Iodine is a trace mineral your thyroid gland uses to make its two main hormones, thyroxine (T4) and triiodothyronine (T3), so getting enough iodine supports normal thyroid function. Most adults need 150 mcg a day, and the safe upper limit from all sources combined is 1,100 mcg. Seafood, dairy, eggs and iodized salt supply most of it, while kelp and other seaweeds vary so widely that a single serving can go past the upper limit.
Key Takeaways
- Iodine is an essential part of the thyroid hormones T4 and T3, which help regulate metabolism and normal development.
- The NIH RDA is 150 mcg a day for adults, 220 mcg in pregnancy and 290 mcg while breastfeeding. The adult upper limit is 1,100 mcg a day from food and supplements combined.
- Cod, seaweed, dairy, eggs and iodized salt are good sources. Sea salt, kosher salt and most salt in processed foods are usually not iodized.
- Kelp and seaweed products vary enormously in iodine; some kelp supplements exceed the upper limit in one dose.
- Too much iodine can disturb thyroid function, much like too little. Selenium also matters: the thyroid holds more selenium per gram than any other organ.
- If you have a thyroid condition or take thyroid medication, talk to your doctor before using iodine or kelp supplements.
Evidence grade: Strong. Iodine's role as a building block of thyroid hormones and its RDA and upper limit are established NIH reference values.
What Iodine Does in the Thyroid
Iodine's best-known job is in the thyroid, the butterfly-shaped gland at the front of your neck. According to the NIH Office of Dietary Supplements (ODS), iodine is an essential component of thyroxine (T4) and triiodothyronine (T3). These hormones regulate many biochemical reactions, including protein synthesis and enzyme activity, and they are key drivers of metabolic activity [1].
Thyroid hormones are also required for normal skeletal and nervous system development in fetuses and infants [1]. That is why iodine needs rise during pregnancy and breastfeeding.
A healthy adult holds about 15–20 mg of iodine, 70%–80% of it in the thyroid. The body absorbs iodide from food almost completely, and most of what the thyroid does not use leaves in the urine [1].
How the body keeps thyroid hormone steady
The pituitary gland releases thyroid-stimulating hormone (TSH), which tells the thyroid to take up iodine and release T3 and T4 [1]. When iodine intake falls below about 100 mcg a day, TSH rises so the gland can trap more iodine [1]. If intake stays low for a long time, the thyroid can enlarge as it works harder; this enlargement is called a goiter. Very low intakes, roughly 10–20 mcg a day, are not enough to keep thyroid hormone production normal [1].
How Much Iodine Do You Need?
The Food and Nutrition Board sets the Recommended Dietary Allowance (RDA) and the Tolerable Upper Intake Level (UL). The UL is the highest daily intake unlikely to cause harm, and it covers food and supplements together [1].
| Life stage | RDA (mcg/day) | Upper limit (mcg/day) |
|---|---|---|
| Children 1–8 years | 90 | 200 (ages 1–3); 300 (ages 4–8) |
| Children 9–13 years | 120 | 600 |
| Teens 14–18 years | 150 | 900 |
| Adults 19+ years | 150 | 1,100 |
| Pregnancy (19+) | 220 | 1,100 |
| Breastfeeding (19+) | 290 | 1,100 |
The World Health Organization recommends 250 mcg a day in pregnancy [1]. A review in Endocrine Reviews describes an optimal adult intake range of roughly 150–250 mcg a day [3].
Most people in the United States get enough. The FDA's Total Diet Study estimated average intake at 216 mcg a day, not counting iodized salt added at the table, although some pregnant women may fall short [1].
