Consumption of soy phytoestrogens of the isoflavone series, representing an important component of food in Eeast-Asian populations, is believed to possess a number of beneficial effects on humanhealth.Onthe other hand, recent studies demonstrated that these compounds may act as inhibitors of peroxidases involved in the biosynthesis of thyroid hormones. The objective was to find out, whether an association between actual levels of phytoestrogens and laboratory markers of thyroid function exist. In serum samples from 196 males and females, obtained during screening of iodine deficiency, two major isoflavonoids, daidzein and genistein were determined along with the principal laboratory markers of thyroid function,TSH, fT3 and fT4. According to the questionnaire completed by all probands, including questions recording the food consumed within the last 24 hours, probands were selected who ate soy or other pulses within 24 h before blood collection. Actual levels of phytoestrogens in the groups without- and after soy meals were correlated with the levels of thyroid hormones, with the following results: In 14 subjects eating soy products with phytoestrogen levels above 2 nmol/l a highly significant negative correlation was found between fT3 and genistein levels, and a border-line positive correlation of the latter phytoestrogen with TSH. The average actual levels of both phytoestrogens ± S. D. in all probands was 0.76 ± 0.46 nmol/l. In all the subjects a positive correlation was obtained between both phytoestrogens and TSH. The results differed according to sex. The results confirm the hypothesis that dietary isoflavonoids, especially genistein,may decrease biosynthesis of thyroid hormones.

        Key words: thyroid hormones, phytoestrogens, daidzein, genistein.