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.
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
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
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.
What does this mean?
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.
How does this effect you?
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, patients should
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, get your thyroid hormone levels checked by
It's quick, easy, and can help to optimize your management.
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