Iodine is required in only trace amounts, but its role in thyroid hormone production makes it absolutely essential — and both too little and, less commonly, too much can cause real problems. Here’s what you need to know.
What Does Iodine Do?
Iodine is a trace mineral that the thyroid gland uses to produce thyroid hormones (T3 and T4), which regulate metabolism, growth, and development throughout the body. Without adequate iodine, the thyroid simply cannot produce enough of these hormones, regardless of how well everything else is functioning.
Thyroid hormones influence nearly every cell in the body, affecting metabolic rate, heart rate, body temperature, and — critically during pregnancy and early childhood — brain development.
Food Sources
Because iodine content in plant foods depends heavily on the iodine content of the soil they’re grown in, dietary sources can vary. Reliable sources include:
- Iodized salt (a major source in many countries specifically because of historical fortification efforts)
- Seaweed and sea vegetables (some of the richest natural sources, though content varies widely by type)
- Dairy products (partly due to iodine-based sanitizers used in dairy farming)
- Seafood and fish
- Eggs
Why Iodized Salt Became a Public Health Measure
In the early 20th century, iodine deficiency was a significant public health problem in many regions with iodine-poor soil, leading to widespread goiter (enlarged thyroid) and, in severe cases, developmental issues in children born to iodine-deficient mothers. The introduction of iodized salt in the 1920s dramatically reduced these issues in many countries and remains one of the most successful public health nutrition interventions in history.
Signs of Iodine Deficiency
Goiter. An enlarged thyroid gland, which develops as the thyroid works harder to capture whatever iodine is available.
Hypothyroidism symptoms. Fatigue, weight gain, cold intolerance, dry skin, and depression can all result from underactive thyroid function due to insufficient iodine.
Developmental effects. Iodine deficiency during pregnancy is particularly concerning, as it can affect fetal brain development and, in severe cases, cause significant cognitive impairment in the child — one of the most important reasons pregnant individuals are often advised to ensure adequate iodine intake.
Who’s at Risk of Deficiency Today?
While iodized salt has greatly reduced deficiency rates in many countries, certain groups remain at higher risk:
- People who avoid iodized salt in favor of non-iodized sea salt or kosher salt without compensating through other sources
- People following strict vegan diets without seaweed or fortified foods
- Pregnant and breastfeeding individuals, who have increased iodine requirements
- People living in regions with naturally iodine-poor soil and limited access to iodized salt or seafood
How Much Do You Need?
Recommended daily intakes are:
- Adults: 150 mcg
- Pregnant individuals: 220 mcg
- Breastfeeding individuals: 290 mcg
Can You Get Too Much Iodine?
Yes — and this is a less commonly discussed side of the iodine story. Excessive iodine intake, often from high-dose supplements or very high consumption of iodine-rich seaweed, can paradoxically also disrupt thyroid function, potentially causing either hypothyroidism or hyperthyroidism, particularly in people with pre-existing thyroid conditions. This is a good example of a nutrient where both too little and too much can cause similar categories of problems.
The Bottom Line
Iodine plays an irreplaceable role in thyroid hormone production, and adequate intake is especially critical during pregnancy for healthy fetal brain development. For most people using iodized salt and eating a varied diet, meeting iodine needs isn’t difficult — but those avoiding iodized salt or following strict plant-based diets may want to pay closer attention to reliable sources.
This article is for informational purposes only and is not a substitute for professional medical advice.
