QUESTION: I am A and hypothyroid. You state soy is excellent for A's but, from my researching and reading, most sources state that hypothyroid should avoid soy. Any suggestions as how to get protein and vitamins without using soy?
ANSWER: In a recent response to many of the off-handed accusations about the effects of soy on the thyroid Clare Hasler, Ph.D., one of the world's experts on soy and human nutrition had the following to say:
"There is no convincing evidence that soy protein has an adverse effect on thyroid function, particularly at the moderate level of consumption (25 grams) that would occur due to the approval of a health claim for coronary heart disease.
There is evidence that animals exposed to large amounts of soy protein (e.g., 40%) will develop goiter, particularly when fed an iodine deficient diet (Kimura et al., 1976; Filisetti and Lajolo, 1981). The mechanism for this effect can be explained by the fact that the principal isoflavones in soy, genistein and daidzein, have been shown to inhibit thyroid peroxidase (Divi et al., 1997) and 5'-deiodinase (Cody et al., 1989), key enzymes involved in thyroid hormone biosynthesis. The inhibition of these enzymes results in decreased levels of circulating thyroid hormones (e.g., T4 and T3) which leads to increased secretion of thyroid stimulating hormone (TSH) by the anterior pituitary. The increased levels of TSH provides a growth stimulus to the thyroid, resulting in goiter. It must be emphasized, however, that this occurs only with large amounts of soy isoflavones in the diet and/or when the diet is low in iodine. Furthermore, soy isoflavones are not the only dietary flavonoids that can inhibit thyroid peroxidase. A variety of other flavonoids have also been shown to be even more potent in inhibiting the activity of this enzyme, including kaempferol, naringenin, and quercetin (Divi and Doerge, 1996) . Such flavonoids are widely distributed in plant-derived foods and would be consumed daily at relatively high levels (possibly up to 1 gram or more per day) by vegetarians or semi-vegetarians, yet such individuals do not have a significant increased incidence of goiter. Goiter has also been reported in infants where soy has served as the sole source of food (Hydovitz, 1960). However, this situation is hardly comparable to adults consuming soy protein in moderate amounts as a means to lower total or LDL cholesterol levels.
In sum, soy products have been consumed as a dietary staple in Asian countries for hundreds of years with no significant occurrence of goiter in that population. Goiter is primarily due to a deficiency of dietary iodine, not the consumption of moderate amounts of soy protein incorporated into a nutritionally sound diet. That goiter would result in adults consuming 25 grams of soy protein per day in response to an approved health claim for coronary heart disease is ludicrous."
In essence, it you don't consume 40% of your body weight in soy protein daily, you've not got much to worry about.
Cody V, Koehrle J and Hesch RD. Structure-activity relationships of flavonoids as inhibitors of iodothyronine deiodinase. In: Environmental Goitrogenesis, Gaitan, E. (ed), pp. 57-69, CRC Press, Boca Raton, FL, 1989.
Divi RL, Chang HC and Doerge DR. Anti-thyroid isoflavones from soybean. Biochem. Pharmacol 54:1087-1096, 1997.
Divi RL and Doerge DR. Inhibition of thyroid peroxidase by dietary flavonoids. Chem. Res. Toxicol. 9:16-23, 1996.
Filisetti TM and Lajolo FM. Effect of the ingestion of soybean fractions, raw or autoclaved, on the rat thyroid. Arch. Latinoam. Nutr. 31:287-302, 1981.
Hydovitz JD. Occurrence of goiter in an infant on a soy diet. N Engl. J. Med. 262:351-353, 1960.
Kimura S, Suwa J, Ito B and Sato H. Development of malignant goiter by defatted soybean with iodine-free diet in rats. Gann 67:763-765, 1976.