Acute effects of Estradiol (E2), Genistein (Gen), Resveratrol (Rsv) and 8-Prenylnaringenin (8-PN) on serum LH, TSH and thyroid hormone concentrations

Phytoestrogens (particularly the soy derived isoflavonoid Gen) are increasingly consumed as nutritional supplements in artificially high doses, but also Rsv (a polyphenol) is promoted with anti-carcinogenic properties and 8-PN, a flavanone, is advertised for human breast enlargement properties. All three compounds bind to both estrogen receptor subtypes (ERα and ERβ) and Gen is known to stimulate uterine growth in unphysiologically high concentrations. Under chronic conditions we had shown earlier that some isoflavones have an impact on the hypothalamo-pituitary-thyroid axis. Therefore we studied the effects of these three phytoestrogens after acute administration (50mg Gen and Rsv and 8-PN or 5mg of E2 given per gavage). One day prior to administration the animals were implanted with a jugular vein catheter through which blood samples were withdrawn at -1, -0.5, 0, 0.5, 1, 2, 3 and 4 hours after oral administration of a test compound. Animals were sacrificed by decapitation 6 hours after application. Blood was collected from the trunk, uteri removed and weighed. Pituitary and thyroid hormones were determined by a RIA in the serum.

Serum LH concentrations were decreased by the test compounds with the following strength: E2>8-PN>Gen while Rsv had no effect on LH. Uterine weights were similarly stimulated by the compounds (p<0.05) in each case. E2>8PN>Gen stimulated TSH concentrations while Rsv had no significant or an even slightly decreasing effect on TSH levels and decreased serum T4 and T3.

It is concluded that under acute conditions not only E2 but also Gen and 8-PN but not Rsv have estrogenic effects in the uterus and on pituitary LH release, while the acute effects of phytoestrogens were different from those of E2 on the hypothalamo-pituitary-thyroid axis. Hence other than estrogenic mechanisms may become acutely functional under the influence of phtyoestrogens in the hypothalamo-pituitary-thyroid axis.