Can Soy Mess Up Metabolism?

Jerry Brainum

How soy affects thyroid function in men

One frequent criticism of soy products is that they interfere with thyroid function. Isoflavones, which are found in soy, are known to adversely affect thyroid-gland function in animals by inhibiting an important enzyme called thyroid peroxidase. Researchers from Canada and Seattle teamed to explore the effects of soy intake on thyroid functioning in young men.1

Thirty-five healthy men, average age 27, supplemented their diets with one of the following:

1) Milk protein isolate

2) Low-isoflavone soy protein isolate

3) High-isoflavone soy protein isolate

They used the supplements for 57 days, separated by four-week washout periods when they took no supplements. The results: None of the soy-based supplements had any adverse effect on any measure of thyroid hormone activity.                                            


1 Dillingham, B.L., et al. (2004). Soy isoflavone consumption does not influence serum thyroid hormones in young men. (Presented at Experimental Biology 2004, April 2004, in Washington, D.C.)