Iodine deficiency stems from a combination of factors: soil that’s been stripped of iodine, diets low in seafood and dairy, and a global food system that increasingly relies on non-iodized salt. Adults need about 150 micrograms of iodine daily, and pregnant women need 220 mcg. Falling short of those amounts over time leads to deficiency, but the reasons people fall short vary widely.
Where You Live Shapes Your Risk
Iodine originates in the ocean and reaches inland soil through rain, but that cycle doesn’t distribute it evenly. Mountainous regions, areas prone to flooding, and inland territories far from the coast tend to have iodine-depleted soil. Heavy rainfall and erosion wash iodine from the topsoil, leaving crops grown in those areas with very little of the mineral. Research in the lower Himalayan region of Pakistan found that elevated, flood-prone areas had significantly lower soil iodine levels than flat plains nearby, along with higher rates of thyroid disorders in the local population.
This pattern repeats globally. Children living in mountainous parts of Nepal show lower iodine levels than those in the plains. Historically, the “goiter belts” of the Alps, the Andes, and central Africa were named for the swollen thyroid glands common among people who ate almost exclusively local food grown in depleted soil. If the soil where your food is grown lacks iodine, the food will too, no matter how balanced your diet looks on paper.
Diets Low in Seafood, Dairy, and Eggs
The richest food sources of iodine are marine fish, dairy, and eggs. A single serving of haddock delivers around 352 mcg of iodine, more than double the daily adult requirement. A glass of cow’s milk provides about 60 mcg, a cup of yogurt around 62 mcg, and two eggs roughly 50 mcg. Fruits, vegetables, nuts, and grains, by contrast, contribute very little. An 80-gram portion of fruits or vegetables contains only about 3 mcg, and a serving of nuts about 5 mcg.
This gap matters most for people following strict plant-based diets. Vegans who don’t supplement are at high risk of deficiency because no combination of plant foods reliably meets the daily requirement. Even people who simply swap cow’s milk for unfortified plant-based alternatives take a hit: a glass of unfortified oat or almond milk contains roughly 2 mcg of iodine compared to 60 mcg in cow’s milk. One UK study found that people who exclusively consumed plant-based milk alternatives had lower iodine intake (94 vs. 129 mcg per day) and lower urinary iodine levels than cow’s milk drinkers, putting them below the threshold the WHO uses to define population-level deficiency.
The Processed Food Problem
Salt iodization has been one of public health’s biggest success stories. About 88% of the global population now uses iodized salt, and 124 countries have mandatory iodization laws. But there’s a significant loophole: in many countries, those laws apply only to table salt sold for home cooking and seasoning. Salt used by restaurants and commercial food manufacturers is often not iodized.
That distinction matters more every year. As people eat more processed and restaurant food and cook less at home, a shrinking share of their total salt intake comes from the iodized container in their kitchen. The food industry has resisted switching to iodized salt in manufacturing, citing concerns about product discoloration and taste changes. Countries have been slow to update regulations to close this gap, and nearly one billion people worldwide still lack access to adequately iodized salt.
Foods That Block Iodine Uptake
Certain foods contain compounds called goitrogens that interfere with how your thyroid uses iodine. The effect is usually minor if your iodine intake is adequate, but it can tip the balance toward deficiency when intake is already low.
Cruciferous vegetables like broccoli, cabbage, and cauliflower contain compounds that break down into a substance called thiocyanate, which competes with iodine for entry into thyroid cells. Soy products contain isoflavones that can inhibit the enzyme your thyroid needs to produce hormones. Cassava, a dietary staple in parts of sub-Saharan Africa, also generates thiocyanate when digested. None of these foods cause problems on their own in people with sufficient iodine, but in regions where iodine intake is borderline, heavy reliance on cassava or soy can push people into clinical deficiency.
Environmental Chemicals That Compete With Iodine
Your thyroid absorbs iodine through a specialized transport channel, and several common environmental contaminants use the same channel. Perchlorate (found in rocket fuel residue and some drinking water supplies), thiocyanate (a byproduct of cigarette smoke), and nitrate (common in fertilizer runoff and processed meats) all block iodine from entering thyroid cells through competitive inhibition. Each of these chemicals individually has a modest effect, but combined exposures, especially in someone whose iodine intake is already marginal, can meaningfully reduce the amount of iodine your thyroid captures from your bloodstream.
Pregnancy Increases Demand Sharply
Pregnant women need about 50% more iodine than other adults because their bodies must produce enough thyroid hormone for both themselves and their developing baby. The recommended intake jumps from 150 mcg to 220 mcg during pregnancy and 290 mcg while breastfeeding.
The timing is critical. Thyroid hormone-dependent brain development in the fetus begins during the second half of the first trimester, a period when the baby is entirely dependent on the mother’s thyroid hormones. This early phase drives neuron production and the beginning of neuron migration in the brain’s cortex and hippocampus. The fetal thyroid doesn’t begin producing its own hormones until around the start of the second trimester, and even then, its reserves remain low. Maternal thyroid hormones continue contributing to the baby’s supply all the way until birth. A woman who enters pregnancy with marginal iodine status can quickly become deficient as demands escalate, with consequences for fetal brain development during windows that can’t be revisited later.
Other Nutrient Deficiencies Make It Worse
Iodine deficiency rarely exists in isolation. In populations where iodine is scarce, deficiencies in iron and vitamin A are often present too, and each one compounds the thyroid problems caused by low iodine. Iron deficiency impairs the enzyme your thyroid uses to produce hormones. Vitamin A deficiency decreases the thyroid’s ability to absorb iodine and assemble it into functioning hormones.
In one study of children with severe iodine deficiency, those who also had vitamin A deficiency were more than six times as likely to have goiter compared to children with adequate vitamin A levels. Animal research has shown that combined iodine and vitamin A deficiency produces greater thyroid impairment than either deficiency alone. This overlap is why public health programs in affected regions increasingly pair iodine supplementation with vitamin A, recognizing that fixing one deficiency without addressing the other yields incomplete results.
Who Is Most at Risk
Several groups face a higher likelihood of iodine deficiency based on the causes above:
- People in inland or mountainous regions without strong salt iodization programs, particularly in South Asia, Central Asia, and parts of Africa.
- Vegans and strict vegetarians who don’t supplement, especially in countries like the UK and Norway where dairy and fish are the primary dietary iodine sources.
- Pregnant and breastfeeding women whose increased requirements outpace their intake.
- People who eat mostly processed or restaurant food in countries where industrial salt is not required to be iodized.
- Populations relying heavily on cassava as a staple food, particularly in sub-Saharan Africa where iodine intake is already marginal.
Globally, 21 countries still have populations with insufficient iodine intake as of 2020. Even in countries considered iodine-sufficient at the national level, pockets of deficiency persist among subgroups whose diets or circumstances place them at higher risk.