Iodine is an element that is used by the thyroid. Humans cannot produce iodine, so it must be consumed. It is added to some foods and also to salt.
Iodine reduces thyroid hormone and can kill fungus, bacteria, and other microorganisms such as amoebas. Iodine deficiency is one of the most common and preventable world health problems. Most iodine is found in the ocean, where it is concentrated by sea life, particularly in seaweed.
Iodine is taken by mouth to prevent and treat iodine deficiency and its consequences, including goiter and some thyroid disorders. A specific kind of iodine called potassium iodide is also US FDA approved to prevent thyroid damage after a radioactive accident. Iodine is also used for pink eye, gum infections, wound healing, and many other conditions, but there is limited scientific evidence to support many of these uses
Uses & Effectiveness
Likely Effective for
- Iodine deficiency. Taking iodine supplements by mouth, including iodized salt, is effective for preventing and treating iodine deficiency.
- Radiation exposure. Taking iodine by mouth can protect the thyroid after exposure in a radiation emergency. But it should not be used for general protection against radiation.
Possibly Effective for
- Pink eye. Using eye drops containing iodine in the form of povidone-iodine seems to reduce the risk of pink eye in newborns. It also seems to help treat pink eye in adults.
- Foot sores in people with diabetes. Applying iodine to foot sores might help treat foot ulcers related to diabetes.
- Swelling (inflammation) of the lining of the uterus (endometritis). Washing the vagina with a solution containing iodine in the form of povidone-iodine before a cesarean delivery reduces the risk of swelling of the uterus lining.
- A type of benign (non-cancerous) breast disease (fibrocystic breast disease). Taking molecular iodine, in a dose of about 3000-6000 mcg daily, reduces breast tenderness and pain. Lower doses of 1500 mcg daily don’t seem to help.
- Swelling (inflammation) and sores inside the mouth (oral mucositis). Rinsing the mouth with an iodine solution seems to prevent soreness and swelling inside the mouth caused by chemotherapy.
- A serious gum infection (periodontitis). Rinsing the mouth with an iodine solution during non-surgical treatments for gum infections can help reduce the depth of infected gum pockets.
- Infection after surgery. Applying iodine in the form of povidone-iodine before or during surgery reduces the risk of infections. But it’s unclear how it compares to other options, such as chlorhexidine, for preventing infections.
- A life-threatening condition caused by excess of thyroid hormone (thyroid storm). Taking iodine by mouth in combination with other treatments can help treat thyroid storm.
- Lumps in the thyroid. Taking iodine by mouth can improve lumps on the thyroid called thyroid nodules.
- Leg sores caused by weak blood circulation (venous leg ulcer). Applying cadexomer iodine to the skin might help leg ulcers heal. But it’s unclear if applying povidone-iodine helps.
Possibly Ineffective for
- Infections in people with catheters. Applying povidone-iodine where a dialysis catheter is inserted doesn’t seem to work as well as chlorhexidine for reducing the risk of a blood infection. Applying povidone-iodine before inserting a urinary catheter doesn’t reduce the risk for infection.
- Growth and development in premature infants. Feeding premature infants iodine supplements doesn’t improve their brain development nor reduce their risk of dying.
There is interest in using iodine for a number of other purposes, but there isn’t enough reliable information to say whether it might be helpful.
When taken by mouth: Iodine is likely safe for most people when taken in doses less than 1100 mcg daily. Large amounts or long-term use of iodine is possibly unsafe. Adults should avoid prolonged use of higher doses without proper medical supervision. Higher intake can increase the risk of side effects such as thyroid problems. Iodine in larger amounts can cause metallic taste, soreness of teeth and gums, burning in mouth and throat, stomach upset, and many other side effects.
When applied to the skin: Iodine is likely safe for most people when appropriately diluted products are used. A 2% iodine solution is an FDA-approved prescription product.
Special Precautions & Warnings:
Pregnancy and breast-feeding: Iodine is likely safe when taken by mouth in recommended amounts or when applied to the skin appropriately using an approved product (2% solution). Do not take more than 1100 mcg of iodine daily if you are over 18 years old; do not take more than 900 mcg of iodine daily if you are 14-18 years old. Iodine is possibly unsafe when taken by mouth in high doses. Higher intake might cause thyroid problems in the baby.
