To Top

Does soy interfere with thyroid function?

When I began training over 40 years ago, the major protein source in commercial protein supplements was soy. The most popular brands at the time were those sold by the York Barbell company, who also published the popular magazines, Strength and Health, followed later by Muscular Development.According to advertising copy and articles published in their magazines, soy was a vegetable-based protein that was unique because it contained all the essential amino acids required to promote muscular growth. In point of fact, natural soy protein was low in the essential amino acid, methionine. In recent years, companies that manufacture soy protein products have added methionine, which has greatly improved the biological value of the protein. So much so, that some emerging studies suggest that the protein found in modern soy supplements is equal to high grade milk proteins, such as whey, in promoting muscle gains. Not all studies have confirmed this,however, and most of the positive studies related to soy have been supported by funds provided by the soy industry. Of the two primary milk proteins, whey and casein, whey is considered a fast-acting, rapidly absorbed protein. In contrast, casein absorption can take as much as 7 hours. This combination of slow and fast proteins makes milk protein ideal for bodybuilding purposes. You want a fast-acting protein before and after training to support muscle protein synthesis and help prevent excess muscle protein breakdown during training. On the other hand, you would favor a slowly digested protein, such as casein, to support an anti-catabolic effect in muscle by its qualities of slow release of amino acids into the blood for an extended time. Soy is considered an intermediate-acting protein, not as rapidly absorbed as whey, but absorbed faster than casein.

This absorption property of soy is being used to promote the use of isolated soy protein (ISP) in combination with milk proteins as being an ideal bodybuilding supplement. But various problems have been often suggested when soy is ingested. Perhaps the most prominent of such allaged adverse reactions of soy is that it lowers testosterone. I discussed this in a recent issue of Ironman. Briefly, the existing evidence shows no adverse effects of soy in relation to testosterone. The charge arose because soy contains isoflavones, such as genistein,which are structurally similar to estrogen. In fact, much of the health benefits attributed to soy are related to its isoflavone content. These health claims include  protection against cardiovascular disease and cancers of the breast and prostate. Indeed, these particular cancers are less prevalent in Asian countries, where soy consumption is higher, and the average intake of isoflavones from soy ranges from 30 to 60 milligrams daily.On the other hand, Asian people are also known to ingest other health protective foods, such as green tea. Even if we discount the testosterone-lowering effect attributed to soy, there is another problem: interference with thyroid function.

The thyroid gland, located in the neck, produces hormones that control the resting metabolic rate. While there are two primary thyroid hormones, only one is considered metabolically active, while the other is considered more of a prohormone. Thyroid hormone itself consists of the amino acid, L-tyrosine complexed with the trace mineral, iodine. Several studies have shown that large amounts of soy foods can interfere with the production of thyroid hormones. The precise mechanism involves an interference with the function of a primary enzyme  involved in thyroid hormone synthesis, namely thyroid peroxidase. When the function of this enzyme is blocked, various thyroid maladies can ensue, ranging from goiter to autoimmune thyroiditis. The isoflavones in soy can interfere with the uptake of iodine into the thyroid gland. The effect is more marked when a low intake of dietary iodine occurs in combination with a large intake of soy foods. In the past,lack of sufficient iodine was common, but this changed with the advent of iodized salt. But in recent years, the focus has been on consuming less table salt fortified with iodine because the sodium in salt is linked to high blood pressure, water retention, and other ills. From the standpoint of food, most fish contains good amounts of iodine, but again, if you don’t eat a lot of fish, you could be in trouble from an iodine standpoint. Most supplements aimed at bodybuilders, including various meal replacements and vitamin-mineral combos, also contain iodine, so lack of iodine is likley rare in most bodybuilder’s diets. But such a lack does set you up for thyroid problems if you ingest either a lot of soy foods or use a ISP supplement.

