Effects of soy on hormones and thyroid function

boost soy thyroid

How eating soy affects Hashimoto’s and the immune health

In the past twenty years soy has become a popular food all around the globe. Although there are many health benefits of eating soy, there are side effects to be aware of.

A possible side effect of soy is that in some people it may affect thyroid function, especially in people with an underactive thyroid as it might interfere with the absorption of thyroid medication (1-4).

Our internal research at BOOST Thyroid found that 1 in 4 people have identified soy as a trigger of their autoimmune flare-ups.

Soy’s nutrition facts

Soy has more protein and fat, and less carbohydrates, than other legumes. The quality of soy protein is also much higher than other plant proteins (5-7). Soy’s low carbohydrate content can make it good food option for some diabetic patients (8).

Soybeans’ carbohydrates can help the growth of good colon bacteria, this is why soy carbohydrates are considered as prebiotics (9-11). The soybean is a good source of vitamins and minerals, including potassium, iron, and calcium, as well as omega-3 essential fatty acids (5, 6, 12-15). It also might be useful to know soybeans’ carbohydrates can increase flatulence (16).

Health benefits of soy (17-28):

  • Protecting heart health

  • Protecting against breast and prostate cancer

  • Protecting bones

  • Relieving menopausal symptoms

  • Helping with diabetes

The impact of soy on thyroid and immune function

Consuming soybeans might inhibit thyroid function in some people by blocking iodide uptake and preventing zinc from functioning. Both zinc and iodide are important nutrients necessary for T3 and T4 production (29, 30).

Additionally, soy might interfere with the absorption of thyroid medication in some people (1-4).


Isoflavones are the main active component of soybeans that produces both hormonal and non-hormonal effects (31).

The chemical structure of isoflavones is similar to the hormone estrogen. Thus, they’re able to partially replace some of estrogen’s functions in various organs including the skin, spleen, pancreas, kidney, thymus, adrenal gland, brain, pituitary, ovaries, and testes (32-37). This could cause problems for some people—having high estrogen levels and then having isoflavones mimic estrogen could further increase inflammation and the risk of certain cancers.

Isoflavones in soy can be problematic for some people with an underactive thyroid and Hashimoto’s. They can block the function of the enzyme thyroid peroxidase (TPO), which is necessary for the thyroid to be able to produce T3 and T4 hormones (38-40).

There’s also clinical evidence that isoflavones can affect immune function (41). Some researchers have hypothesized that consumption of processed foods with high salt content could be a big factor in the triggering of autoimmune conditions (42-47).

A vegetarian diet is usually rich in foods containing soy, and can increase the risk of chance of developing an underactive thyroid (48).

Foods that contain soy:

  • Edamame

  • Vegan meat alternatives

  • Tofu

  • Tempeh

  • Miso

  • Soy-based sauces (shoyu, tamari, and teriyaki)

  • Soy milk

  • Soy nuts

  • Soy flour

If you are affected by soy intolerance, or if you’ve identified soy as a cause of your flare-ups, talk to your medical health care practitioner or nutritionist to find an alternative foods.

If you eat soy, remember to take your medication at least a few hours apart, as soy products can interfere with the uptake of thyroid hormones.

If you’re trying a soy-free diet, track it in BOOST Thyroid app to help you understand if your symptoms improve. In order to discover if soy is your trigger, try the diet for at least ten days to feel the first effects and don’t change your other dietary routines.


  1. Messina M, et al. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature, 2006

  2. Fitzpatrick M. Soy formulas and the effects of isoflavones on the thyroid, 2000

  3. Doerge DR, et al. Goitrogenic and estrogenic activity of soy isoflavones, 2002

  4. Sathyapalan T, et al. The effect of soy phytoestrogen supplementation on thyroid status and cardiovascular risk markers in patients with subclinical hypothyroidism: a randomized, double-blind, crossover study, 2011

  5. Messina MJ. Legumes and soybeans: Overview of their nutritional profiles and health effects, 1999

  6. US Department of Agriculture, Agricultural Research Service, Nutrient Data Laboratory USDA. National Nutrient Database for Standard Reference, Release 28, 2016

  7. Hughes GJ, et al. Protein digestibility-corrected amino acid scores (PDCAAS) for soy protein isolates and concentrate: Criteria for evaluation, 2011

  8. Feinman RD, et al. Dietary carbohydrate restriction as the first approach in diabetes management: Critical review and evidence base, 2015

  9. Inoguchi S, et al. Effects of non-fermented and fermented soybean milk intake on faecal microbiota and faecal metabolites in humans, 2012

  10. Bang MH, et al. Soyoligosaccharide increases fecal bifidobacteria counts, short-chain fatty acids, and fecal lipid concentrations in young Korean women, 2007

  11. Hayakawa K, et al. Effects of soybean oligosaccharides on human faecal flora, 1990

  12. Slavin M, et al. Antioxidant properties, phytochemical composition, and antiproliferative activity of Maryland-grown soybeans with colored seed coats, 2009

  13. Rizzo NS, et al. Nutrient profiles of vegetarian and nonvegetarian dietary patterns, 2013

  14. Davey GK, et al. EPIC-Oxford: Lifestyle characteristics and nutrient intakes in a cohort of 33,883 meat-eaters and 31,546 non meat-eaters in the UK, 2003

  15. Heaney RP, et al Soybean phytate content: Effect on calcium absorption, 1991

  16. Suarez FL, et al. Gas production in human ingesting a soybean flour derived from beans naturally low in oligosaccharides, 1999

  17. Weggemans RM, et al. Relation between soy-associated isoflavones and LDL and HDL cholesterol concentrations in humans: a meta-analysis, 2003

  18. Nestel P. Isoflavones: their effects on cardiovascular risk and functions, 2003

  19. Tsugane S, et al. The JPHC study: design and some findings on the typical Japanese diet, 2014

  20. Yamamoto S, et al. Soy, isoflavones, and breast cancer risk in Japan, 2003

  21. Messina MJ. Emerging evidence on the role of soy in reducing prostate cancer risk, 2003

  22. Lamartiniere CA, et al. Genistein chemoprevention: timing and mechanisms of action in murine mammary and prostate, 2002

  23. Messina M, et al. Skeletal benefits of soy isoflavones: a review of the clinical trial and epidemiologic data, 2004

  24. Gil-Izquierdo A, et al. Soy isoflavones and cardiovascular disease epidemiological, clinical and -omics perspectives, 2012

  25. Jayagopal V, et al. Beneficial effects of soy phytoestrogen intake in postmenopausal women with type 2 diabetes, 2002

  26. Cederroth CR, et al. Soy, phytoestrogens and metabolism: A review, 2009

  27. Kang X, et al. Effect of soy isoflavones on breast cancer recurrence and death for patients receiving adjuvant endocrine therapy, 2010

  28. Hwang YW, et al. Soy food consumption and risk of prostate cancer: a meta-analysis of observational studies, 2009

  29. Tran L, et al. Soy extracts suppressed iodine uptake and stimulated the production of autoimmunogen in rat thyrocytes, 2013