Antihormonal Effects of Plant Extracts

Abstract

The acute administration of Lithospermum officinale (Boraginaceae) freeze dried extracts (FDE) to euthyroid rats is associated with a decrease in serum thyroxine and triiodothyronine concentrations, suggesting a possible direct effect of the plant extract on circulating TSH (hypophyseal hormone blocking activity) and/or on TSH secretion. To further study this possibility plant extracts from Lithospermum officinale, Lycopus virginicus L. (Lamiaceae), Melissa officinalis L. (Lamiaceae) and Thymus serpyllum L. (Lamiaceae) were administered to euthyroid and hypothyroid rats. In the euthyroid rat serum and pituitary TSH-levels were greatly diminished by the plant extracts. In hypothyroid rats circulating TSH was suppressed by Lithospermum officinale without any influence on the hypophyseal TSH-stores whereas Lycopus virginicus induced pituitary TSH repletion. The chronic administration of L. officinale to hypothyroid rats suppressed TSH-levels and correspondingly the goiter weight. These findings that resemble the effect of low doses of thyroxine in euthyroid and hypothyroid rats suggest that the antithyrotropic activity of plant extracts may be explained by two independent factors: a hypophyseal hormone blokking effect and a thyroid hormone like activity at a hypophyseal site. The decline of TSH-serum levels was associated with a strong inhibition of thyroidal secretion as expressed by endocytosis and colloid size of the thyroid follicle. At the same time prolactin serum levels and hypophyseal stores were reduced by the plant extracts. Because of the great influence of thyroid status on prolactin secretion this effect of plant extracts may be due to a thyroid hormone analogue acting at a hypothalamical site initiating dopaminergic reactions responsible for the fall in prolactin and additionally TSH-concentrations.

Key Word Index

Lithospermum officinale - Boraginaceae - TSH - Prolactin - Thyroid gland