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Effects of Soy Protein on Thyroid Function – Research Review

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Title and Abstract

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. (2006) 16:249-58.

Soy foods are a traditional staple of Asian diets but because of their purported health benefits they have become popular in recent years among non-Asians, especially postmenopausal women. There are many bioactive soybean components that may contribute to the hypothesized health benefits of soy but most attention has focused on the isoflavones, which have both hormonal and nonhormonal properties. However, despite the possible benefits concerns have been expressed that soy may be contraindicated for some subsets of the population. One concern is that soy may adversely affect thyroid function and interfere with the absorption of synthetic thyroid hormone. Thus, the purpose of this review is to evaluate the relevant literature and provide the clinician guidance for advising their patients about the effects of soy on thyroid function. In total, 14 trials (thyroid function was not the primary health outcome in any trial) were identified in which the effects of soy foods or isoflavones on at least one measure of thyroid function was assessed in presumably healthy subjects; eight involved women only, four involved men, and two both men and women. With only one exception, either no effects or only very modest changes were noted in these trials. Thus, collectively the findings provide little evidence that in euthyroid, iodine-replete individuals, soy foods, or isoflavones adversely affect thyroid function. In contrast, some evidence suggests that soy foods, by inhibiting absorption, may increase the dose of thyroid hormone required by hypothyroid patients. However, hypothyroid adults need not avoid soy foods. In addition, there remains a theoretical concern based on in vitro and animal data that in individuals with compromised thyroid function and/or whose iodine intake is marginal soy foods may increase risk of developing clinical hypothyroidism. Therefore, it is important for soy food consumers to make sure their intake of iodine is adequate.


Soy protein is one of those topics that seems to be a perennial topic of debate and argument with staunch pro- and anti-soy people out there making all kinds of seemingly good arguments for either the benefits or dangers of soy protein.  As is usually the case with extremist positions, I find that the reality lies somewhere in the middle.

Now, I think that part of the problem, as I explained in a seminar a few weeks ago, is how people, at least those in the United States (I can’t speak to the rest of the world) tend to approach things.  Folks are prone to extremes in the first place and nowhere is this more prevalent than in the health field.

Whenever some nutrient is discovered to be ‘healthy’, invariably people figure that more must be better and start mega-dosing it.  This invariably leads to some sort of backlash as people learn (often the hard way) that more is, in fact, not better.  Then they invariably go on a crusade against that nutrient not realizing that their own extreme behavior (rather than the nutrient itself) was the actual cause of the problem.

One of my favorite examples is that of oat bran back in the 80’s.  Discovered to improve blood lipid levels, people starting eating mountains of the stuff, 50+ grams per day.  People were putting down horse-doses of the stuff because, you know, more is better.  Until it was found that such massive fiber intakes, especially from isolated sources, had the potential to cause vitamin and mineral deficiencies by binding them up before they could be absorbed.

Which brings me in a roundabout way to the issue of today’s research review on soy protein and thyroid function.  As per usual, there are camps on both sides of the debate for soy protein having either a beneficial or negative effect on thyroid hormones.  And also as per usual, the truth of the matter, in terms of how soy protein affects thyroid hormones lies somewhere in the middle and depends on other factors.  Today’s review looks at them.


To give readers a brief background on the topic, the thyroid gland releases two primary hormones T4 and T3 (thyroxine and trio-iodothyronine respectively) in a ratio of roughly 80:20 in response to the signal sent by TSH (thyroid stimulating hormone).  That is to say, most of the thyroid released from the thyroid gland itself is the relatively inactive T4.  Most T3 is actually made in other tissues (especially the liver but also in many other cells) from the metabolism of T4 via an enzyme called 5′-deiodinase.

Now, I imagine most readers think of T3 in terms of its effects on body weight or body fat and it’s certainly true that T3, along with the catecholamines (adrenaline/noradrenaline or epinephrine/norepinephrine depending on which side of the pond you’re on) are two of the primary regulators of human metabolic rate.  Of course, thyroid controls about a billion other things in the body too and, as one example, low T3 status can cause depression.

