Soy may cause goiters in healthy people, especially the elderly

This study was published in Nihon Naibunpi Gakkai Zasshi 1991 May 20;67(5):622-9

Study title and authors:
The effects on the thyroid gland of soybeans administered experimentally in healthy subjects.

Ishizuki Y, Hirooka Y, Murata Y, Togashi K.
Ishizuki Thyroid Clinic.

This study can be accessed at: http://www.ncbi.nlm.nih.gov/pubmed/1868922

This study investigated the effects of soy consumption on thyroid function. The study included 37 healthy subjects who were given 30g of soybeans everyday and divided into 3 groups.

(i) Group 1 were given soybeans for one month.
(ii) Group 2 (average age 29 years old) were given soybeans for three months.
(iii) Group 3 (average age 61 years old) were given soybeans for three months.

The study found:
(a) In all groups thyroid stimulating hormone (TSH) levels rose significantly (high TSH levels may indicate an underactive thyroid gland).
(b) TSH levels in group 3 increased more significantly than that in group 2.
(c) Symptoms such as 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.

The author of the study, Dr. Yoshimochi Ishizuki of Aichi Medical University in Japan, concludes: "These findings suggested that excessive soybean ingestion for a certain duration might suppress thyroid function and cause goiters in healthy people, especially elderly subjects".