Food Sources of Iodine
Seaweed, fish and other seafood, and eggs are among the best food sources. Dairy also contributes, though its iodine varies: 59 samples of nonfat milk ranged from 38 to 160 mcg per cup [1]. Most fruits and vegetables are poor sources, and their iodine depends on the soil they grew in [1].
| Food | Iodine (mcg) | % Daily Value |
|---|---|---|
| Cod, baked, 3 ounces | 146 | 97% |
| Seaweed, nori, dried, 2 tablespoons flaked (5 g) | 116 | 77% |
| Oysters, cooked, 3 ounces | 93 | 62% |
| Greek yogurt, plain, nonfat, 3/4 cup | 87 | 58% |
| Milk, nonfat, 1 cup | 84 | 56% |
| Iodized table salt, 1/4 teaspoon | 78 | 52% |
| Egg, hard-boiled, 1 large | 31 | 21% |
| Tuna, canned in water, 3 ounces | 7 | 5% |
| Sea salt, non-iodized, 1/4 teaspoon | 0 | 0% |
Values are approximate [1]. Bread can be a large source only when the baker uses an iodate dough conditioner, which about 20% of U.S. bread products list on the label [1].
Iodized salt
U.S. salt makers have added iodine to table salt since the 1920s, though it is still voluntary [1]. Labeled iodized salt carries 45 mcg of iodine per gram [1]. But most salt in the American diet comes from processed foods, which almost always use non-iodized salt, and specialty salts such as sea salt, kosher salt, Himalayan salt and fleur de sel are not usually iodized [1]. If you use a specialty salt, as in our guide to how much Celtic sea salt to use per day, check the label or get iodine from other foods.
Kelp and seaweed: why the numbers vary so much
Seaweed is rich in iodine, but the amount is hard to predict. ODS notes that commercially sold seaweeds have ranged from 16 to 2,984 mcg of iodine per gram [1]. A 2004 analysis of 12 species found nori at about 16 mcg per gram and one kelp granule product above 8,000 mcg per gram, with further variation from harvest, cooking and storage [5].
A 2021 Norwegian study tested 96 seaweed-containing products, including 14 supplements. Supplements ranged from 5 to 5,600 mcg of iodine per daily dose. For 54 of the products, one portion or dose would exceed the UL, and labeling of iodine content was inadequate or inaccurate for several of them [6]. A kelp product is only as predictable as its stated iodine amount.
Who May Fall Short on Iodine
Overt iodine shortfall is now uncommon in North America, thanks largely to iodized salt and a national food supply [1]. ODS lists these groups as more likely to have low iodine status [1]:
- People who do not use iodized salt.
- Pregnant women, whose RDA rises to 220 mcg a day; see key nutrients for fetal development.
- Vegans and people who eat little dairy, seafood or eggs; see nutrients often missing in vegan diets.
- People living where soils are low in iodine, unless they use iodized salt or eat foods grown elsewhere.
Breastfeeding raises needs to 290 mcg a day [1]; see our breastfeeding diet guide.
Selenium: Iodine's Partner Mineral
Selenium is an essential mineral built into about 25 selenoproteins. These proteins play important roles in thyroid hormone metabolism, DNA synthesis, reproduction and protection from oxidative damage [2].
The thyroid contains more selenium per gram than any other organ in the body [2]. Selenium-containing enzymes help convert T4, the less active prohormone, into T3, the active form. Two other selenoproteins, glutathione peroxidase and thioredoxin reductase, help shield the gland from the hydrogen peroxide it produces while making hormones [2].
How much selenium you need
The adult RDA for selenium is 55 mcg a day, rising to 60 mcg in pregnancy and 70 mcg while breastfeeding. The adult UL is 400 mcg a day [2]. In 2023 the European Food Safety Authority set a lower adult upper level of 255 mcg a day [2].
Seafood (yellowfin tuna has about 92 mcg per 3 ounces), meat, poultry, eggs, grains and dairy all provide selenium. Brazil nuts are the standout: about 544 mcg in one ounce (6–8 nuts), or 68–91 mcg in a single nut [2]. ODS notes that eating them regularly could cause selenium toxicity [2].
