Myth: Soya Inhibits Thyroid Function

Myth: Soya Inhibits Thyroid Function

In this article, we look at the definitions of thyroid disease and look to disprove the myth that Soya inhibits Thyroid Function. 

The thyroid gland

The thyroid gland is described as a butterfly-shaped gland about the size of a walnut that weighs less than 30g, sits just above the collarbone and below the Adam’s apple, and is part of the endocrine system (3).

The thyroid gland uses dietary iodine to produce the hormones thyroxine and triiodothyronine, also known as T3 and T4, respectively. Thyroid hormones affect virtually every cell in the body, from protein synthesis and enzymatic activity to playing a critical role in the determination of energy expenditure.

An overactive thyroid producing excess T3, T4, or both results in hyperthyroidism, with frequent bowel movements and weight loss.

An underactive thyroid gland results in hypothyroidism and is associated with constipation and unexplained weight gain.

Women have a higher occurrence of thyroid disease than men and experience symptoms such as fatigue, depression and anxiety, constipation or loose bowels, and menstrual irregularities. These symptoms are often treated individually and not diagnosed as thyroid disease (3).

The most common cause of hyperthyroidism is Grave’s disease, while Hashimoto’s thyroiditis is the most common cause of hypothyroidism. Both are autoimmune diseases.

Cruciferous vegetables and soybean-related foods contain goitrogens, which interfere with thyroid hormone production or utilization. Goiter can be a response to an overactive or underactive thyroid gland.

Unless there is a coexisting iodine deficiency, these foods generally are of no clinical significance according to the Institute of Medicine (3). Goitrogens are inactivated by heating and cooking (3).

Therefore, the use of cruciferous vegetables (cabbage, broccoli, spinach etc) and soy foods can be used daily in the case of thyroid disease, where there is no coexisting iodine deficiency (this latter condition is extremely rare).


Consult with a Registered Dietician who will use nutrition assessment data to develop an energy-controlled diet to promote weight control.

The emphasis in the diet will be on foods high in fiber along with adequate fluid, as well as soya-based foods, taking into consideration potential drug-nutrient interactions.

Adequacy of vitamin and mineral intake will be assessed to determine the need for multivitamin-mineral supplementation.


Once again, consult with a Registered Dietician.

Typically in the case of hyperthyroidism, a patient’s energy needs may be increased by 50% to 60% (or 10% to 30% in mild cases), and so a high-energy diet will likely be necessary.

Increase milk or equivalent foods to 4 cups per day to supply adequate calcium, phosphorous, and vitamin D.

The adequacy of the diet for vitamin and mineral intake will be assessed to determine whether a general multivitamin supplement may be beneficial.

The diet will also be monitored for iodine content.

Precautionary advice for those with hypo- OR hyperthyroidism:

A study presented at a recent annual meeting of the American Thyroid Association suggests over-the-counter “thyroid support” pills (commonly used for weight loss and to fight fatigue) are mostly ineffective. The study analyzed 10 thyroid supplements and found all but one contained amounts of two thyroid hormones intended only for prescription drugs because they can cause increased heart rate, palpitations, and diarrhea (3). Because this study was presented at a medical meeting, the data and conclusions should be viewed as preliminary until published in a peer-reviewed journal.

Monitoring the outcomes of dietary recommendations will help ensure patients’ success in incorporating individualized nutrition management of thyroid disease.

Soy foods, soy supplements and thyroid disease

In recent years, the impact of soy foods and supplements upon human health has become increasingly controversial among the general public. No one has conducted a broad evaluation of the scientific evidence supporting or refuting popular perceptions of the health effects of soy consumption, until 2014, where a group of scientists conducted a comprehensive assessment of the literature surrounding the health effects of soy consumption that are of greatest interest.

This review has focused on 5 health benefits – relief of menopausal symptoms and prevention of heart disease, breast cancer, prostate cancer, and osteoporosis, and 5 health risks – increased risk of breast cancer, male hormonal and fertility problems, hypothyroidism, antinutrient content, and harmful processing by-products. Systematic reviews of human trials, prospective human trials, observational human studies, animal models, in vitro studies, and laboratory analyses of soy components were included for review.

This literature review revealed that soy foods and isoflavones may provide relief from menopausal symptoms and protect against breast cancer and heart disease.  Soy supplementation also appears to affect thyroid function in an inconsistent manner, as studies have shown both increases and decreases in the same parameters of thyroid activity. Soaking, fermentation, and heating may reduce problematic antinutrients contained in soy.

