Soy. Things You Should Know
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by Jade Teta on December 5, 2009
Jade Teta ND, CSCS. Keoni Teta ND, LAc, CSCS. Jillian Sarno ND.

A recent discussion about Soy from Natural Triad Magazine:

In response to Dr. Tabors concerns about our recent article on soy and hypothyroid: He raises several valid points that we would like to clarify and expand on. Additionally, we would like to offer several more reasons why we do not recommend soy to our clients and look to soy consumption as a potential cause of weight loss resistance in people with hypothyroidism.

As we pointed out in the article, there is far more to thyroid function than normal levels of thyroid hormone. Additionally, there are a significant number of people out there with undiagnosed thyroid issues and/or insufficient iodine intake. These people do not have “normal” thyroid function and could be impairing their thyroid further with soy consumption. It is also well established in the literature that soy interferes with thyroid medication, making it less effective. There is also anecdotal evidence that excessive soy consumption has the potential to cause thyroid problems in previously “normal” individuals. Now we will discuss these concepts point by point.

Dr. Tabor is correct in saying that the article we cited was not a human study. However, it along with other studies does show, in our opinion, “isoflavones in soy are potentially damaging to adults and especially worrying in children”. Let us discuss soy’s actions on the thyroid. The study we quoted, along with another published the previous year by the same authors, clearly shows the anti-thyroid effects of soy. Dr. Tabor himself quotes two more articles, which clearly show soy acts as a goitrogen. (1,2) Although these studies are just examples, we feel that a strong precedent has been established in the literature for soy as a thyroid disruptor.

It has been shown conclusively that soy disrupts thyroid peroxidase (TPO), the chief enzyme involved in the synthesis of thyroid hormone. As a matter of fact, it was reported as far back as the 1930s that chickens and other animals fed soy developed goiter (20, 21, 22, 23). Furthermore, many studies showing no changes in thyroid hormone levels are short feeding studies and likely were not long enough to see the effects if they did occur. There are few if any long-term studies on this subject that we are aware of. The fact that not ALL research demonstrates soy has detrimental thyroid effects should not make us less concerned.

There is a general consensus among researchers, as Dr. Tabor points out, that soy does not seem to affect thyroid hormone levels in normal thyroid people despite the disruption of the thyroid enzyme systems. However, our article was more focused on those with abnormal thyroid function.

It is our opinion and clinical observation that soy can and does affect not only thyroid enzymes but thyroid hormone levels in susceptible individuals. There is also at least one study that shows this as well (5). In our article, “Tried Every Diet & Still Can’t Lose Weight? Consider Your Thyroid Function,” published in last month's Natural Triad Magazine our intention was to highlight the potential of underlying thyroid dysfunction in those with weight loss resistance.

There are those with diagnosed or hidden thyroid issues, and/or inadequate iodine intake and/or subclinical hypothyroid that we specifically were trying to address. The two latter groups, by the way, are not an insignificant part of the population. It is estimated that 25% of the world population is iodine insufficient including up to 10% of the United States, and it is estimated that 8% of American women and 3% of American men has a subclinical thyroid condition (8). Other researchers go further. The endocrinologist Ridha Arem, M.D., out of Baylor College of Medicine published a book on thyroid issues and believes this number is much higher saying 1 in ten Americans are dealing with low thyroid. This would mean almost 20 million people do NOT have normal thyroid function and may be at risk from soy consumption (6,7). This is becoming a real issue because iodized salt, the number one source of iodine in the American diet, is frequently avoided due to concerns of blood pressure.

Furthermore, most salt used today does not contain iodine, unless it specifically states “Iodized Salt,” and even this is no guarantee (15). Adequate iodine intake is important because it seems to be the factor that protects against the soy-induced disruption of the thyroid. As Dr. Tabor has pointed out in his citations, soy does indeed block thyroid production unless there is adequate iodine intake. This has led many to assume it is a simple matter of increasing iodine along with thyroid. However, there is a narrow window of iodine intake beyond which it will also begin to disrupt thyroid function (19).

