There is little doubt that the foods we eat dramatically affect our health. As healthcare providers who support a holistic approach to wellness, massage therapists often discuss lifestyle factors with their clients – including proper nutrition. Although a bodyworker’s ideas on eating a healthy diet may not be backed by a formal degree, the trusting relationship between patient and practitioner could place significant importance on suggested foods. As such, massage therapists who are knowledgeable about the controversy around soy foods are best prepared to address this growing dietary concern.

Soy Benefits

While tofu, soymilk and other soybean products have long been considered to be health foods, concern about soy’s negative impact on health is growing. Soy food’s benefits typically fall into one of the following two categories:

  1. Endorsed by the American Heart Association, soy is a healthier, low-fat protein alternative to meat and poultry.
  2. Soy is a natural alternative to estrogen replacement for some women to reduce the risk of breast cancer and symptoms of menopause.

However, the long-term effect of a diet emphasizing soy products has turned up some disturbing news for those with hypothyroidism.

Thyroid Troubles

Although the phytoestrogens or isoflavones in soy are supposed to protect us from heart disease, cancer, osteoporosis and the discomforts of menopause, isoflavones have been confirmed to depress thyroid function. Confirming this understanding, healthcare practitioners across the globe have reported cessation of their patient’s hypothyroid symptoms upon eliminating soy from their diet.

According to writer for Cambridge Scientific Abstracts, Jennifer A. Phillips, estrogens (including the phytoestrogens in soy) can block the efficiency of thyroid hormones. Since women have more estrogen then men, this explains why women need more thyroid hormones and are more prone to thyroid troubles. Symptoms of low thyroid function or hypothyroidism include:

  • Memory loss
  • Confusion
  • Slowness
  • Depression
  • Fatigue
  • Hair loss
  • Menopause difficulties
  • Digestive problems
  • Constipation
  • Infertility
  • Brittle bones

Often times, these symptoms indicate other types of ailments or co-existing health conditions such as fibromyalgia or chronic fatigue syndrome. However, helping one problem often improves other conditions regardless of the client’s presenting diagnosis.

Soy’s Danger to the Thyroid

Below are several reports documenting soy’s danger to the thyroid:

  • According to Theodore Kay at Kyoto University in Japan, “Thyroid enlargement in rats and humans, especially children and women, fed with soybeans has been known for half a century.” Kay’s 1988 study attempted to determine the amount of iodine required to prevent goiter in populations consuming soy foods. He found that small amounts of iodine could prevent noticeable thyroid enlargement, but even large amounts did not prevent pathological changes to the thyroid gland.
  • In the April 1990 edition of the Journal of American College of Nutrition, New York researchers found that the frequency of feedings with soy-based milk formulas in early life was noticeably higher in children with autoimmune thyroid disease, and thyroid problems were almost triple in those soy formula-fed children compared to their siblings and healthy unrelated children.
  • In the November 1997 issue of Biochemical Pharmacology, The National Center for Toxicological Research identified that the isoflavones (the estrogen-like compounds found plentifully in the soybean) are capable of suppressing thyroid function, and causing or worsening hypothyroidism. Additionally, they found that high consumption of soy products could cause goiter.
  • The March 1999 issue of Natural Health magazine has a feature on soy that quotes Daniel R. Doerge, Ph.D., a researcher at the Food and Drug Administration’s National Center for Toxicological Research. Dr. Doerge has researched soy’s anti-thyroid properties and said, “…I see substantial risks from taking soy supplements or eating huge amounts of soy foods for their putative disease preventive value. There is definitely potential for interaction with the thyroid.”

Not-So-Hasty Conclusions

While the studies listed above are dated, they did provide enough concern to prompt 21st century research into isoflavones’ effect on the thyroid. In the March 2006 edition of Thyroid, the official journal of the American Thyroid Association, investigators looked collectively at 14 trials evaluating soy’s impact on thyroid function. While some evidence suggested that soy foods inhibit the absorption of the thyroid hormone typically taken to balance hypothyroidism, only one of the 14 studies demonstrated a connection between the two. Clearly, there is conflicting evidence surrounding soy’s affect on thyroid function.

Although the inconclusiveness of soy’s affect on the thyroid could inspire dismissal of troubling claims, an abundance of case studies are proving otherwise. Because those with signs of hypothyroidism (see list above) often see a reduction of their symptoms by removing soy from their diet, being wary of this food deserves consideration. Deciding if eating soy foods is good – or not so good – for your health is not definite. Its benefits on heart health and menopausal symptoms may outweigh any potential interference with the thyroid. However, if you have a client who has hypothyroidism or demonstrates signs of a low-functioning thyroid, their experimenting with soy food elimination for several weeks couldn’t hurt. It may even bring them enormous relief.


Messina M, Redmond G., 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., How is Hypothyroidism related to FMS and CFS?, Deanna Couras Goodson, Minerva WebWorks LLC, 2008., Do Soy Foods Negatively Affect Your Thyroid?, Mary Shomon, February 2008., Soy: The Dark Side of America’s Favorite “Health” Food, Sally Fallon, Mary G. Enig, PhD, The Weston A. Price Foundation, 2008.