Can your thyroid be thrown off balance by getting too much or too little iodine?

Iodine is essential for the production of thyroid hormones, but both too little and too much can disrupt the thyroid. A severe deficiency causes the thyroid to enlarge (goitre), and prolonged severe deficiency can cause permanent neurological damage or cretinism in children. Iodine supplementation largely prevents this. During pregnancy the need is especially high: the World Health Organization recommends a minimum of 200 micrograms per day, because iodine deficiency during that period can permanently impair a child's brain development.

Too much iodine at once causes the thyroid to automatically apply the brakes: hormone production drops temporarily. In healthy people this lasts at most a few days, after which the gland recovers on its own. But in vulnerable groups, such as newborns, people with Hashimoto's disease, or people who have previously undergone thyroid treatment, this suppressed state can persist and eventually cause an underactive thyroid. This sometimes resolves on its own once iodine exposure stops, but in some people it can become permanent.

In people with multiple nodules in the thyroid (multinodular goitre), a surge in iodine can trigger the opposite effect: uncontrolled overactivity in which the thyroid produces too much hormone. This is known as the Jod-Basedow phenomenon. A concrete and common example is amiodarone, a heart rhythm medication that contains very large amounts of iodine. Approximately 15 to 20 percent of users develop a thyroid problem as a result, in the form of too little or too much hormone.

Whether broccoli, cabbage and Brussels sprouts harm the thyroid is a frequently asked question. A large systematic analysis of 123 studies gives a clear answer: as long as you get enough iodine, these vegetables have no clinically relevant adverse effect on the thyroid. With chronically excessive iodine intake there are observational indications of an increased risk of autoimmune thyroid diseases and certain forms of thyroid cancer, but causality has not yet been established.