The thyroid effects of soy go beyond disrupting TPO. Soy isoflavones act as weak estrogens and therefore impact thyroid hormone indirectly (18). It is well known that women tend to have bigger thyroids (9) than men because of estrogen’s action on the thyroid. In light of this, it is questionable in our opinion, to recommend the consumption of soy, as it can have estrogen-like effects on the thyroid. If research shows that soy decreases the action of thyroid hormone and primary care physicians and other clinicians consistently observe that soy decreases the action of thyroid medication and have observed hypothyroid effects in presumably normal patients, then how can we in good conscience recommend that our clients consume soy, especially the ones that are experiencing difficulty with weight loss?

To us this opens up a whole other can of worms. If we are going to acknowledge that soy interrupts the action of thyroid hormones given as medication, then it is reasonable to assume that soy impacts the action of endogenous thyroid hormone. In those who are weight loss resistant, which our article was focused on, soy would seem to have no place in the dietary recommendations.

Furthermore, there is much debate and discussion about what is actually “normal” as far as the thyroid is concerned. As we mentioned above, undiagnosed, subclinical hypothyroidism is a real concern and deserves careful attention. The actual definition of subclinical hypothyroid is someone who has a normal thyroid hormone with slightly elevated levels of TSH. However, many health practitioners and researchers question this definition since we frequently see clients who are completely normal in their thyroid numbers and have overt hypothyroid symptoms.

The American Association of Clinical Endocrinologists (AACE) released a statement in Jan 2001 that said: “Even though a TSH level between 3.0 and 5.0 uU/ml is in the normal range, it should be considered suspect since it may signal a case of evolving thyroid underactivity.” Therefore the number of those with compromised thyroid function could be even larger. This is a major point we were attempting to make in our article. Weight loss resistance is increasing and there seems to be an increase in those with thyroid issues as well as issues with iodine insufficiency. As healthcare providers why would we want to recommend soy, a known thyroid disruptor, to these people especially when the thyroid is by far not the only issue where we feel soy’s health benefits should be questioned?

Soy is a known endocrine disruptor. These effects have been seen for some time and have been reported consistently in the literature (10, 11). It has been shown in animals from rats to monkeys, and in humans, that isoflavones disrupt sexual maturation in males (12) and speed it up in females (13, 14). The thyroid issues of soy may be much worse in human children suggested by two recent reviews on soy isoflavones and their thyroid effects related to thyroid auto-antibodies and autism (16, 17). While none of this can be seen as conclusive, taken as a whole the vast amount of research should make us more cautious not less. Researchers and doctors like to make arguments as to whether these effects occur consistently in humans, but we tend to take the stand of Dr. Richard Sharp who is the Director of the Medical Research Centre for Reproductive Biology in Edinburgh Scotland who said in regard to soy: “I’ve seen numerous studies showing what soy does to female animals and until I have reassurances that it doesn’t have this effect in humans, I will not give soy to my children”.

Isoflavones in soy are not the only issue – soy is also high in manganese and can be contaminated with aluminum. The excess estrogen, manganese, and aluminum are, in our opinion, extremely detrimental to children and their brain and sexual development. It appears the health departments of several countries, such as Great Britain, New Zealand and Israel agree with us, strongly recommending against the use of soy and soy formula (24, 25, 26). The reason we single out soy as “perhaps one of the worst goitrogenic foods” is that unlike other goitrogenic foods whose goitrogens are neutralized through cooking, soy’s goitrogens are not degraded, resulting in the full delivery of thyroid interrupting chemicals to the body (3,4)

Let us recap the major points we were trying to make about soy. Genistein, the major isoflavone present in soy, is a strong TPO inhibitor. As we have pointed out this may or may not be of concern for normal thyroid people, but it most certainly is for those with thyroid issues and those taking thyroid medications. In addition, a large number of people have as-of-yet undiagnosed thyroid conditions and/or iodine intakes that are insufficient, for which soy consumption would be a risk. We have personally worked with those who have been, in our opinion, extremely negatively impacted by soy consumption. We strongly feel that soy has no place at all in the diets of children.

Many of the soy-based products on the market today are not only made from GMO (genetically modified organism) soy, which raises several additional questions beyond the scope of this paper, but also concentration levels of soy isoflavones that have been purposely elevated to such a high level that does not occur in nature and would never be eaten as a food. This creates yet another big question mark regarding what is or is not safe as far as soy is concerned.