The authors found that consuming moderate amounts of traditionally prepared and minimally processed soy foods may offer modest health benefits while minimizing potential for adverse health effects. However, additional studies are necessary to elucidate the variable thyroid response to soy supplementation, and more rigorous studies are required to assess dose-response relationships, the relationship between intestinal-flora composition and the response to soy, and the unknown long-term health effects of consuming highly processed modern soy foods. (2)

In 2011 a well-known study was carried out, with conclusions indicating that there is a 3-fold increased risk of developing overt hypothyroidism with a dietary supplement of 16mg of phytoestrogens. BUT a supplement of 16mg phytoestrogens SIGNIFICANTLY reduced the insulin resistance, cardiovascular disease risk markers AND blood pressure in these patients. (4)

And yet, more recently, in 2014 another specific study was carried out as follows:

Background: Post-menopausal women under treatment with levothyroxine (thyroid medication) for their medical conditions may take concomitantly dietary supplements containing soy isoflavones in combination to treat their post-menopausal symptoms. The aim of this study was to investigate the effect of a fixed combination of soy isoflavones on the oral bioavailability of levothyroxine in post-menopausal female volunteers.

Methods: 12 healthy post-menopausal females, who were on stable oral levothyroxine as replacement/supplementation therapy for hypothyroidism, received a single recommended oral dose of a food supplement containing 60 mg of soy isoflavones (>19% genistin and daidzin) concomitantly with (test) and 6 h later (reference) the administration of levothyroxine in a randomized, open label, crossover fashion. Plasma concentrations of levothyroxine and soy isoflavones (daidzin, daidzein, genistin, genistein, S-equol) were determined.

Conclusions from this specific study: The combination of soy isoflavones used in the present investigation does not affect the rate and extent of levothyroxine absorption when administered concomitantly in post-menopausal women.

It must be noted that in infants with congenital (genetically inherited) hypothyroidism. Soy products do interfere with levothyroxine absorption and can endanger infants and young children with this condition who are at risk for developmental and growth delays.(1) This does NOT apply to adults with hypothyroidism, where the benefits of soy products outweigh any possible negative consequences of using soy products together with thyroid medications. (2) 

What do we make of all this research into the link between soy products and thyroid disease?

FACT:    SOY FOODS contain concentrated forms of phytoestrogens which have known health benefits conclusively proven in research, such lowering incidences of certain cancers, heart health, insulin resistance, relieving menopausal symptoms, and many other related health benefits.

FACT:    A 2014 study showed that the combination of soy isoflavones does not affect the rate and extent of levothyroxine absorption when administered concomitantly in post-menopausal women.

FACT:    The use of cruciferous vegetables (cabbage, broccoli, spinach etc) and soy foods can be used daily in the case of thyroid disease, where there is no coexisting iodine deficiency (this latter condition is extremely rare).

Whilst there is ongoing research into this link between thyroid disease and soy food consumption, we can safely say that soy-based foods, such as Soya Life Porridge, Soya Life Instant Meal replacement Drink, and Soya Milks, as well as fermented soy foods such as tofu and miso, when used by sufferers of thyroid disease, and who are on thyroid medication, should still experience T3 and T4 hormone stabilisation brought about by this medication, and furthermore will experience definite improvements in insulin resistance, cardiovascular disease markers (including weight gain) and blood pressure where these are also a problem.


  1. Unawareness of soy intake in the management of congenital hypothyroidism. Fruzza, A.G. etal. PEDIATRICS 2012 Sept Vol 130(3) pp 699-702.
  2. (English) By: D’Adamo CR; Sahin A, Alternative Therapies in Health and Medicine [Altern Ther Health Med], ISSN: 1078-6791, 2014 Winter; Vol. 20 Suppl 1, pp. 39-51; Publisher: InnoVision Communications; PMID: 24473985.
  3. Are there foods that support thyroid health? Eleese Cunningham RD. American Association of Clinical Endocrinologists. Jan 2012.
  4. The effect of soy phytoestrogen supplementation on thyroid status and cardiovascular risk markers in patients with subclinical hypothyroidism: a randomized, double-blind, crossover study. Sathyapalan, T. etal. J.Clin. Endocrinol Metab. 2011 May. Vol 96(5) pp1442-9.
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