The Effect of Soy Phytoestrogen Supplementation on Thyroid Status and Cardiovascular Risk Markers in Patients with Subclinical Hypothyroidism: A Randomized, Double-Blind, Crossover Study, The Journal of Clinical Endocrinology and Metabolism, May 2011
"There is a 3-fold increased risk of developing overt hypothyroidism with dietary supplementation of 16 mg soy phytoestrogens with subclinical hypothyroidism. However, 16-mg soy phytoestrogen supplementation significantly reduces the insulin resistance, hsCRP*, and blood pressure in these patients."The good news was that soy isoflavones lowered insulin resistance, CRP (CRP is a marker for inflammation or infection somewhere in the body), and blood pressure. The bad news was that soy increased the risk of hypothyroidism in women who already had marginally low thyroid activity. Camille from Bloom and Grow had a great review.
* Highly sensitive C-reactive protein
There are other phytoestrogens (plant-derived compounds with estrogen activity) besides those found in soy. Clover and alfalfa sprouts contain a phytoestrogen called coumestrol. But our diets are so rich in soy, when it comes to phytoestrogens, isoflavones in soy are often the primary source.1
So, how easy is it to consume 16 mg of soy phytoestrogens (in the form of isoflavones)? The Linus Pauling Institute (LPI) assembled this table from data in that USDA database I referenced above.
About thyroid function, the LPI says:
"In cell culture and animal studies, soy isoflavones have been found to inhibit the activity of thyroid peroxidase, an enzyme required for thyroid hormone synthesis. However, high intakes of soy isoflavones do not appear to increase the risk of hypothyroidism as long as dietary iodine consumption is adequate. Since the addition of iodine to soy-based formulas in the 1960s, there have been no further reports of hypothyroidism in soy formula-fed infants. Several clinical trials, mostly in premenopausal and postmenopausal women with sufficient iodine intakes, have not found increased consumption of soy isoflavones to result in clinically significant changes in circulating thyroid hormone levels."Here's the reference for the bolded part:
Effects Of Soy Protein And Soybean Isoflavones On Thyroid Function In Healthy Adults And Hypothyroid Patients: A Review Of The Relevant Literature, Thyroid, March 2006
"Thus, collectively the findings provide little evidence that in euthyroid, iodine-replete individuals, soy foods, or isoflavones adversely affect thyroid function. In contrast, some evidence suggests that soy foods, by inhibiting absorption, may increase the dose of thyroid hormone required by hypothyroid patients. However, hypothyroid adults need not avoid soy foods. In addition, there remains a theoretical concern based on in vitro and animal data that in individuals with compromised thyroid function and/or whose iodine intake is marginal soy foods may increase risk of developing clinical hypothyroidism. Therefore, it is important for soy food consumers to make sure their intake of iodine is adequate."So, it appears that for some people, the consumption of isoflavones from soy might reduce inflammation, reduce insulin resistance, reduce blood pressure, and, as long as intake of iodine is sufficient (~150 mcg)2, won't hurt thyroid function.
2 The Dietary Reference Intake (DRI) for iodine for men and women over the age of 14 is 150 micrograms per day. Source: USDA DRI Tables