Children: Iodine is likely safe when taken by mouth in appropriate doses depending on age. Doses should not exceed 200 mcg daily for children 1 to 3 years old, 300 mcg daily for children 4 to 8 years old, 600 mcg daily for children 9 to 13 years old, and 900 mcg per day for adolescents.
A type of rash called dermatitis herpetiformis: Taking iodine can make this rash worse.
Thyroid disorders: Prolonged use or high doses of iodine might make certain thyroid disorders worse, including hypothyroidism, an enlarged thyroid gland (goiter), or a thyroid tumor. Also, people with autoimmune thyroid disease might be especially sensitive to the harmful effects of iodine.
People with thyroid conditions can’t manage their condition through diet. But, eating the wrong foods or taking the wrong supplements can cause trouble.
Among the foods to go easy on are soy, kelp and dietary supplements like iodine and selenium, says endocrinologist Christian Nasr, MD. “There is a lot of literature on what people shouldn’t do,” he says.
Should people with thyroid problems avoid these foods?
Soy: If you have hypothyroidism, yes. Eating too much soy causes problems only for those with hypothyroidism, which occurs when your thyroid gland does not make enough thyroid hormones, Dr. Nasr says.
The main problem is that soy may hinder absorption of the hormones that such patients take.
“Some studies show that if you eat a lot of soy, or drink a big glass of soy milk, within one hour of taking a thyroid hormone, it might affect absorption,” Dr. Nasr says. “Many individuals depend on a consistent absorption of those hormones to achieve a steady state.”
Generally, experts recommend that people who have a borderline thyroid — one that’s a little underactive but you’re still trying to preserve thyroid function — do not to consume large amounts of soy every day, he adds.
Turnips and root vegetables: No. These vegetables are sometimes thought to cause thyroid problems, but that’s not the case, Dr. Nasr says. They are good for your diet, regardless of any thyroid issues.
One root vegetable that is the exception is cassava, a common staple in certain parts of Africa. This plant “is known to produce toxins that can slow an already underactive thyroid, especially in the presence of an iodine deficiency,” Dr. Nasr says. “But that’s not relevant here in the United States, unless you cook cassava and you eat it every day.”
Kelp: No, but don’t take it in supplement form. People with thyroid issues should not have more than an average daily recommended intake of 158 to 175 micrograms of kelp per day, Dr. Nasr says. The concentration of kelp in foods is generally not enough to cause a problem, but a kelp capsule can contain as much as 500 micrograms, he says. “Those recommendations to go easy on kelp are for people who don’t understand and take three capsules per day. If you eat an average amount of kelp once a day, that’s not a problem.” Pregnant women especially should avoid ingesting large amounts of kelp, as it may put them at risk for developing fetal goiter, he adds.
Cabbage and cruciferous veggies: Yes. Even though they are good for us, cabbage and other cruciferous vegetables eaten raw in large quantities, especially in the context of iodine deficiency or borderline iodine levels, can result in hypothyroidism. These vegetables generate a substance that competes with the uptake of iodine by the thyroid.
Should people with thyroid problems avoid these supplements?
Iodine: Yes. Avoid it as a supplement whether you have hyperthyroidism or hypothyroidism. The effect of iodine supplements can vary by person, causing the thyroid to produce either too much or too little hormone.
Certain alternative medicine websites or doctors tell patients that iodine is good for your thyroid, Dr. Nasr says, but “if there is anybody who shouldn’t take iodine, it is thyroid patients.” Such claims are made because iodine deficiency is the No. 1 cause of thyroid conditions in the world, he says. But that’s not true in the U.S., where we have iodine in our diets. Iodine is added to many foods, and not just salt, he says.
However, Dr. Nasr says that people on restrictive diets may consider adding a daily multivitamin that contains the recommended daily allowance of iodine.
He also assures patients not to worry that you are getting too much iodine from everyday foods. “You would have to eat a ton of it to cause problems,” he says. “It’s not, ‘don’t eat anything with iodine.’ It’s, ‘don’t eat a bunch of iodine.’ And patients should be careful with iodine-concentrated supplements.”
Selenium: No, but don’t take more than 200 micrograms per day. Selenium, which is needed to support efficient thyroid function, is not something you would typically find at the grocery store, but an alternative medicine doctor might prescribe it, Dr. Nasr says. You can also get it in foods like fish, Brazil nuts, meat and poultry. A selenium supplement is OK to use “as long as you’re not overdoing it,” he says.