Which form of iodine is best?

The forms

• Potassium iodide and sodium iodide. These are the usual forms in supplements.^[1] The stomach and small intestine absorb iodide quickly and almost completely.^[1] Of 34 US prenatal vitamins with iodine, 25 used potassium iodide.^[1]
• Potassium iodate. WHO recommends it for iodized salt, because it is more stable in warm, damp climates.^[1] The gut changes iodate to iodide before it is absorbed.^[1]
• Molecular iodine (I2). A trial for breast pain gave 1.5, 3 or 6 mg a day of this form.^[3] In 33 men who took high doses for 14 days, molecular iodine and iodide had similar effects on thyroid hormones.^[4]
• Kelp and other seaweed. Seaweed sold whole or in sheets contains 16 to 2,984 µg of iodine per gram.^[1] Kelp can also contain heavy metals such as arsenic, cadmium and lead.^[2]
• Iodized oil. Trials in areas with iodine deficiency gave one large dose by mouth: 100 mg to infants and 400 to 490 mg to children.^[5] The trials in Indonesia and Albania used iodized oil.^[6][7]

Kelp and seaweed supplements

The amount of an ingredient in a supplement often differs from the amount on the label.^[2] Some sources advise against seaweed while breastfeeding, because of its high iodine content and possible heavy metals.^[2] Doctors reported thyroid disease in adults and newborns after high intake of seaweed products.^[8]

Multivitamins and prenatal vitamins

Many multivitamins contain 150 µg of iodine.^[1] Not every prenatal vitamin does: of 59 popular US prenatal vitamins checked in 2016 and 2017, 34 contained iodine. Their median amount was 150 µg a day, with a range of 25 to 290 µg.^[1] The Supplement Facts label shows whether a product has iodine and how much.^[1]

Iodine from salt and food

In the US, iodized salt contains 45 µg of iodine per gram, by its label.^[1] A quarter teaspoon of iodized salt gives 78 µg, a cup of nonfat milk 84 µg and 3 ounces of cod 146 µg.^[1]

Which forms trials used

• pregnancy: 150 to 225 µg a day of potassium iodide^[1][9]
• children with mild deficiency: a daily tablet of 150 µg^[10]
• children with moderate deficiency: one dose of 400 mg as iodized oil^[7]
• breast pain: 1.5 to 6 mg a day of molecular iodine^[3]

Who should be careful

Supplements with only iodine often contain high doses, sometimes above the US upper limit of 1,100 µg a day.^[1] People with autoimmune thyroid disease can have side effects at lower amounts.^[1] Some sources advise women who are breastfeeding not to use seaweed, because of its high iodine and possible heavy metals.^[2] Ask a doctor or pharmacist first if you have a thyroid disease or take an antithyroid medicine.^[1]

With food or without

No trial found in PubMed searches compared iodine taken with food and without food. Iodide from both food and supplements is absorbed almost completely.^[1]