Iodine is a trace mineral your body needs primarily to make thyroid hormones, which regulate metabolism, growth, and brain development. Most adults need 150 micrograms (mcg) per day, roughly the amount in half a teaspoon of iodized salt. Beyond its role in the thyroid, iodine is used as a skin antiseptic in medical settings and as an emergency medication to protect the thyroid from radiation exposure.
How Your Thyroid Uses Iodine
Nearly all the iodine in your body ends up in your thyroid gland, a butterfly-shaped organ at the base of your neck. The thyroid absorbs iodine from your bloodstream and combines it with a large protein called thyroglobulin to build two hormones: T4 (thyroxine) and T3 (triiodothyronine). T4 contains four iodine atoms, T3 contains three. Your thyroid stores these hormones and releases them into your blood as needed.
These hormones act on virtually every cell in your body. They set the pace of your metabolism, influencing how quickly you burn calories and how warm you feel. They help maintain your heart rate, support bone turnover, and keep your digestive system moving. In children, thyroid hormones are essential for normal growth and brain maturation. Without enough iodine, the thyroid simply cannot produce adequate amounts of T3 and T4, and the downstream effects touch nearly every organ system.
Why Iodine Matters During Pregnancy
Iodine needs increase substantially during pregnancy and breastfeeding. The recommended intake jumps to 220 mcg per day for pregnant women and 290 mcg per day while breastfeeding. The World Health Organization recommends even more for pregnancy: 250 mcg daily.
This increase exists because a developing fetus depends entirely on maternal thyroid hormones during early pregnancy, before its own thyroid gland becomes functional. Severe iodine deficiency during pregnancy can cause irreversible brain injury in the child, a condition historically called cretinism, which involves profound intellectual disability, hearing loss, and motor problems. Even mild to moderate deficiency during pregnancy has been linked to lower IQ scores, delayed language development, poorer executive function, and reduced educational achievement in children. These effects can persist into adulthood, making iodine one of the most consequential nutritional factors in fetal brain development.
What Happens When You Don’t Get Enough
When iodine intake falls short, the thyroid gland works harder to capture whatever iodine is available. Your pituitary gland responds by releasing more thyroid-stimulating hormone (TSH), which pushes the thyroid to grow. Over time, this leads to goiter, a visible or palpable enlargement of the thyroid. Most people with goiter notice a gradual swelling at the front of the neck but otherwise feel fine, because the enlarged gland often still produces enough hormone. In more severe cases, large goiters can press on the windpipe or esophagus, causing a choking sensation, difficulty swallowing, coughing, or hoarseness.
If iodine deficiency becomes severe enough, the thyroid can no longer compensate, and hypothyroidism develops. Symptoms include fatigue, weight gain, cold intolerance, dry skin, constipation, and mental sluggishness. Globally, the number of iodine-deficient countries has dropped from 113 in 1990 to 21 in 2020, largely thanks to salt iodization programs. Nearly 90% of the world’s population now uses iodized salt. Still, communities in parts of Asia and Africa served by small-scale salt producers remain at higher risk.
Best Food Sources of Iodine
Iodine content varies enormously across foods. Seaweed is by far the richest source: dried nori contains roughly 2,320 mcg per 100 grams, which means even a small sheet can deliver several times the daily requirement. This also means it’s easy to overshoot with seaweed-heavy diets.
More moderate sources include:
- Cod: about 131 mcg per 100 grams (raw), making a single serving nearly a full day’s worth
- Greek yogurt: roughly 51 mcg per 100 grams
- Eggs: about 49 to 51 mcg per 100 grams, so two large eggs provide around half the daily target
- Milk: approximately 34 mcg per 100 grams, meaning one cup covers about half the RDA
- Cheese: processed American cheese contains about 59 mcg per 100 grams
- Salmon: around 26 mcg per 100 grams
- Canned tuna: only about 8.5 mcg per 100 grams, much lower than many people assume
Iodized table salt remains one of the most reliable sources, containing roughly 5,070 mcg per 100 grams. In practical terms, a quarter teaspoon provides about 75 mcg. Keep in mind that most sea salt, kosher salt, and specialty salts are not iodized unless the label specifically says so. If you rely heavily on these alternatives, your iodine intake may be lower than you think.
Foods That Interfere With Iodine Use
Certain plant foods contain compounds called goitrogens that can interfere with how your thyroid uses iodine. Cruciferous vegetables like broccoli, cabbage, cauliflower, and kale contain substances that get converted to thiocyanates in the body. These block iodine transport into the thyroid and reduce its incorporation into thyroid hormones. Cassava and sweet potatoes contain a different class of goitrogens with similar effects.
For most people eating a varied diet with adequate iodine, these foods pose no real problem. The concern arises when someone already has borderline iodine intake and eats large quantities of cruciferous vegetables or cassava regularly. In those situations, the combination of low iodine and high goitrogen intake can amplify the strain on the thyroid.
Iodine as an Antiseptic
Outside of nutrition, iodine has a long history as a germ-killing agent. Povidone-iodine solutions (the familiar brown liquid applied before surgery or on minor wounds) work by releasing free iodine that destroys bacteria, viruses, and fungi on contact. It remains widely used for preoperative skin preparation, wound cleaning, and as a gargle or nasal rinse in some clinical settings.
That said, a landmark trial published in the New England Journal of Medicine found that chlorhexidine-alcohol solutions reduced surgical-site infections by 41% compared to povidone-iodine, with an overall infection rate of 9.5% versus 16.1%. As a result, many hospitals have shifted toward chlorhexidine-based preps for surgery, though povidone-iodine is still common for wound care and in settings where chlorhexidine isn’t suitable.
Radiation Protection in Emergencies
During a nuclear accident, radioactive iodine can be released into the environment. If inhaled or swallowed, it concentrates in the thyroid gland, significantly increasing the risk of thyroid cancer. Potassium iodide (KI) tablets work by flooding the thyroid with stable, non-radioactive iodine so it has no room to absorb the dangerous form.
Timing is critical. KI is most effective when taken within 24 hours before or 4 hours after exposure to radioactive iodine. A single dose protects for 24 hours. The adult dose is 130 mg for people ages 18 through 40. Adults over 40 are generally advised to take KI only when predicted exposure levels are very high, because the risk of thyroid cancer from radiation decreases with age while the risk of side effects from high-dose iodine increases.
How Much Is Too Much
The tolerable upper intake level for iodine is 1,100 mcg per day for adults. Regularly exceeding this amount can paradoxically cause the same problems that deficiency does: both hypothyroidism and hyperthyroidism. The American Thyroid Association recommends against taking iodine or kelp supplements containing more than 500 mcg daily.
People with pre-existing thyroid conditions are especially vulnerable to excess iodine. This includes those with Hashimoto’s thyroiditis, Graves’ disease, thyroid nodules, or a history of thyroid surgery. Infants, the elderly, and pregnant or lactating women are also more susceptible to adverse effects. During pregnancy, expert recommendations for the upper limit range from 500 to 1,100 mcg daily, depending on the source. There is no established thyroid benefit to taking more iodine than the standard recommended amount.