Iodine's Role in Thyroid Health

Iodine is an element that's required for the thyroid gland to produce thyroid hormones. Since the body does not produce iodine on its own, it needs to come from dietary sources—and striking the right balance is key. Inadequate levels or overconsumption of iodine can lead to or worsen thyroid disease, as well as cause other significant health concerns.

The Importance of Iodine

When you consume iodine, it is quickly absorbed and entered into your bloodstream. Your thyroid, which has tiny cells that capture the circulating iodine, takes in and oxidizes it so it can begin to be used to create triiodothyronine (T3) and thyroxine (T4)—thyroid hormones that make their way throughout the body to regulate metabolism and ensure healthy functioning of the heart, brain, and other organs. While the major portion of iodine is concentrated in the thyroid gland, the nonhormonal iodine is found in a variety of body tissues including the mammary glands, the eyes, the gastric mucosa, the cervix, and the salivary glands. 

Levels of T3 and T4—as well as thyroid stimulating hormone (TSH), which is influenced by the two—that are out of normal ranges can lead to issues such as hypothyroidism and hyperthyroidism, and the complications related to having an underactive or overactive thyroid.

This can occur for a number of reasons, including taking in too little or too much iodine.

How Much Iodine You Need

According to the Food and Nutrition Board of the National Academies (formerly the National Academy of Science), the recommended dietary allowance (RDA) for iodine in the United States ranges anywhere from 90 mcg per day for toddlers to 150 mcg for teens and adults.

Considering that one cup of plain low-fat yogurt contains about 75 mcg, 3 ounces of fish sticks contain about 54 mcg, a cup of cooked pasta contains about 27 mcg, and a quarter teaspoon of iodized salt contains about 71 mcg, that's generally an easy amount for most people to consume.

Pregnant and Breastfeeding Women

The American Thyroid Association recommends that all pregnant and breastfeeding women in the United States and Canada take a prenatal vitamin containing 150 mcg of iodine a day as part of an overall recommended intake of 220 mcg/day and 290 mg/day, respectively. Excess iodine, however, can be particularly dangerous in these women.

Iodine Sources

Most Americans have no trouble meeting the recommended intake of iodine because of the iodization of salt in the United States and incorporation of iodine-rich foods such as the following:

• Cod (3 ounces): 99 mcg
• Plain low-fat yogurt (1 cup): 75 mcg
• Reduced fat milk (1 cup): 56 mcg
• White enriched bread (2 slices): 45 mcg
• Shrimp (3 ounces): 35 mcg
• Enriched macaroni (1 cup): 27 mcg
• Egg (1 large): 24 mcg
• Canned tuna in oil (3 ounces): 17 mcg
• Dried prunes (5 prunes): 13 mcg
• Cheddar cheese (1 ounce): 12 mcg
• Raisin bran cereal, (1 cup): 11 mcg
• Apple juice (1 cup): 7 mcg
• Frozen green peas (1/2 cup): 3 mcg
• Banana (1 medium): 3 mcg

Supplements (e.g. potassium iodide, sodium iodide, kelp) and iodine-containing herbs, such as bladderwrack, are other sources that can be considered.

Iodine Deficiency

Since iodine is needed to make thyroid hormone, diminished levels can lead to hypothyroidism (low thyroid function). Iodine deficiency is also linked to the development of goiter (thyroid enlargement).

The impact of too little iodine reaches further. Children born to mothers with severe iodine deficiency can suffer from stunted growth, severe and irreversible intellectual disabilities, and problems with movement, speech, and hearing.

Even mild iodine deficiency during pregnancy can lead to subtle intellectual deficits, although many children improve with iodine supplementation. Mild iodine deficiency can also cause miscarriage.

Fibrocystic breast disease, a benign condition characterized by lumpy, painful breasts mostly in women of reproductive age, is also associated with iodine deficiency.

Risk Factors

The World Health Organization (WHO) estimates that two billion people, including 285 million school children, are iodine deficient. Among them, iodine deficiency disorders (IDD) affect some 740 million.

In the United States, however, the risk of iodine deficiency is relatively low; incidence of IDD has dropped significantly since the iodization of salt first began in the 1920s. To further stem the risks of thyroid disease worldwide, the American Thyroid Association (ATA) called for the universal iodization of salt in 2017.

That said, there are certain risk factors for iodine deficiency that everyone should be aware of no matter where they live:

• Pregnancy
• A low- or no-salt diet
• An iodine-poor diet high in goitrogenic foods such as soy, cassava, and cruciferous vegetables (e.g. cabbage, broccoli, and cauliflower), which may significantly impact your body's ability to utilize any iodine it does get

Preventive Supplementation When You Have Thyroid Disease

Some healthcare providers are almost knee-jerk in their insistence that anyone with a thyroid problem requires iodine supplementation; alternative healthcare providers may recommend iodine-containing herbs, like kelp or seaweed.

This can be particularly risky, in part because iodine supplements can interact with several types of drugs, including anti-thyroid drugs used to treat hyperthyroidism. Taking high doses of iodine with anti-thyroid medications can have an additive effect and could cause hypothyroidism.

If iodine deficiency isn't the cause of hypothyroidism, then iodine supplements won't be helpful.

Excess Iodine

Given the strong link between iodine and thyroid health, it's reassuring to learn that iodine deficiency is rare in the United States and other developed countries where iodized salt is used. Indeed, as an International Journal of Molecular Sciences study reported in 2014, iodine excess is currently a more frequent occurrence in these places. This, though, is not without concern.

For some people with abnormal thyroid glands, excessive iodine can trigger or worsen hypothyroidism. While initially, you may have more energy, high doses can cause an "iodine crash" that leaves you feeling exhausted and achy within a few days.

That's because high iodine intake can initiate and exacerbate infiltration of the thyroid by lymphocytes, the white blood cells that accumulate due to chronic injury or irritation.

In addition, large amounts of iodine block the thyroid's ability to make thyroid hormones. A 2014 study in the journal Endocrinology and Metabolism found that more-than-adequate or excessive iodine levels are unsafe and may lead to hypothyroidism and autoimmune thyroiditis (Hashimoto's thyroiditis, chronic lymphocytic thyroiditis), especially for people with recurring thyroid disease.

Women who take too much supplemental iodine during pregnancy may give birth to babies with congenital hypothyroidism, a thyroid deficiency that, if left untreated, can lead to mental, growth, and heart problems, according to a 2012 study published in The Journal of Pediatrics.

Are You Getting Enough or Too Much?

While iodine can be detected in urine, relying on such a test is not helpful, since 90 percent of the iodine you ingest is quickly expelled. Rather, healthcare providers use thyroid tests to determine if your iodine intake is concerning or not.

In addition, iodine deficiency is typically suspected based on the development of goiter, hypothyroidism, or congenital hypothyroidism (low thyroid function at birth).

Be sure that any adjustments you make to your iodine intake, whether you have a circumstance that seems to call for them or not, are cleared by your healthcare provider first.