I should mention that iodine intake plays a crucial role in thyroid metabolism with inadequate intake of iodine causing thyroid problems.  There are many other micronutrients that are involved in this conversion process as well; these include selenium and iron (iron deficiency can impair thyroid conversion, yet another reason to eat red meat while dieting).

Now soy proteins are known to contain hormonal mimics called phytoestrogens. This include genistien, daidzein and others.  A great deal of controversy exists over the impact of these types of compounds in the human diet; while phytoestrogens may have some beneficial effects (especially in post-menopausal women for whom low estrogen can predispose towards heart disease and bone loss) other research shows negative impacts.  A lot of whether positive or negative impacts are seen depends on what’s being looked at and, of course, the dose studied.

The effect of phytoestrogens in men is far less studied and understood.  While many are concerned that the phytoestrogens present in soy may negatively impact on testosterone levels the reality is that the studies done to date, using moderate doses of soy/phytoestrogens, have found little to no impact (higher doses are often seen to cause issues).

There is likely to be a sex and population specific response to these compounds and whether or not soy has an impact on anything at all depends heavily on the amount being consumed.  Small amounts of soy protein tend to have minimal or no effects on most things studied (such as testosterone levels) while large daily amounts are often seen to have an effect.

There’s an old saw in medicine that the dose makes the poison and this is certainly one of those situations.

And, as I noted in the introduction, I think part of the backlash against soy has more to do with the human nature of people thinking more is better than with the nutrient itself.  As I mention below, the actual soy intake among Asian cultures doesn’t appear to be that high in the first place (and anyone who is worried about the impact on testosterone levels might consider that Asians, as a whole, don’t seem to be having many problems with fertility or making babies).

Again, I’m not going to focus on all of the potential effects of soy (e.g. on hormones such as testosterone) here; I only want to look at the impact, or potential impact of soy protein on thyroid hormone metabolism.

The Paper

So with that background out of the way, on to today’s research review, a review paper on the impact of soy protein on thyroid hormone status and metabolism.  As I noted above, there are two primary thyroid hormones, T4 and T3 and it appears that soy may have an impact on both.

Early work, in animals, had supported the idea that soy proteins could actually increase thyroid (mainly T4) output and this is likely where a lot of the pro-soy claims come from in terms of thyroid status (e.g. some will claim that soy will help fat loss by raising thyroid hormones).

However, as the paper points out,  studies suggests a very different effect in humans with soy protein having little to no direct impact on thyroid hormone output.  This is yet another place where extrapolating from animal research just doesn’t pan out.  Of course, there is also animal research suggesting a negative impact of soy protein, primarily the phytoestrogens, on animal thyroid status, something the pro-soy folks seem to ignore when they claim that soy will increase thyroid output.

Beyond that, at least in individuals with normal thyroid function, soy protein appears to have little to no impact on overall thyroid status.  The review examined 14 different studies (as noted above, 8 in women, 4 in men and 2 in both) and, with one exception, found little to no impact of soy intake on any measure of thyroid hormone status.  I’ll spare you all of the details, only the punchline is of any real importance.  Again, that’s in individuals with otherwise normal thyroid function.

However, in individuals with pre-existing low-thyroid (hypothyroid) symptoms, soy proteins can cause problems. Research has shown that soy protein intake may increase the dose of thyroid medication needed (the soy appears to impair uptake of thyroid medication) and individuals who are on thyroid hormones may need to avoid soy protein immediately around the intake of their medication.

Another review (Doerge DR. Goitrogenic and estrogenic activity of soy isoflavones. Environ Health Perspect. (2002) 110 Suppl 3:349-53.2002) has shown, using mainly animal work, that the phytoestrogens in soy can impair the enzyme (thyroid peroxidase) responsible for proper thyroid hormone production. That same review found that while soy protein itself could not induce a hypothyroid state, a high phytoestrogen intake coupled with a low iodine intake could.

