Can too much iodine harm the thyroid?

How too much iodine affects the thyroid

In some people, excess iodine leads to an underactive thyroid and a goiter:^[1]

1. Excess iodine blocks the making of thyroid hormone.^[1]
2. The pituitary gland releases more TSH.^[1]
3. High TSH makes the thyroid grow, and a goiter can form.^[1]

Excess iodine can also cause hyperthyroidism, an overactive thyroid, even when iodine is given to correct a deficiency.^[1] This happens most often in regions that were iodine deficient in the past. There, more people have thyroid nodules that make hormone on their own.^[3] Studies have also linked high intakes to thyroiditis (inflammation of the thyroid) and to papillary thyroid cancer.^[1]

What the studies found

In China, 3,018 people in three regions were followed for 5 years. Median urine iodine was 84 µg/L in the first region, 243 µg/L in the second and 651 µg/L in the third. Mild (subclinical) hypothyroidism developed in 0.2%, 2.6% and 2.9% of people, and autoimmune thyroiditis in 0.2%, 1.0% and 1.3%.^[4]

Doctors reported thyroid disease in adults who drank a soy milk made with kombu seaweed. They also reported hypothyroidism in newborns whose mothers had eaten seaweed soup.^[5]

In a 14-day study, 33 men took iodide or molecular iodine at 0.3 or 1 mg per kg of body weight. The higher dose of both forms raised TSH.^[6]

Where too much iodine comes from

Sources of excess iodine include iodized salt, seaweed, iodine supplements, drinking water, iodine contrast media used in X-ray scans, and amiodarone, a heart rhythm medicine.^[3] Seaweed sold whole or in sheets contains 16 to 2,984 µg of iodine per gram.^[1] Many supplements with only iodine contain high doses, sometimes above the upper limit.^[1]

Upper limits

US upper limits, from food and supplements:^[1]

• 1 to 3 years: 200 µg a day^[1]
• 4 to 8 years: 300 µg a day^[1]
• 9 to 13 years: 600 µg a day^[1]
• 14 to 18 years: 900 µg a day, also in pregnancy and breastfeeding^[1]
• adults: 1,100 µg a day, also in pregnancy and breastfeeding^[1]

Infants under 1 year should get iodine only from breast milk, formula and food.^[1] The EU upper limits are 200 µg a day at 1 to 3 years, rising to 500 µg at 15 to 17 years, and 600 µg for adults.^[2] The limits do not apply to people who take iodine as a treatment, and these people should be under a doctor's care.^[1]

Acute poisoning

Acute iodine poisoning is rare and usually needs doses of many grams. Its signs are burning of the mouth, throat and stomach, fever, stomach pain, nausea, vomiting, diarrhea, a weak pulse and coma.^[1]

Who should be careful

People with autoimmune thyroid disease or iodine deficiency can have side effects at amounts that are safe for others.^[1] In a Spanish study, pregnant women who took 200 µg a day or more had a higher risk of a raised TSH.^[1] Large amounts of seaweed while breastfeeding can cause short-term hypothyroidism in the infant.^[7] Ask a doctor or pharmacist before you take more than the upper limit.^[1]