In 2006, a review published in the journal,Thyroid, examined 14 studies related to the effect of soy on thyroid function. The study found that 13 of the 14 studies examined found either no or only modest effects of soy on thyroid function. One problem with this particular study was that the lead author was a paid consultant for the soy industry. He could have thus handpicked studies showing that soy ingestion didn’t affect the thyroid, while ignoring those that suggested otherwise. But a consensus of independent studies shows that ingesting 30 milligrams a day or less of soy isoflavones doesn’t affect thyroid function.But 50 grams of ISP can contain as much as 84 milligrams of isoflavones, far more than the suggested limit. The most recent study in this regard featured older women, who ingested an average of 54 milligrams a day of soy isoflavones for three years. This dose of isoflavones had no effect on the women’s thyroid function. This is significant because thyroid problems are common in older women, with over 50% having such problems. Any potential problems related to soy ingestion and thyroid function can be prevented by limiting your intake of soy, and also ensuring that you are ingesting an adequate amount of iodine. If you are worried about the iodine aspect, consider an old supplement called kelp. Kelp is seaweed, and it’s loaded with iodine. Taking a couple of kelp tablets can ensure a sufficient intake of iodine. But don’t go overboard on this: too much kelp or iodine has a reverse effect, in that it can paradoxically  depress thyroid output. The same holds true for the mineral selenium. Selenium is vital for thyroid hormone synthesis, but too much, or more than 400 micrograms daily, can depress thyroid function.

Bitto, A ,et al. Genistein aglycone does not affect thyroid function: results from a three-year, randomized, double-blind, placebo-controlled trial.J Clin Endocrin Metabol 2010: in press.

If you enjoyed the information presented here, check out my e-book, Natural Anabolics, available at

Anabolic Stack - Gains In Bulk


  1. Dave

    April 20, 2010 at 4:18 pm

    Thanks, Jerry, for another very interesting article. I also want to mention that soy isolate is significantly higher in glutamine and arginine than whey. That coupled with the fact that it does not negatively affect testosterone levels seems to make it a pretty solid bodybuilding protein. Yet, soy remains perhaps the most denigrated of all foods by bodybuilders! …..By the way, did you do an article on the study concerning protein utilization topping off at 20-25 grams per meal? No doubt, that’s another one that will not be well received (or even believed) by most bodybuilders.

  2. Jerry Brainum

    April 20, 2010 at 6:46 pm

    Your statements about soy in relation to amino acid content is correct, although whey is considerably higher in branched-chain amino acids compared to soy. My feeling is that soy, as an intermediate-acting protein source in relation to whey (rapid) and casein(slow) makes it compatible to use in a protein supplement featuring all three protein sources. I have written about the 20-25 gram protein intake level maximizing muscle protein synthesis. I agree that many BBs will continue to ingest over 400 grams a day, which is a waste. On the other hand, dogma has held that the body cannot use more than 30 grams of protein in one meal for years, which I also was skeptical about, until I studied the nuances of protein absorption in the body.

You must be logged in to post a comment Login

Leave a Reply

More in Blog Post

  • Get A Good Workout with Mike Rashid

    Muscle sensation Mike Rashid, whose motto is “Train Your Mind as Hard as You Train Your Body,” explains his shoulder massifier step...

    Cindy MamJuly 16, 2018
  • Why Your Big Toe Is Your Key To Great Calves!

    If you’re looking to really build your calves, this one little piece of advice will help tremendously. Yet you rarely, if...

    Nick NilssonApril 6, 2016
  • Create Big-Ass Shoulders With Mike Rashid

    His motto “Train Your Mind as Hard as You Train Your Body,” may test your muscles and your mind.

    Iron Man MagazineMarch 21, 2016
  • The Workout and Happiness

    Happy New Year—a perennial cliché!  Can anything or anybody make you happy? To paraphrase a well-known line from Shakespeare’s Julius Caesar,...

    John BalikFebruary 11, 2015
  • New Year Resolutions

    January first, the birth of a new year.  Time to reflect on the old and plan for the new! Historically, January...

    John BalikJanuary 15, 2015
  • Do You Know Squat ?

    Have you heard any of the following recommendations? “Don’t let your knees go over your toes when you squat, because it’s...

    Doug BrignoleDecember 30, 2014
  • Another Rewarding Olympia Weekend

    A couple of weeks ago I was at the Mr. Olympia competition, the 50th rendition of that iconic event. What started...

    John BalikDecember 15, 2014
  • Muscle Beach Breakfast Club

    I have been a part of Muscle Beach Venice since competing there in the mid ’60s. Much has changed in the...

    John BalikNovember 15, 2014
  • The Science Behind My 2 Week Body Transformation into a Mythological Minotaur

    Sometimes getting in shape can mean perfect timing.  In order to make a 2 week body transformation, science can play a...

    curtis.fisherNovember 14, 2014