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Nutrition Hot Topics - Soy and Thyroid Function
It has been
suggested that too much soy can cause thyroid problems, and that soy is contraindicated for individuals on thyroid medications.
Soy intake has not been observed to adversely effect normal thyroid function in human studies. While it is unlikely that the
level of soy consumed by most adults would affect dosage levels of thyroid medications, potential interaction exists. One
strategy is to take soy protein and medication at separate times. It is a good idea to also let one's physician know that
supplemental soy is being taken so that thyroid levels can be routinely monitored. Effects of soybeans and foods vs. soy protein
isolatesA number of foods, including raw soy beans, members of the cabbage family, and others, contain pro-goitrogens. These
substances stimulate the thyroid gland to enlarge, causing what is known as goiter. While unprocessed soy contains these goitrogenic
factors, heat applied during the processing of soy protein isolate results in their removal. Soy infant formula and thyroid
dysfunctionWhile goitrogens can enhance the effects of iodine inadequacy, iodine deficiency is the primary cause of goiter.
In the 1960's, there were some reports of goiter inducing-effects observed in soy-formula fed infants. This was related to
the inadequate content of iodine in these formulas, not the soy itself. Today, iodine is routinely added to soy formulas.
Apart from the iodine content, the use of soy formula feeding in infancy has been associated in retrospective analyses with
autoimmune thyroid disease, and there are ongoing theoretical questions and concerns about soy formula. There are several
reports of medication dosage problems in soy formula fed infants with congenital or hereditary hypo-thyroidism who were treated
with thyroid replacement medication (e.g. synthroid). Soy apparently bound the thyroxine drug in the gastro-intestinal tract
and decreased its absorption, requiring an increase in the drug dose. It does not appear that these effects apply to adults
for a number of reasons: 1) Infants respond differently to medications than do adults. 2) The sole diet of these infants was
soy, while adults eat a varied diet. 3) There is not a high incidence of thyroid abnormalities in Asian countries where soy
intake is common. Effects of isoflavones and protein components of soyIn vitro (test tube) studies suggest that soy isoflavones
may inhibit the action of enzymes involved in thyroid metabolism, namely thyroid peroxidase. It has been speculated that these
actions might lead to thyroid problems. To date, however, no consistent changes in thyroid function as measured in several
studies have been observed, and no metabolic dysfunction has been reported from human studies. Soy protein contains a high
ratio of arginine to lysine, two naturally occurring amino acid components of all protein. This ratio is thought to stimulate
synthesis of the thyroid hormone, thyroxine. In animals (gerbils, hamsters and rats), increase in serum thyroxine levels has
been noted when high levels of soy have been fed. In human studies, however, no clear relationship between thyroid hormone
concentrations and soy protein ingestion has been noted. Most importantly, in human studies, serum thyroxine levels have never
been observed to rise outside the normal range. Soy researchers have stated that soy does not appear to impact thyroxine levels
in humans.
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