Short-term effect of soy consumption on thyroid hormone levels and correlation with phytoestrogen level in healthy subjects.

OBJECTIVE: Since soy isoflavones may influence the thyroid hormone feedback system by interference with their biosynthesis, secretion and metabolism, we tested whether their controlled shortterm consumption affects thyroid function.

METHODS: Eighty six volunteers--university students (32 males and 54 females) were eating unprocessed boiled natural soybeans (2 g/kg body weight/day) for 7 consecutive days. Thyrotropin, free thyroid hormones, antibodies to thyroid peroxidase and to thyroglobulin, and actual levels of unconjugated major soy phytoestrogens, daidzein and genistein, were measured in sera collected before, at the end and one week after finishing soy meal consumption.

RESULTS: Both phytoestrogens increased significantly (p<0.0001) at the end of soy-diet and fell down after its termination nearly back to the initial values. No significant changes were found in female group, while in males a significant transitory increase of thyrotropin (p<0.0001) was recorded. When actual levels of phytoestrogens were related to thyroid parameters, the only significant correlations were found between basal levels of daidzein and thyrotropin, daidzein and antithyroglobulin at the end of soy consumption in males, and between daidzein and free thyroxine at the end of the soy ingestion in females.

CONCLUSION: Though only modest and transitory effects on thyroid parameters occurred after controlled short-term soy consumption, some actual thyroid hormone parameters do correlate with actual isoflavone levels.