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Thyroid & Iodine….What You Should Know

Why does the body need iodine?

The major function of the thyroid gland is to produce thyroid hormone in an amount sufficient to meet the body’s needs. To make thyroid hormone, the thyroid uses iodine. If iodine is not available in the diet, the thyroid may produce an insufficient amount of hormone.

How much iodine does the body need?

Areas in the United States where iodine deficiency occurs are scarce. In North America, iodine is added to salt and bread. It is also present in additives, water sources, medications, and dietary supplements. The daily iodine intake varies widely throughout the world. A minimum of 60 micrograms of elemental iodine per day is required to make thyroid hormone. The following lists examples of average iodine intake in various countries and the recommended amount of iodine consumption:

Typical Iodine Intakes

North America 200-700 micrograms/day
Germany 20-150 micrograms/day
Chile 50-150 micrograms/day
Switzerland 130-180 micrograms/day
Recommended Daily Intake
Adults 150 micrograms/day
Children 90-120 micrograms/day
Pregnant Women 200 micrograms/day

In North America, the higher values are mainly due to an increased intake in salt. In Japan, where foods rich in iodine are consumed regularly, the intake may be as high as over 1000 micrograms/day. Although iodine consumption is generally lower in Europe, the people in these countries do not usually develop thyroid disease. However, when they are exposed to unaccustomed, large amounts of iodine (such as moving to North America and increasing their iodine intake), they can develop thyroid disease. This occurs particularly in people who have an underlying predisposition to developing thyroid disease.

How is iodine used by the thyroid?

The process by which the thyroid uses iodine is actually quite complicated and certain steps are still unclear. Essentially, iodine is converted to its free elemental form, called iodide. Iodide enters the thyroid gland through a special transport mechanism. Iodide then undergoes a process called oxidation and is incorporated into intermediate hormones called MIT (Monoiodotyrosine, which contains 1 iodide) and DIT (Diiodotyrosine, which contains 2 iodides.) These compounds then combine to form the active hormones, tri-iodothyronine (T3) and thyroxine (T4). T3 is the most biologically active thyroid hormone. It is formed by combining a MIT with a DIT (so the total of iodides in the molecule is 3). T4 is formed in much greater quantity by combining a DIT with another DIT (so that the total of iodides in the molecule is 4). These hormones are then stored in the thyroid gland and released into the blood stream.

Based on the above summary, it is evident that thyroid hormone is actually made up of iodide/iodine directly. So you can see the importance of iodine in relation to the function of the thyroid gland. Whew! Glad the physiology stuff is over!!

I hope the above information helps to provide a basic understand of why iodine is important, how much iodine is needed, and how it is used by the thyroid gland. With this background, we can now continue on to part 2, which will address specific problems if too much or too little iodine intake occurs. Stay tuned!

Thyroid & Iodine Part 2

Disease Prevention in Women Slideshow
Medical Author: Ruchi Mathur, M.D.
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.

Iodine Deficiency

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.

Iodine Excess

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 wiseeverything in moderation!


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