Iodine during pregnancy: too little and too much can affect baby’s brain development

When people think about nutrition during pregnancy, nutrients like folate, DHA, iron, and choline tend to get most of the attention. Iodine is discussed much less often, despite playing an essential role in thyroid hormone production and early brain development.

A recent 2025 review estimated that iodine intake remains inadequate in many parts of the world, particularly during pregnancy and lactation.

At the same time, this is not a nutrient where more is automatically better. Emerging research suggests that both inadequate and excessive iodine intake may be associated with less favorable child developmental outcomes, making overall intake and context worth paying attention to.

Why does iodine matter so much for babies?

Iodine is required to make thyroid hormones (T3 and T4), which help regulate growth, metabolism, and neurologic development.

During pregnancy, thyroid hormones play an especially important role in early brain development, including neuronal growth, migration, and myelination.

Without enough thyroid hormone during those early critical windows, brain development can be permanently impaired. Severe iodine deficiency is actually the leading preventable cause of brain damage worldwide, which is a sobering statistic. But most families reading this aren’t facing severe deficiency. The more relevant question is what happens with mild, everyday deficiency, and it turns out that even subclinical deficiency, the kind that doesn’t necessarily show up on a standard thyroid test, can affect a child’s cognitive development, language skills, and IQ. This is important because standard thyroid screening measures TSH, and mild iodine deficiency can quietly reduce thyroid hormone availability without significantly moving that number.

The first trimester is the most critical window

Here’s something that often surprises people: a baby’s thyroid doesn’t even start working on its own until around 12 weeks of pregnancy. That means for the entire first trimester, a baby is completely dependent on maternal thyroid hormones for brain development.

If your iodine levels are low when you get pregnant, there’s a real risk that your baby’s brain development in those first three months is affected before you’ve even had your first prenatal appointment or started most prenatal supplements. Standard thyroid screening can miss this entirely, because mild iodine deficiency can reduce thyroid hormone availability without significantly moving TSH, which is the marker most providers use.

This is why researchers are increasingly recommending that iodine status be assessed and optimized before conception, not in the second trimester when most of these conversations typically happen.

How much iodine do you actually need?

The World Health Organization recommends 250 µg per day during both pregnancy and breastfeeding, which is about 50% more than the 150 µg recommended for non-pregnant adults. Most prenatal vitamins contain between 150 and 220 µg, and whether that’s enough for you depends largely on what you’re eating.

This is where a common snag comes in: many health-conscious households have switched to sea salt, Himalayan pink salt, or kosher salt, none of which contain significant amounts of iodine. If that describes your kitchen, your dietary iodine may be considerably lower than you’d expect, especially if you’re also dairy-free or eat little seafood.

Good dietary sources

Can you actually get too much iodine?

Yes, and this is where the conversation becomes more nuanced.

A large Australian study followed nearly 800 pregnant women and found that children born to mothers who consumed more than about 370 µg of iodine per day during pregnancy had worse cognitive and language outcomes, similar in degree to children born to mothers who were deficient. The ideal range appeared to be roughly 185 to 370 µg per day, which is actually a fairly narrow window when you consider how easy it is to stack a prenatal vitamin on top of an already iodine-rich diet.

This same U-shaped pattern has been observed in studies from the UK, Norway, Portugal, Greece, and Bangladesh.

The reason excess iodine causes problems comes down to thyroid physiology. When the thyroid is flooded with excess iodine, it temporarily shuts down hormone production in a process called the Wolff-Chaikoff effect. In healthy adults, the thyroid self-corrects within a day or two. But a developing fetal thyroid doesn’t have that ability yet, and if it gets suppressed at the wrong moment, it can interfere with exactly the brain development it was supposed to be supporting.

Watch out for seaweed

Seaweed and kelp are among the most common unrecognized sources of iodine excess. Some varieties contain 10 to 100 times the daily recommended amount in a single serving, and they’re easy to overlook because they’re often marketed as health foods. Daily kelp capsules, seaweed powders, or frequent large portions of nori or wakame can push iodine intake well beyond the safe range, especially in combination with a prenatal vitamin. “Thyroid support” supplements also frequently contain significant iodine. If you use seaweed-based products regularly or take any thyroid supplement, it’s worth mentioning to your provider.

What about breastfeeding?

Iodine needs don’t end at birth. Your body actively concentrates iodine into your breast milk via a specialized transporter in the mammary gland, which is a remarkable system designed to prioritize your baby’s needs. But it only works if you have enough iodine to begin with. If you are deficient, your milk will be too, and iodine deficiency in the first six months of life can impair cognitive and motor development in ways that may not be fully reversible even if status is corrected later.

A couple of things worth checking: many postpartum supplements don’t contain iodine, even ones specifically marketed for breastfeeding, so it’s worth looking at the label of whatever you’re taking. And if your baby is formula-fed, iodine content in formula varies by brand, so it’s worth verifying that yours meets adequate levels. The WHO recommends continuing iodine supplementation through the entire breastfeeding period, not just during pregnancy.

If you have a thyroid condition

If you have Hashimoto’s thyroiditis, any form of thyroid autoimmunity, or a history of postpartum thyroiditis after a previous pregnancy, iodine supplementation during pregnancy deserves extra attention. Women with underlying thyroid conditions are more sensitive to changes in iodine intake, and even increases within the normal recommended range can sometimes trigger thyroid hormone fluctuations. This doesn’t mean avoiding iodine, because deficiency is also harmful, but it does mean that supplementation decisions are best made in partnership with a provider who knows your thyroid history, with closer monitoring of your levels throughout pregnancy.

The bottom line for parents

• Check your prenatal vitamin. Does it contain iodine, and how much? Many do, but some food-based or whole-food prenatal vitamins don’t include it at meaningful levels.
• Think honestly about your diet. If you are dairy-free, vegan, eating low amounts of seafood, or cooking with non-iodized salt, your dietary iodine is probably lower than you’d assume.
• Don’t over-supplement. If your prenatal contains 200 µg and you eat a diet with regular dairy and seafood, you likely don’t need a separate high-dose iodine supplement on top of that.
• Watch the seaweed. Occasional seaweed consumption is generally fine, but daily kelp capsules or large portions of seaweed are worth discussing with your provider, especially in combination with a prenatal vitamin.
• Keep supplementing while breastfeeding, and take a look at whether your postpartum supplement actually includes iodine, because many don’t.
• Ideally, think about this before you get pregnant. The first trimester is the highest-risk window and it often passes before iodine is ever mentioned at a prenatal visit.

Research source: Ma ZF, Brough L. “Effect of Iodine Nutrition During Pregnancy and Lactation on Child Cognitive Outcomes: A Review.” Nutrients. 2025. PMID: 40573127. Open access.

Clinical summary by Erika Krumbeck, ND, FABNP for naturopathicpediatrics.com.

A note: This post is for educational purposes and is not a substitute for personalized medical advice. Everyone’s nutritional needs during pregnancy are different. If you have questions about your specific iodine status or supplementation, please talk with your naturopathic doctor, midwife, or OB.