Iodine
Also known as: I, dietary iodine
Medically reviewed by Nano Health Insights Editorial Team · Last reviewed 2026-06-29
Iodine is an essential trace mineral needed to make thyroid hormones; adults need 150 mcg per day.
Iodine is an essential trace mineral needed to make thyroid hormones, and the recommended dietary allowance for most adults is 150 mcg per day.
What it is
Iodine is a nutrient the body needs in small amounts to produce the thyroid hormones thyroxine (T4) and triiodothyronine (T3). These hormones help regulate metabolism, growth, temperature control, and many enzyme-driven processes. Iodine is also especially important during pregnancy and infancy because normal thyroid hormone levels are required for fetal and early brain development.
The body does not make iodine, so it must come from food, iodized salt, or supplements. Too little iodine can lead to iodine deficiency disorders, including goiter and hypothyroidism. Too much iodine can also disturb thyroid function, especially in people with thyroid disease or in those exposed to large supplemental doses.
Common dietary sources include seafood, dairy products, eggs, and iodized salt. The amount in plant foods varies widely because it depends on the iodine content of the soil and water where food is produced. This matters in regions where soils are iodine-poor, including some mountainous areas such as the Himalayas. In India, iodized salt has been a major public health measure to reduce iodine deficiency.
A quick guide to daily needs:
| Life stage | Recommended intake |
|---|---|
| Adults | 150 mcg/day |
| Pregnancy | 220 mcg/day |
| Lactation | 290 mcg/day |
How it works
After iodine is absorbed, the thyroid gland takes it up from the bloodstream and uses it to make T4 and T3. This process is regulated mainly by thyroid-stimulating hormone, or TSH, from the pituitary gland. When iodine intake is low, the thyroid has less raw material to make hormones. TSH may rise, pushing the gland to work harder and sometimes enlarge, which can cause a goiter.
Thyroid hormones affect many organs. They influence energy use, heart rate, protein synthesis, and normal growth. During fetal life and infancy, they are critical for brain and nervous system development. That is why iodine deficiency in pregnancy is a major concern even when the pregnant person has only mild symptoms.
Most ingested iodine is absorbed efficiently. The body does not store large excess amounts outside the thyroid, and much of the iodine not used is excreted in urine. For that reason, urinary iodine is often used to assess iodine status in populations.
Evidence and uses
Iodine is not a general wellness supplement. Its established role is prevention and treatment of iodine deficiency and support of normal thyroid hormone production when intake is inadequate.
Situations where iodine matters most include:
- Preventing iodine deficiency: Adequate intake prevents goiter and other deficiency disorders.
- Pregnancy and lactation: Needs rise because the fetus and infant depend on maternal iodine supply.
- Public health fortification: Iodized salt is the main strategy used worldwide to reduce deficiency.
- Documented low intake diets: People who avoid dairy, seafood, eggs, and iodized salt may be at higher risk.
Evidence strongly supports iodine sufficiency as a public health priority. Severe deficiency is known to impair growth and neurodevelopment, and milder deficiency can still affect thyroid function. However, taking extra iodine when intake is already adequate does not usually improve energy, weight, or thyroid health.
Some supplements market kelp or iodine for breast health, immunity, or metabolism. Evidence for these uses is limited or inconsistent in humans. Reviews note that iodine has biological roles beyond the thyroid, but these possible benefits are not established enough to support routine supplementation for those purposes.
Food sources vary a lot. Seaweed can contain very high and unpredictable amounts, while processed foods may not use iodized salt. Specialty salts such as sea salt, Himalayan salt, and kosher salt are often not iodized unless the label says so.
Safety and interactions
Both deficiency and excess can be harmful. For adults, the tolerable upper intake level is 1,100 mcg per day from food, supplements, and other sources combined. Regularly exceeding this level can increase the risk of thyroid dysfunction.
Potential problems from too much iodine include hypothyroidism, hyperthyroidism, thyroiditis, and goiter. People with autoimmune thyroid disease, nodules, prior iodine deficiency, infants, and older adults may be more sensitive to excess intake.
Important safety points:
| Issue | Why it matters |
|---|---|
| Kelp supplements | Iodine content can be highly variable and sometimes excessive |
| Thyroid disease | Extra iodine can worsen hypo- or hyperthyroidism in some people |
| Pregnancy | Adequate intake is essential, but high-dose supplements are not automatically safer |
| Specialty salts | Many are not iodized |
Iodine can interact with some medicines. Antithyroid drugs used for hyperthyroidism can have additive effects with high iodine intake. Amiodarone contains large amounts of iodine and can cause either hypothyroidism or hyperthyroidism. Potassium iodide may also interact with certain blood pressure medicines such as ACE inhibitors or potassium-sparing diuretics by increasing the risk of high potassium in some settings.
If you are considering an iodine supplement, especially above the amount in a standard multivitamin, talk to a clinician or pharmacist first. This is particularly important if you have thyroid disease, are pregnant, or take thyroid-related medicines.
When to see a clinician
See a clinician if you have symptoms that could suggest thyroid problems, such as unexplained fatigue, weight change, constipation, heat or cold intolerance, palpitations, neck swelling, or menstrual changes. Pregnant people, those planning pregnancy, and people on restrictive diets may also need guidance on whether their iodine intake is adequate.
Medical advice is also important before using iodine drops, kelp tablets, or high-dose supplements bought online. More is not better with iodine, and self-treating thyroid symptoms with supplements can delay proper diagnosis.
Limitations and open questions
Iodine intake is hard to estimate for an individual because food content varies widely by geography, farming practices, food processing, and salt type. Seaweed products are especially inconsistent. A single urine iodine test is useful for population studies but is less reliable for judging one person's long-term iodine status.
There are also open questions about the health effects of mild iodine deficiency in some groups, the best supplementation strategies in pregnancy across different regions, and the non-thyroid roles of iodine. Evidence in humans is limited for claims that iodine supplements improve general energy, immunity, fibrocystic breast symptoms, or weight when deficiency is not present.
The practical goal is adequacy, not excess. For most people, that means getting iodine from a balanced diet and iodized salt when appropriate, while avoiding unnecessary high-dose supplements.
FAQs
What does iodine do in the body?
Iodine is needed to make the thyroid hormones T4 and T3. These hormones help control metabolism, growth, body temperature, and normal brain development. During pregnancy and infancy, adequate iodine is especially important because thyroid hormones support fetal and early-life nervous system development.
How much iodine do adults need each day?
Most adults need 150 mcg of iodine per day. Needs increase to 220 mcg per day during pregnancy and 290 mcg per day during lactation. The adult tolerable upper intake level is 1,100 mcg per day, so high-dose supplements are not a routine choice.
What foods are good sources of iodine?
Seafood, dairy products, eggs, and iodized salt are common sources of iodine. Seaweed can contain a lot of iodine, but the amount is often unpredictable and can be excessive. Plant foods may contain little or more iodine depending on the iodine content of the soil and water where they were grown.
Can you get too much iodine?
Yes. Too much iodine can trigger hypothyroidism, hyperthyroidism, goiter, or thyroid inflammation, especially in people with underlying thyroid disease. Kelp and other seaweed supplements are a common source of unexpectedly high intake because their iodine content can vary widely.
Who is more likely to have low iodine intake?
People who do not use iodized salt and eat little seafood, dairy, or eggs may be at higher risk. Pregnant people need about 50% more iodine than nonpregnant adults, so low intake matters more in that period. Risk can also be higher in areas with iodine-poor soils, including some Himalayan regions, which is one reason iodized salt programs are important in India and globally.