Do you need an iodine supplement in pregnancy?

Why the need rises

In early pregnancy, the thyroid of the fetus is not yet complete. The fetus depends on the mother's T4 hormone, and so on the mother's iodine.^[1] The mother makes about 50% more T4 in pregnancy.^[1]

The US recommended intake is 220 µg a day in pregnancy and 290 µg when breastfeeding. WHO advises 250 µg for both.^[1] EFSA, the EU food safety authority, sets 200 µg for both.^[4]

US surveys from 2003 to 2014 found that a large share of pregnant women had too little iodine. WHO sets enough iodine in pregnancy at a median urine iodine of 150 to 249 µg/L.^[1] Pregnant women who ate no dairy had lower levels.^[1]

What the studies found

A pooled analysis looked at 6,180 mothers and children from the Netherlands, Spain and the UK. Children of mothers with low iodine in the first trimester had a lower verbal IQ at 1.5 to 8 years.^[1] The data came from three birth cohorts, not from trials of supplements.^[1]

Trials of supplements found these results:

• A 2020 review of women with mild to moderate deficiency included 10 trials. Most found that iodine lowered thyroglobulin, a blood marker of iodine intake, and 3 found that it limited the growth of the mother's thyroid.^[2][5]
• In the same review, 2 trials that tested the children found no effect on thinking, language or movement scores.^[2]
• A 2021 review of 14 trials pooled 5 of them and found no difference in weight, length or head size at birth, or in development in the first 2 years.^[3]
• A 2017 Cochrane review of 11 trials with more than 2,700 women found too few data for firm conclusions. Iodine lowered the risk of hyperthyroidism after the birth by 68% (3 trials). In one small trial, more women had digestive problems with iodine. The quality of the evidence was low or very low.^[6]

In an Italian trial, 90 women took 225 µg a day of potassium iodide or a placebo, from before week 12 until 8 weeks after the birth. The thyroid grew by 7.43% with iodine and by 11.17% with placebo. Iodine did not harm thyroid function.^[5]

Breastfeeding

The iodine in breast milk depends on the mother's iodine.^[7] Vegan and vegetarian mothers appear to have less iodine in their milk.^[7] A US survey of 2011 to 2014 found that only 19% of breastfeeding women used a supplement with iodine.^[7]

In a trial in Morocco, where iodine intake was low, mothers got one dose of 400 mg of iodine as iodized oil after the birth. Their infants got enough iodine through breast milk for at least 6 months.^[8]

What the guidelines say

The American Thyroid Association advises 150 µg of iodine a day as potassium iodide for women who are planning a pregnancy, pregnant or breastfeeding. The American Academy of Pediatrics advises at least 150 µg a day and the use of iodized salt.^[1] Of 59 popular US prenatal vitamins checked in 2016 and 2017, only 34 contained iodine.^[1]

Who should be careful

In a Spanish study, pregnant women who took 200 µg a day or more had a higher risk of a raised TSH than women who took less than 100 µg.^[1] Large amounts of seaweed while breastfeeding can cause short-term hypothyroidism in the infant.^[7] Women with a thyroid disease should ask their doctor before they take iodine.^[1]