Issue: April 2011
April 01, 2011
1 min read

Soy phytoestrogens increased risk for overt hypothyroidism

Sathyapalan T. J Clin Endocrinol Metab. 2011;doi:10.1210/jc.2010-2255.

Issue: April 2011
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Dietary supplementation with 16 mg of soy phytoestrogens, equivalent to the level found in a vegetarian diet, was associated with a threefold increased risk for overt hypothyroidism among patients with subclinical disease. However, supplementation at this dosage reduced insulin resistance, highly sensitive C-reactive protein and blood pressure.

A randomized, double blind, crossover study included 60 patients aged 44 to 70 years with subclinical hypothyroidism and normal free thyroxine. Half of the patients began treatment with 2 mg phytoestrogen, equivalent to the level found in a Western diet, and 30 g soy protein powder; the other half received 16 mg phytoestrogen and 30 g soy protein powder. After an 8-week washout period, patients received alternative supplementation for an additional 8 weeks.

Overt hypothyroidism developed in 10% (n=6) of patients who received high-dose phytoestrogen, all of whom were women. Progression to overt hypothyroidism did not occur in any patients who received low-dose phytoestrogen. Despite this difference in disease advancement, mean thyroid-stimulating hormone, free T4 and free triiodothyronine levels were not statistically different between the two groups.

TSH levels increased by 57% among the six women who developed overt hypothyroidism (P<.05) and free T4 levels decreased by 25% (P<.05). Phytoestrogen supplementation at 16 mg was associated with a significant reduction in systolic and diastolic BP; however, 2 mg supplementation was associated with a reduction only in systolic pressure. Supplementation at 16 mg vs. 2 mg was also associated with significant improvements in insulin resistance (homeostasis model assessment-insulin resistance, 3.5 vs. 2.6; P<.02) and high-sensitivity C-reactive protein (4.9 vs. 3.9; P<.01).

“This suggests that female vegetarian patients with subclinical hypothyroidism may need more careful monitoring,” the researchers wrote. “Conversely, phytoestrogen supplementation showed a beneficial dose-dependent effect on the cardiovascular risk profiles in this patient population.”

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