Thyroid gland function in ovariectomized ewes exposed to phytoestrogens

Phytoestrogens are by definition plant-derived substances that are able to activate the mammalian oestrogen receptors. We examined the possible effects of phytoestrogens on the secretion of thyroid hormones as well as on the immunoreactivity to oestrogen receptor alpha (ERα) in the thyroid glands of ovariectomized ewes. Eight ovariectomized ewes were fed 3.5 kg of 100% red clover silage for 14 days. Blood samples were collected before and on day 14 of exposure to phytoestrogens. After 5 months, four of the ewes were re-exposed to red clover silage as described above and the other four served as controls. Blood samples were collected as above. All ewes were slaughtered at the end of the experiment and the thyroid glands were weighed and examined for macroscopical changes. Tissue samples were taken for immunohistochemistry and image analysis. Ewes exposed to red clover silage had significantly higher plasma concentrations of total T3 and free T3 than ewes fed hay. The cross-section area of thyroid follicles tended to be larger in ewes fed red clover silage than in the control animals. ERα immunoreactivity was stronger in thyroid glands from ewes exposed to phytoestrogens than in ewes fed hay. In conclusion, daily ingestion of 81–95 mg phytoestrogens per kg body weight for 14 days stimulated secretion of thyroid hormones and tended to increase follicle size and ERα immunoreactivity of thyroid glands of ovariectomized ewes.