To elucidate whether soybeans would suppress the thyroid function in healthy adults, we selected 37 subjects who had never had goiters or serum antithyroid antibodies. They were given 30g of soybeans everyday and were divided into 3 groups subject to age and duration of soybean administration. In group 1, 20 subjects were given soybeans for 1 month. Groups 2 and 3 were composed of 7 younger subjects (mean 29 y.o.) and 10 elder subjects (mean 61 y.o.) respectively, and the subjects belonging to these groups received soybeans for 3 months. The Wilcoxon-test and t-test were used in the statistical analyses. In all groups, the various parameters of serum thyroid hormones remained unchanged by taking soybeans, however TSH levels rose significantly although they stayed within normal ranges. The TSH response after TRH stimulation in group 3 revealed a more significant increase than that in group 2, although inorganic iodide levels were lowered during the administration of the soybeans. We have not obtained any significant correlation between serum inorganic iodide and TSH. Hypometabolic symptoms (malaise, constipation, sleepiness) and goiters appeared in half the subjects in groups 2 and 3 after taking soybeans for 3 months, but they disappeared 1 month after the cessation of soybean ingestion. These findings suggested that excessive soybean ingestion for a certain duration might suppress thyroid function and cause goiters in healthy people, especially elderly subjects.