DIETARY GOITROGENS AND THYROID FUNCTION: IS THERE A LINK?

Read in 4 minutes
This blogpost was prompted by a comment on a Whatsapp group about dietary goitrogens and thyroid disease. That discussion was related to soy. 
If you have hypothyroidism,the most important question that you’d like to be answered is:
Should you avoid food like soy and cruciferous vegetables (dietary goitrogens)?
 
WHAT ARE GOITROGENS?
According to Wikipedia “Goitrogens are substances (whether in drugs, chemicals, or foods) that disrupt the production of thyroid hormones by interfering with iodine uptake in the thyroid gland. “
( In this post i will not discuss drugs or endocrine disrupting chemicals that affect thyroid function).
What are the common food that well-meaning healthcare practitioners sometimes ask you to avoid if you have hypothyroidism?
The commonest ones are soy and cruciferous vegetables. Cabbage, kale,broccoli, brussels sprouts,bok choy, cauliflower are the commonest cruciferous vegetables that you eat.
SOY AND GOITRE
The alleged association of soy with goitre has probably originated in animal studies and few isolated case studies reported in the sixties ,where few infants fed a soy formula had developed goitre.
What is the true association of soy and thyroid function? In an article in the journal Thyroid (2006) the authors looked at 14 trials (though thyroid function was not the primary health outcome in any trial). They commented that “the findings provide little evidence that in euthyroid, iodine-replete individuals, soy foods, or isoflavones adversely affect thyroid function.”
What about soy interfering with supplemental thyroid hormone absorption?
An interesting case study reported in Endocrine Practice: May 2001 discusses a patient who needed very high doses of thyroid hormone replacement after she had undergone a thyroid gland removal for cancer.On enquiry it turned out that she was taking a “soy-cocktail “protein supplement immediately after taking her synthetic thyroid hormone! Once the intake of both were separated, she needed a lesser dose of thyroid hormone!So this was not a problem of soy being a goitrogen, but a problem of food interfering with  thyroid pill absorption.
Is it only soy that interferes with supplemental thyroid hormone absorption? NO!
Iron, calcium, proton-pump inhibitors like Omeprazole,coffee, cholestyramine and other food also interfere with thyroid absorption. 
This is why we recommend that you should take your thyroid pills on an empty stomach. Preferably avoid eating or drinking anything (other than water) one hour before and one hour afer taking your thyroid meds.Another suggestion from my practice partner Dr.Natasha Iyer is to take your thyroid meds in the middle of the night if you wake up to use the washroom. But no nibbling then!
What about cruciferous vegetables as goitrogens?
In animal studies very high intake of cabbage and turnip have been associated with hypothyroidism.However, animal studies cannot be extrapolated to humans!
A study reported in the New England Journal of Medicine, May 2010 talks about an 88 year old Chinese woman who was brought to the emergency department in myxedema coma (severe hypothyroidism leading to coma).She did not have thyroid dysfunction before. She was taking 1 to 1.5 kilograms of raw bok choy everyday for several months in the belief that it would take care of her diabetes! I don’t know whether her diabetes was affected of but it sure sent her into coma!
However, a small study which  looked at 10 people who ate 150 grams of cooked Brussels Sprouts everyday for 4 weeks, did not show any adverse effects on thyroid function.
IMPORTANT TAKEAWAYS
  • If you have hypothyroidism you can eat soy, cabbage, bok choy, cauliflower, broccoli, kale as long as you do not over-indulge!
  • Supplemental thyroid hormones should be taken away from food or supplements for best absorption.
You may be interested in reading the following posts:
(Watch for post on iodine and thyroid to be posted later).
REFERENCES
  1. Bell, MB, FACE, David SH, and Fernando Ovalle, MD. “Use of soy protein supplement and resultant need for increased dose of levothyroxine.”Endocrine Practice 7.3 (2001): 193-194.
  2. Chu, Michael, and Terry F. Seltzer. “Myxedema coma induced by ingestion of raw bok choy.” New England Journal of Medicine 362.20 (2010): 1945-1946.
  3. McMillan, M., E. A. Spinks, and G. R. Fenwick. “Preliminary observations on the effect of dietary brussels sprouts on thyroid function.” Human & Experimental Toxicology 5.1 (1986): 15-19.