Whether people who have hypothyroidism should avoid soy consumption is a matter of debate.
Hypothyroidism is usually treated with synthetic thyroid hormone, and it has long been believed that soy interferes with the body's ability to absorb the medication. However, there is no evidence that people who have hypothyroidism should completely avoid soy consumption.
In total, 14 trials (thyroid function was not the primary health outcome in any trial) were identified in which the effects of soy foods or isoflavones on at least one measure of thyroid function in presumed healthy subjects were assessed. ; eight involved women only, four involved men, and two men and women. With only one exception, no or only very modest effects were seen in these trials. Taken together, therefore, the findings provide little evidence that in euthyroids, iodine-replete individuals, soyfoods, or isoflavones negatively affect thyroid function. Conversely, some evidence suggests that soyfoods, by inhibiting absorption, may increase the dose of thyroid hormone required by hypothyroid patients. However, adults with hypothyroidism do not need to avoid soy-based foods. Additionally, there is theoretical concern based on in vitro and animal data that in individuals with compromised thyroid function and/or whose iodine intake is marginal, soyfoods may increase the risk of developing clinical hypothyroidism. Therefore, it is important for consumers of soy-based foods to ensure that their iodine intake is adequate.
If you have hypothyroidism, take thyroid hormone replacement medication as directed by your doctor. The medicine can be taken at any time that is most comfortable for you, on an empty stomach or with food, as long as you do the same thing every day.
In general, it's best to wait four hours after taking thyroid medication before consuming products that contain soy. The guidelines apply to other products that can affect the body's ability to absorb thyroid medication, including iron and calcium supplements, and antacids that contain calcium or aluminum hydroxide.