Iodine is essential for your health. Your body cannot make it, so you need to get it from food, and without enough of it, your thyroid gland can’t produce the hormones that regulate your metabolism, body temperature, and brain development. Most adults need about 150 micrograms per day, a tiny amount that has an outsized impact on how your body functions.
What Iodine Does in Your Body
Your thyroid gland, the butterfly-shaped organ at the base of your neck, is the primary consumer of iodine. Thyroid cells actively pull iodide from your bloodstream using a specialized pump on their surface. Once inside, an enzyme combines iodine with a protein called thyroglobulin to build two critical hormones: T4 (which contains four iodine atoms) and T3 (which contains three). These hormones circulate through your blood and influence virtually every cell in your body.
T3 and T4 control how fast your cells burn energy, how quickly your heart beats, how well your brain functions, and how your body maintains its temperature. Without a steady supply of iodine, this production line stalls. Your thyroid compensates by working harder, which can cause it to swell into a visible lump in the neck called a goiter.
Signs You’re Not Getting Enough
When iodine intake drops too low, the thyroid can’t keep up with hormone demand, and you develop hypothyroidism. The symptoms tend to creep in slowly: persistent tiredness, unexplained weight gain, feeling cold when others are comfortable, dry skin, constipation, a slow heartbeat, and even depression. Many people chalk these up to aging or stress before discovering their thyroid is the root cause.
Iodine deficiency remains a global problem. Roughly 181 million people worldwide were living with iodine deficiency in 2021, with the highest burden in sub-Saharan Africa and South Asia. Women and people between the ages of 10 and 45 are most affected. The introduction of iodized salt in many countries has lowered the per-capita rate over the past three decades, but the total number of cases has actually increased as populations have grown.
Why Iodine Matters Most During Pregnancy
Iodine’s role in brain development makes it especially critical during pregnancy. A developing fetus depends entirely on the mother’s thyroid hormones during the first trimester, before its own thyroid starts functioning. Low maternal iodine during this window can impair the child’s cognitive and language development in ways that show up months or years later.
A study of 699 pregnant women found a clear sweet spot for iodine intake. Children scored lowest on cognitive and language assessments at 18 months when their mothers consumed either too little or too much iodine during pregnancy. Below about 185 micrograms per day, developmental scores dropped. Above roughly 350 to 370 micrograms per day, scores declined again. Pregnant and breastfeeding women generally need 220 to 290 micrograms daily, higher than the standard adult recommendation but well within this safe range.
Best Food Sources of Iodine
You can meet your daily needs through common foods, though the amounts vary widely. Here are some of the richest sources, based on NIH data:
- Baked cod (3 ounces): 146 mcg
- Dried nori seaweed (2 tablespoons, flaked): 116 mcg
- Greek yogurt, plain nonfat (¾ cup): 87 mcg
- Nonfat milk (1 cup): 84 mcg
- Iodized table salt (¼ teaspoon): 78 mcg
- Hard-boiled egg (1 large): 31 mcg
- Cheddar cheese (1 ounce): 14 mcg
A single serving of cod or a glass of milk with a quarter-teaspoon of iodized salt already gets you past the 150-mcg mark. Dairy is a major source for many people because iodine-based sanitizers used in milk processing leave trace amounts behind. If you avoid dairy, seafood, and iodized salt (choosing sea salt or Himalayan salt instead, which contain negligible iodine), you may fall short without realizing it.
When Too Much Becomes a Problem
Iodine follows a U-shaped curve: too little is harmful, but too much creates its own set of problems. Excess iodine can actually trigger the same condition that deficiency causes, hypothyroidism, because the thyroid shuts down hormone production as a protective response to being flooded. In people who are genetically predisposed, high iodine intake may also trigger Hashimoto’s disease, an autoimmune condition where the immune system attacks the thyroid.
The upper limit for adults is 1,100 micrograms per day. Most people eating a normal diet won’t come close to that number, but it’s surprisingly easy to exceed it with certain seaweed products. Some varieties of kelp contain thousands of micrograms per serving. If you eat seaweed regularly, especially kombu or kelp supplements, checking the iodine content on the label is worth your time. Nori, the type used in sushi rolls, tends to be much lower in iodine and is generally safe in normal portions.
Iodine Supplements: Who Needs Them
If you eat dairy, seafood, or use iodized salt regularly, you likely get enough iodine from food alone. Supplements become more relevant for people who follow vegan or dairy-free diets, those who use non-iodized salt exclusively, and pregnant or breastfeeding women who may not be meeting the higher requirement through food.
Most prenatal vitamins contain 150 mcg of iodine, which bridges the gap between a typical diet and the elevated needs of pregnancy. Standard multivitamins for the general population often contain iodine as well, typically in the range of 75 to 150 mcg. If you’re already getting plenty from food, adding a high-dose supplement on top can push you past the beneficial range without providing any extra benefit.
Iodine’s Role in Radiation Emergencies
You may have heard of potassium iodide tablets in the context of nuclear accidents. This is a specific, emergency use of iodine that works because of a simple trick: your thyroid can’t distinguish between regular iodine and radioactive iodine released during a nuclear event. Taking a large dose of non-radioactive iodine saturates the thyroid so completely that it can’t absorb the dangerous radioactive version for the next 24 hours. This is a targeted medical intervention, not a reason to stockpile iodine supplements for everyday use.