Iodine and the Thyroid: The Connection You Should Know About

By J . Woody Sistrunk, MD, FACE, ECNU

To many, iodine (EYE-eh-dine) is simply element #53 on the periodic table you learned about in high school chemistry class or an antiseptic used to disinfect a nasty case of road rash after a bicycling mishap. In fact, iodine is the most essential ingredient needed to make T3 (triiodothyronine) and T4 (thyroxine), two thyroid hormones that affect virtually every cell in the body and are essential in regulating metabolism. Your thyroid concentrates the iodine and uses it to make thyroid hormone much like the Ford Motor Company uses steel to make cars. Without iodine, the production of thyroid hormone simply does not happen, thus throwing the body’s function into a tailspin.

Iodine was identified in the year 1811, when French chemist Bernard Courtois discovered it accidentally while mixing burnt seaweed (kelp) with sulfuric acid to create potassium nitrate, a vital component used in gunpowder. Soon thereafter, Swiss physician and researcher Jean Francois Coindet found that iodine could reduce goiters (enlargement of the thyroid gland) and began treatment of the condition with iodine. And the rest is history? Well, not quite.

Iodine Deficiency ...how common is it?

Iodine is present naturally in seawater and in soil. But a stable source of the element does not exist in many parts of the world. And since the body cannot make the substance, iodine must come from an outside source. Consequently, control of iodine deficiency disorders is an integral part of most national nutrition strategies. In the United States, salt producers cooperated with public health authorities in the 1920s and added iodine to cooking salt to correct the then-present epidemic of goiter (salt was used as the carrier because it was an easy, spoil-free method of getting iodine into the food chain). So, if you live in the United States, the chance of having simple goiter from iodine deficiency is rare. Most thyroid enlargement in the United States these days is a result of thyroid nodules (multinodular goiter) or other autoimmune thyroid disease such as Graves’ disease or Hashimoto’s thyroiditis.

Still, the population in many parts of the world where there is little iodine in the diet (remote mountainous areas, semi-arid equatorial climates and certain parts of Europe) continue to be at risk for iodine deficiency, which affects about two billion people worldwide.

Pregnant women are particularly at risk for the condition. This is a result of increased thyroid hormone production during pregnancy , which a fetus needs in vitro (inside the womb) to reach optimal development. Thus, the body requires more iodine than what would typically be considered a baseline amount of iodine. And because expectant mothers typically experience increased excretion of iodine through the kidneys – a normal part of any pregnancy – the need for iodine intake is enhanced further. Iodine deficiency during pregnancy may result in cretinism (mental retardation in children), deafness, autism, and delayed brain development and is the leading cause of preventable mental retardation.

As the body’s iodine levels fall, hypothyroidism may develop, since iodine is essential for making thyroid hormone. While this is uncommon in the United States, iodine deficiency is the most common cause of hypothyroidism worldwide.

Daily Dose Guidelines

Although most table salt available today is fortified with iodine as part of the manufacturing and refining process, because we are an on-the-go society that often partakes in processed and so-called “fast” foods – almost none of which contain iodized salt – concern about continued iodine deficiency has been raised by groups such as The Salt Institute and the International Council for the Control of Iodine Deficiency Disorders (ICCIDD).

The current Institute of Medicine Guideline for iodine for men and women is 150 micrograms (mcg) per day. Most multi-vitamins contain this amount of iodine. For pregnant women, 220 micrograms/day is recommended. For women who are breast feeding, 290 micrograms of iodine is recommended. Surprisingly, many prenatal vitamins contain no iodine. So, if you are considering pregnancy or are pregnant, check your prenatal multivitamin’s contents to make sure you are getting the iodine you and your developing fetus will need for a healthy pregnancy and baby.

Your Health and Iodine Excess

Although iodine is essential to life, taking too much iodine can cause problems. This is especially true in those individuals who already have thyroid conditions.

