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Your question on soy and thyroid function is a very good one, and involved a bit of research! The effects of soy on thyroid function in the medical literature are variable, and depend on which species is being studied, and which component of soy is being given.
Soy consists of a number of different compounds. The most well publicized are those compounds with estrogen like activity (hence the use of soy in the treatment of hot flushes). These are called isoflavones. In addition, soy contains protein, and other compounds as well.
Studies have shown that in animals fed soy protein, there is a lower energy utilization and lower protein utilization compared to milk protein fed animals. Circulating thyroid hormone levels (T4) were lower following soy feeding. Remember, these animals were rodents, and they were fed soy as their only source of protein. In another animal study, rats fed soy isoflavones did not demonstrate goiter formation or increased sensitivity to conditions that may lead to low thyroid hormone iodine deficiency). Finally, in another animal study, rats fed soy had lower activity of an enzyme implicated in goiter. This enzyme is known as TPO- which is responsible for making thyroid hormone.
Soy has long been implicated in diet induced goiter. Since the consumption of soy products is increasing dramatically (soy latte anyone?) researchers are trying to define the component responsible. While all the studies mentioned above were on animals, we do know that in babies who have hypothyroidism at birth and require replacement therapy, we need to increase the amount of replacement they are on if we switch their feeds to soy milk. Whether this is because of an absorption problem, or a direct interference of soy with the action of the hormone remains unclear.
In a short summary, soy does seem to have an effect on thyroid hormone status, in animal studies, and by observing children on soy milk with underlying congenital thyroid problems. We don't know the exact mechanism of the interaction with soy and the thyroid. We also have no studies that appropriately address the issue of thyroid replacement therapy and soy consumption.
A good rule to remember is that too much of anything is not healthy. At present we have no reason to say that soy should be avoided in patients on synthroid. And don't forget the health benefits associated with soy, such as reduced cholesterol levels, and improvement in menopausal symptoms. In general, I recommend patients consume no more than 60g of soy a day, and this may vary if there is a history of breast cancer. If you are concerned about your dosage of replacement therapy, or you have made a dramatic dietary change recently, I suggest that you get your thyroid hormone levels checked by your physician. It's quick, easy, and can help to optimize your management.
Medically reviewed by John A. Daller, MD; American Board of Surgery with subspecialty certification in surgical critical care
"Treatment of primary hypothyroidism in adults"