Objective: The ability of soy isoflavones to inhibit thyroid peroxidase and induce goiter in animals has generated concern regarding their potential antithyroid effects in humans. The purpose of this study was to determine the effects of soy protein isolates of varied isoflavone content on circulating thyroid hormones in healthy young men.
Design: Thirty-five healthy men (27.9 +/- 5.7 years old) supplemented their habitual diets with milk protein isolate (MPI), low-isoflavone soy protein isolate (low-iso SPI; 1.64 +/- 0.19 mg iso/day), and high-isoflavone SPI (high-iso SPI; 61.7 +/- 7.4 mg iso/day) for 57 days each, separated by 4-week washouts in a randomized crossover design. Blood was collected on days 1, 29, and 57 of each treatment for analysis of total triiodothyronine (T3), free T3, total thyroxine (T4), free T4, thyroid stimulating hormone (TSH), and thyroid binding globulin (TBG). Twenty-four hour urines were collected at the end of each treatment for analysis of isoflavones.
Main outcome: Results revealed no significant effects of the low-iso or high-iso SPIs on serum total T3, free T3, total T4, free T4, TSH, or TBG when compared with the MPI on either study days 29 or 57. Urinary data revealed that isoflavones were significantly increased by the high-iso SPI relative to the low-iso SPI and MPI.
Conclusions: Results of this study demonstrate that soy isoflavones in a protein matrix do not significantly influence circulating thyroid hormones in healthy young men.