Exceeding the recommended daily dose of iodine can cause symptoms such as pain in your mouth or throat or a metallic taste in your mouth, nausea and vomiting, diarrhea and difficulty urinating. More severe symptoms of iodine poisoning can include life-threatening conditions such as seizures, delirium, breathing difficulties, fever, shock and severe confusion. But the amount of iodine for this to occur is quite high -- a consistent intake of over 2,000 micrograms of iodine daily.

When iodine-rich seafood or seaweed is a large part of the diet, the thyroid might become overactive because excess iodine can promote extra thyroid hormone production. The thyroid counteracts this action through a very specific mechanism present in the body known as the Wolff Chaikoff effect. Once a threshold of iodine is reached in the body, a decrease in the output of thyroid hormone occurs. This effect lasts just over one week, after which the body will then resume the production of thyroid hormone.

Dietary iodine is rarely a source of iodine excess . Most iodine excess is either from the abnormal heart rhythm prescription medication amiodarone or from the use of iodinated contrast dye, a form of intravenous, iodinecontaining radiographic dye used for x-ray studies. When exposure to these substances is ongoing, the body continues the production of thyroid hormone.This is called “escape from the Wolff-Chaikoff effect.”

There are special circumstances in which avoiding excessive iodine intake is essential.

When therapy with radioactive iodine is planned either for hyperthyroidism or thyroid cancer, avoidance of regular iodine is of critical importance as the body does not discriminate in the forms of iodine that it takes in. By not following a low iodine diet prior to radioactive iodine therapy or testing for thyroid cancer, the radioactive iodine may not reach the cells to be destroyed.

Also, if someone has become hypothyroid either by disease, surgery, or other forms of therapy and is on thyroid hormone replacement, additional iodine will not help and may interfere with the way any remaining thyroid tissue works.

Radioactive Iodine

Since the 1940s, the radioactive isotope Iodine-131 has revolutionized the evaluation and treatment of thyroid disorders. The once-fatal illness of Graves’ disease (hyperthyroidism) can almost always be treated with a single capsule of radioactive iodine. The thyroid takes up the radioactive iodine, just as it would normal iodine, and the radioactivity in the iodine destroys most or all of the tissue in your thyroid gland, but does no harm any other parts of your body. Radioactive iodine is also useful in destroying both normal and malignant tissue in thyroid cancer following thyroid surgery. Although these therapies are approaching 70 years since their widespread use in clinical medicine, they remain the standard of therapy for both hyperthyroidism and thyroid cancer.

If you are allergic to iodine and are scheduled for thyroid testing that uses radioactive iodine, make certain you clarify with your physician the nature of your allergy. For example, a reaction to shellfish does not equate to an allergy to iodine, nor does a prior reaction to a large dose of intravenous contrast exclude the use of radioactive iodine for diagnostic or therapeutic purposes. A usual dose of radioactive iodine contains roughly the same amount of iodine as a piece of bread. With this, an allergic reaction to radioactive iodine is very rare.

Conclusion

Due to its important role in thyroid function, as well as fetal and infant development, iodine is critical for proper health at all life stages. Thus, adequate iodine intake remains of paramount importance in prevention of thyroid disease. The history of understanding the importance of iodine is the history of understanding many thyroid diseases. To better appreciate how this simple element has been entwined in the management and treatment of thyroid disease throughout time, you can view the complete American Thyroid Association (ATA) Thyroid Timeline at: http://thyroid.org/ events/thyroid-cancer-tumor-history/. For additional information on thyroid diseases, their diagnosis and treatment, visit the American Association of Clinical Endocrinologists’ (AACE) thyroid website at: http://thyroidawareness.com.

J. Woody Sistrunk, MD is an endocrinologist in Jackson, Mississippi. His practice, Jackson Thyroid & Endocrine Clinic, is a full-service thyroid clinic offering an on-site CLIA-certified lab, thyroid ultrasound/FNA biopsy, thyroid cytology and thyroid nuclear medicine. He currently serves as the chair of the American College of Endocrinology’s Endocrine Certification in Neck Ultrasound (ECNU) Certification Committee. He is also a charter member and current president of the Academy of Clinical Thyroidologists. He has a strong interest in the history of thyroidology and is the Chair of the American Thyroid Association’s History and Archives Committee.