotransformation in the rat, increased in a statistically significant and dose-dependent manner; conversely, hepatic 5′-monodeiodinase activity trended downward with dose. Bile flow as well as the hepatic uptake and biliary excretion of [125I]T4were increased following exposure to FOE 5043. Thyroidal function, as measured by the discharge of iodide ion in response to perchlorate, and pituitary function, as measured by the capacity of the pituitary to secrete thyrotropin in response to an exogenous challenge by hypothalamic thyrotropin releasing hormone, were both unchanged from the controlled response. These data suggest that the functional status of the thyroid and pituitary glands has not been altered by treatment with FOE 5043 and that reductions in circulating levels of T4are being mediated indirectly through an increase in the biotransformation and excretion of thyroid hormone in the liver.

Evidence of increased hepatic metabolizing capacity coupled with reductions in serum thyroxine (T4) levels were noted in the rat during preliminary toxicity studies with FOE 5043, an oxyacetamide with herbicidal properties. These findings were consistent with reports in the literature suggesting that declines in T4 as a result of exposure to various classes of chemicals may be mediated extrathyroidally, such as through chemical induction of hepatic thyroid hormone metabolism. To examine this question with respect to FOE 5043, male rats were surgically thyroidectomized and provided thyroid hormone replacement therapy via implanted osmotic minipumps capable of maintaining a T4/triiodothyronine (T3) serum concentration for approximately 4 weeks at a level comparable to that of euthyroid controls. Seven days after minipump implantation, thyroidectomized + T4/T3 (TX + T4/T3) and nonthyroidectomized intact rats (NTX) were fed diets containing 0, 25, 1000, or 3000 ppm FOE 5043 for up to 3 weeks. Dose-related and equivalent declines in total and free serum T4 levels in both TX + T4/T3 and NTX rats were measured at Weeks 1, 2, and 3. Alterations in thyrotropin, total, free, and reverse serum T3 levels were also noted in both TX and NTX animals; however, a compound-related trend was difficult to discern and, when compared to the T4 response, the changes were markedly less consistent with respect to both time and dose. Additionally, dose-related increases in absolute and relative liver weights were measured in both TX + T4/T3 and NTX animals. As the only source of thyroid hormone in the TX + T4/T3 animals was that provided by the pump, these data suggest that FOE 5043-induced alterations in serum thyroid hormone levels, most notably T4, are being mediated indirectly, possibly as a result of increased hepatic metabolism, rather than through a direct effect on the thyroid gland.

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