Is it safe for me to eat soy foods if I have hypothyroidism?

Dear Alice,

They say soy food is good for you, for decreased chance of osteoporosis and for an alternative to meat products. I'd like to try soy but wonder if it will affect my hypothyroidism and the prescription I use — I've read different studies on it and am confused.

Can I or can I not eat soy?

confused in Minnesota

Dear confused in Minnesota,

Soy, the question about whether this food is appropriate for you (as you manage your hypothyroidism) has bean weighing on your mind. You’re not the only one who’s mulling it over — the question of soy’s effect on the thyroid gland has puzzled scientists for some time. To that end, there is evidence that soy protein may make it more difficult for your gut to absorb thyroid hormone, however, it does not seem to be the case that soy interferes with the action of thyroid hormone that’s circulating in your body. Although some of the research (conducted over the better part of a century) is slightly contradictory, the bottom line is that it may not be necessary to avoid soy completely if you are being treated for hypothyroidism. Talking with your health care provider can help you determine whether soy products would be an appropriate compliment to your diet.

Before discussing how soy may impact the thyroid, it’s helpful to talk hormones! The thyroid is a butterfly-shaped gland that sits at the base of your neck and produces thyroid hormone, combining molecules of the amino acid tyrosine with either atoms of iodine to make T3 (triiodothyronine, a highly active form of the hormone) or T4 (thyroxine, a lesser active form) respectively. People who do not make enough T3 or T4 must take it as a supplement to avoid being clinically hypothyroid. When a health care provider begins a patient on thyroid replacement hormone, s/he will regularly measure levels of thyroid stimulating hormone (TSH), which will help indicate whether thyroid levels are being regulated on the medication.

Here’s where soy comes into play: soy contains active compounds called isoflavones (genistein and daidzein, to be exact) which compete for the enzyme that makes thyroid hormone. Research indicates that the interference of isoflavones with thyroid hormone production is a problem only when there is underlying iodine deficiency. So, if you are getting enough iodine through your diet, it’s unlikely that eating soy will contribute to hypothyroidism. In studies, the one group that saw a need for an increase in thyroid replacement hormone with high soy intake were infants who were born with thyroid disease and who were being fed soy formula. Additionally, because soy may impact the successful absorption of the prescription you take to manage your condition, some sources recommend that you wait four hours to eat soy after taking thyroid medication.

All this to say, if your iodine levels are in order and you consider the timing of when you take your medication when deciding to enjoy a bit of soy, it seems as if you’ll likely be able to add it to your diet without much impact. However, if you are concerned about your thyroid levels or combining soy with your medication, your health care provider will be able to advise you further based on your specific case.

Happy soy sampling!

Last updated Jul 02, 2015
Originally published Jun 21, 2002

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