I bring up this last point because one of the main providers of iodine in the modern diet is iodized salt and even there, diet surveys have shown a downward trend in overall iodine intake (due to a reliance on processed food and less iodinization of salt).  It’s worth noting that seaweed (another stable in Asian culture) is another good source of iodine. Even if Asian cuisine did contain a tremendous amount of soy, it would seem that the intake of seaweed, by providing iodine, would help to prevent problems from occurring.

Basically, I could see how a high intake of soy products (which are being used to fortify many foods such as cereals and protein bars, in addition to the use of soy protein powders) coupled with a misguided attempt to reduce salt intakes excessively (as is often seen in many ‘health-conscious’ individuals) could potentially cause proteins with overall thyroid metabolism.

But, and this goes to my comments earlier in the article, this is only an issue with people insistent on taking aspects of their diet to extremes.  People who are really consuming a massive amount of soy protein on a daily basis (that intake level not being seen in the Asian cultures in the first place) who also are trying to minimize sodium intake could be putting themselves at potential risk. And this is moreso the case if there is a pre-existing problem.

Summing Up

So we have several different issues at stake here in terms of how soy protein might impact on thyroid hormone status.

Clearly individuals with no pre-existing thyroid problems don’t need to worry much about soy.  And, no, I’m not saying that folks should therefore eat as much of it as possible.  Just that they needn’t go out of their way to avoid any and all source of soy in their diet.

But what about people who do have a pre-existing thyroid problem?

First and foremost, anybody who is on thyroid medication should avoid consuming soy immediately before or after taking their medication as soy protein appears to impair absorption of thyroid medication.  Individuals insistent (for whatever reason) on consuming soy near to the intake of their thyroid medication will need to increase their dose to compensate.  This, of course, should be dealt with through your medical provider/health professional.

As well, individuals with pre-existing thyroid problems (and it’s worth mentioning that females, which tend to be the primary target for soy foods, are more likely to have thyroid problems than men) may need to limit their soy intake on a day to day basis.  This is especially the case for individuals intent on reducing their sodium intake.


Trying to avoid every last bit of soy intake (for example, a typical soy protein fortified cereal may contain a few grams at most of soy) seems misguided to me, most studies examining a variety of endpoints find that it’s only when soy protein intake is excessive that any sorts of problems start. Even in the case of hypothyroid individuals, soy only appears to be a problem when iodine intake is insufficient in the first place.

Clearly, living on nothing but soy foods and soy fortified products is misguided as well.  As I mentioned above, the soy intake among most Asian cultures isn’t actually that massive in the first place and I suspect that much of the backlash against soy is primarily to do with people taking a little of a good thing, assuming a lot was better, and causing themselves problems because of it.

To me a happy medium seems the best; assuming no pre-existing thyroid problems, soy products are probably safe in moderation.  What’s moderation?  In The Protein Book, I suggested a daily maximum of perhaps 20-25 grams of soy protein on a daily basis.  Based on the average phytoestrogen content of most soy proteins (generally 2-3 mg phytoestrogen per gram of soy protein), that will keep most people below the threshold where any sorts of issues start to crop up.

I should mention that many foods are currently being fortified with soy protein (check the labels) and people may already be consuming soy protein in some amounts without knowing it.  Adding more (e.g. through a soy protein powder) may very well take people above the level I suggested above.

Frankly, unless someone is a fairly strict vegetarian or vegan, there are enough other high quality protein sources (such as meat, dairy products, whey, etc.) that I don’t see the need to consume massive amounts of soy in the first place.  But neither do I think it’s a horrible protein that no-one should ever eat because it will make their testosterone drop and give men boobs.

Soy, like all proteins has a variety of pros and cons and, in moderation, can make up part of a healthy or sports oriented diet.  Thinking that it is evil and must be eliminated is as silly as thinking it’s the best protein ever and people should consume tons of it.

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