Medical Author: Ruchi
Medical Editor: William C. Shiel, Jr., MD, FACP, FACR
Part 1 of this article focused on the mechanisms of iodine and iodide as related to thyroid hormone production. We outlined the amount of iodine needed for normal thyroid function and we discussed the availability and consumption of iodine throughout the world. In this section, we will discuss the effects of iodine deficiency and excess on thyroid function.
Most animals, including humans, have an ability to conserve the iodine within their bodies if there is a deficiency of iodine consumed in food. If an inadequate intake continues, however, the ability to make thyroid hormone is slowly depleted. Many cellular processes occur to keep the thyroid as efficient as possible and the thyroid gland often enlarges in an attempt to maintain function. Subsequently, a goiter may form as the thyroid is stimulated to try to make more thyroid hormone.
Basically, the changes in hormone levels (namely T4, T3, and TSH) are similar to those that occur in patients who develop low thyroid hormone blood levels (hypothyroidism) from an underlying disease, such as Hashimoto's disease.
In making thyroid hormone, the body responds to increasing doses of iodine intake by first increasing hormone production and then decreasing production by blocking the incorporation of iodine into thyroid hormone. This blockage is protective. The body simply cannot allow all of the iodine received to turn into hormone without regulation. If this happened, there could be too much hormone produced, thereby resulting in toxic levels of thyroid hormone.
While this regulation is complicated, a decrease in the utilization of iodide is called the "Wolff-Chaikoff" effect. If a patient has an underlying problem with the thyroid gland, such as Hashimoto's disease or Graves' disease, this protective mechanism may actually be detrimental. In these cases, the gland already is diseased, and, on top of that, the Wolff-Chaikoff effect takes place. In such situations, a goiter can develop or hypothyroidism can occur if large amounts of iodide are given for long periods of time. The gland can sometimes overcome this effect by "escaping" or adapting in a successful way. In these cases, the blockage of hormone formation may be partially relieved and the patient can regain some thyroid function.
In large quantities, iodine can reduce the release of thyroid hormones from the thyroid gland. If the hormones are not released, their effects won't be seen. Occasionally, doctors use this mechanism to control very active thyroid glands that produce too much thyroid hormone. This type of therapy is difficult and is not used as commonly today. An excess of iodine also decreases the blood flow and growth of the thyroid gland, which is characteristic of Graves' disease. As a result, iodine may be used to reduce the thyroid hormone level in conjunction with surgery to remove some or all of the thyroid gland tissue and assure a good outcome.
While I've just explained how too much iodine exposure can actually shut down thyroid hormone production, in other situations, an excess of iodine may actually cause an over-production of thyroid hormone and hyperthyroidism.
If a person has been exposed to relatively little iodine and then consumes a diet rich in iodine, the individual can develop an excess production of thyroid hormone (iodine-induced hyperthyroidism). In general, this occurs in people who have an underlying thyroid disorder that has not yet manifested clinically. Iodine-induced hyperthyroidism is important in areas of the world where iodine intake is high. We actually see this quite commonly in the United States among populations who have emigrated from countries such as Iran and Africa. The name for this response is the " Jodbasedow" effect, which actually occurs only in a small fraction of people at risk. Even though it is relatively uncommon, it is important since administering iodine-containing dyes for medical procedures (such as CT scans, barium procedures, etc.) can trigger this effect.
What you need to know:
It is easy to see why thyroid physiology often makes a medical student's head swim... and it has the same effect on many practicing doctors too!
It is difficult to predict how a particular individual will respond to thyroid depletion or excess. The family history, country of origin, and other factors in the individual's medical history may help determine what effect, if any, will be seen. In general, it is best to take a moderate approach to iodine consumption. There is no specific reason to advocate kelp or iodine supplementation in our society. Likewise, there is no general reason to discourage eating sushi or seaweed-containing products. If you have questions about specific supplements or food products, or if you have known thyroid disease and have questions, you should consult your doctor.
I hope this review has answered some of your questions on iodine intake and thyroid disease. A word to the wise...everything in moderation!
For Part One of this series, please see Thyroid & Iodine...What You Should Know - Part 1.