By Rachel Neifeld, RD, CDN

With a healthy diet and the help of a daily prenatal vitamin, most pregnant and breastfeeding women are sure to get enough of all the essential vitamins and minerals that they and their baby’s need, but they may not be worried about consuming too much of them. In the case of one mineral, new research proves that more isn’t better.
According to a new study published in Journal of Pediatrics, women who took too much supplemental iodine during pregnancy gave birth to babies that suffered from congenital hypothyroidism- a thyroid deficiency that if left untreated, can lead to mental and/or growth retardation, as well as heart problems. The moms in this study took 11 times more than the safe upper limit while pregnant and/or breastfeeding. That may sound like a lot, but if a woman is consuming a prenatal vitamin, plus additional supplemental iodine, along with foods high in iodine such as kelp (seaweed), she could be consuming too much. 

Like most things in life, nutrition is all about balance. Just as excess iodine consumption can be harmful to a fetus, a diet too low in iodine can also cause congenital hypothyroidism leading to cognitive delays and mental retardation. Fortunately, iodine deficiency is not of much concern in the US since salt is iodized, with just ¼ teaspoon (equivalent to the maximum 1500 mg of salt recommended per day) containing 76 micrograms of iodine. When consuming a food that contains sodium, check the label to see if the salt used was iodized, as some processed foods use non-iodized salt.

The recommended amount of iodine for pregnant women is 220 micrograms, and for breastfeeding women is 290 micrograms. The upper tolerable limit, or the most that can be safely consumed, is 1,100 micrograms. As long as a mother is not taking additional iodine supplements during pregnancy or breastfeeding, it is not likely she will get too much. It may be a good idea to limit consumption of seaweed (kelp), though, as it may contain up to 1,986 micrograms of iodine per tablespoon- above the amount considered safe. 

It’s not necessary to get exactly 200-300 micrograms of iodine each day- women should strive to consume the recommended amount as an average over the course of a few days or a week. It’s hard to know exactly how much iodine is obtained from the foods we consume because a plant’s iodine level depends on the amount in the soil and fertilizer in which it was grown. If a soil is rich in iodine (which usually occurs closer to the ocean), plants and vegetables grown in it will have higher iodine content, and in turn, so will the animals that eat them. You can be sure to get a good dose of this important mineral from codfish (99 micrograms), yogurt (75 micrograms), milk (57 micrograms), and enriched bread (45 micrograms). Unless you follow a vegan diet, a very low salt diet that doesn’t include iodized salt (sea salt is not iodized), or were recommended supplemental iodine by your doctor, then you probably don’t need additional iodine supplements. The best bet to ensure healthy thyroid function in your baby is to stick to a healthy, varied, and balanced diet.

The Journal of Pediatrics
National Institute of Health, Office of Dietary Supplements