One last point, that I think Dr. Tabor will agree with if we consider soy a medicinal food (which we do if soy is used as it has historically been in the Asian culture) then inherent in that definition, any food that is considered a medicine can also be poison. The jury is still out on the safety of soy and in our opinion is not the superfood the media and big industry have made it out to be.

1. Divi RL, Chang HC, Doerge DR. Anti-thyroid isoflavones from soybean: isolation, characterization, and mechanisms of action. Biochem Pharmacol 1997; 54(10):1087-1096.
2. Doerge DR, Sheehan DM. Goitrogenic and estrogenic activity of soy isoflavones. Environmental Health Perspectives 2002; 110(Suppl 3):349-353.
3. Draft report of the COT Working Group on Phytoestrogens, 4. Sources and concentrations of phytoestrogens in foods and estimated dietary intake.
4. Coward L, Smith M et al. Chemical modification of isoflavones in soyfoods during cooking and processing. Am J Clin Nutr, 1998, 68, 1486S-1491S.
5. Hampl R, Ostatnikova D, Celec P, Putz Z, Lapcik O, Matucha P. Short-term effect of soy consumption on thyroid hormone levels and correlation with phytoestrogen level in healthy subjects. Endocrine Regulations 2008; 42:53-61.
6. Arem, Ridha. The Thyroid Solution (Ballantine, 1999).
7. Arem R, Escalante D. Subclinical hypothyroidism: epidemiology, diagnosis and significance. Adv Int Med, 1996, 41, 213-250.
8. Hollowell J, Staehling N et al. Iodine Nutrition in the United States. Trends and Public Health Implications: Iodine Excretion Data from National Health and Nutrition Examination Surveys I and III (1971-1974 and 1988-1994). J Clin End and Metab, 1998, 83 no.10, 3401-3408.
9. Bates Guide to Physical Examination and History Taking (9th ed)
10. Waring RH, Ayers S, Gescher AJ, Glatt HR, Meinl W, Jarratt P, Kirk CJ, Pettitt T, Rea D, Harris RM. Phytoestrogens and xenoestrogens: the contribution of diet and environment to endocrine disruption. J Steroid Biochem Mol Biol. 2008 Feb;108(3-5):213-20.
11. Jefferson WN, Padilla-Banks E, Newbold RR. Disruption of the developing female reproductive system by phytoestrogens: genistein as an example. Mol Nutr Food Res. 2007 Jul;51(7):832-44.
12. West MC, Anderson L, McClure N, Lewis SE. Dietary oestrogens and male fertility potential. Hum Fertil (Camb). 2005 Sep;8(3):197-207.
13. Fortes EM, Malerba MI, Luchini PD, Sugawara EK, Sumodjo L, Ribeiro Neto LM, Verreschi IT. High intake of phytoestrogens and precocious thelarche: case report with a possible correlation. Arq Bras Endocrinol Metabol. 2007 Apr;51(3):500-3.
14. McLachlan JA, Simpson E, Martin M. Endocrine disrupters and female reproductive health. Best Pract Res Clin Endocrinol Metab. 2006 Mar;20(1):63-75
16. Melarova, Et. Al. actual levels of soy phytoestrogens in children correlate with thyroid laboratory parameters. “Clin Chem Lab Med. 2006;44(2):171-4
17. Roman, Et. Al. Autism: transient in utero hypothyroxinemia related to maternal flavonoid ingestion during pregnancy and to other environmental antithyroid agents. “J Neurol Sci. 2007 Nov 15;262(1-2):15-26.
18. Doerge. Et. Al. Goitrogenic and estrogenic activity of soy isoflavones. Environ Health Perspect. 2002 Jun;110 Suppl 3:349-53.
19. Lee Et Al. Too much versus too little: the implications of current iodine intake in the United States.Nutr Rev. 1999 Jun;57(6):177-81.
20. Matrone, G. et al., “Effect of Genistin on Growth and Development of the Male Mouse”, Journal of Nutrition (1956) 235-240.
21. McCarrison R. The goitrogenic action of soybean and ground nut. Indian J Med Res.1933, 21:179
22. Patton AR, Wilgus HS et al. The production of goiter in chickens. Science, 1939, 89, 162
23. Sharpless GR et l. Production of goiter in rats with raw and treated soybean flour. J Nutr, 1939, 17, 545-555