What selenium research in healthy adults shows
Observational studies have linked lower selenium status with a larger thyroid volume in women, but not in men [2]. In a Danish randomized trial of 491 adults aged 60–74 with modestly low selenium intake, 100, 200 or 300 mcg a day of selenium yeast for up to five years produced only very small, dose-related drops in TSH and free T4. The authors concluded that selenium supplements were not warranted for thyroid function in people with marginally low selenium [7].
Where an Iodine Supplement Fits
ODS notes that many multivitamin/mineral supplements already contain 150 mcg of iodine, usually as potassium or sodium iodide, which the body absorbs almost completely. Some iodine-only and kelp supplements contain high doses, sometimes above the UL [1]. Add up the iodine in every product you take.
OPA sells OPA Advanced Iodine+, a capsule with iodine from kelp. Its label lists 300 mcg of iodine per two-capsule serving, which is 200% of the adult RDA. That is below the 1,100 mcg adult UL on its own, and it leaves room for typical dietary intake of around 200 mcg [1]. It is still more than many people need, so count iodine from any multivitamin, iodized salt and seaweed too. The formula also includes a kelp and greens blend (alfalfa, chlorella and milk thistle) and an herbal and mineral blend (turmeric, reishi, guggul, red algae and hyssop). It contains fish and crustacean shellfish ingredients. Check with your doctor first if you have a thyroid condition, take medication, or are pregnant or breastfeeding.
Safety and Interactions
Too much iodine
High iodine intakes can cause some of the same effects as too little, including goiter, raised TSH and lower thyroid hormone output. In susceptible people, excess iodine can also cause an overactive thyroid or inflammation of the thyroid [1]. A review in Nature Reviews Endocrinology notes that intakes above the RDA are generally well tolerated, but older adults, fetuses and newborns, and people with pre-existing thyroid conditions face a higher risk of iodine-induced thyroid changes. [4].
Acute iodine poisoning is rare and usually involves doses of many grams. People with autoimmune thyroid conditions may react to iodine amounts that are safe for most people [1].
Medication interactions (from NIH ODS)
- Antithyroid medicines such as methimazole: high doses of iodine taken with these drugs can have an additive effect and lower thyroid hormone too far [1].
- ACE inhibitors such as benazepril, lisinopril and fosinopril: taking potassium iodide with them can raise blood potassium [1].
- Potassium-sparing diuretics such as spironolactone and amiloride: potassium iodide can also raise blood potassium with these [1].
If you have a thyroid condition or take any thyroid medication, including thyroid hormone, talk to your doctor before using iodine, kelp or seaweed supplements.
Too much selenium
Chronically high selenium intake causes selenosis. Early signs are garlic breath and a metallic taste; the most common signs are hair loss and brittle nails [2]. Stay under 400 mcg a day from all sources. NIH ODS notes that trials of selenium in people with autoimmune thyroiditis have had mixed results, and that the American Thyroid Association recommends against selenium supplements for pregnant women with thyroid autoimmunity [2]. If you take cisplatin, talk to your oncologist before using selenium [2].
Frequently Asked Questions
How much iodine should an adult get each day?
The RDA is 150 mcg a day for adults, 220 mcg during pregnancy and 290 mcg while breastfeeding. The upper limit for adults is 1,100 mcg a day from all sources combined [1].
Which foods are best for iodine?
Cod and other seafood, seaweed, dairy, eggs and iodized salt are the most reliable sources. A quarter teaspoon of iodized salt provides about 78 mcg, while non-iodized sea salt provides virtually none [1].
Is kelp a good way to get iodine?
Kelp is rich in iodine, but its content varies widely between species, batches and products. Testing has found some kelp supplements above the upper limit in a single daily dose [5][6]. Choose a product that states its iodine amount.
Do I need to avoid broccoli, cabbage or soy?
For most people, no. These foods contain goitrogens that can interfere with iodine uptake, but ODS says this mainly matters for people with marginal iodine intake. For people who get enough iodine and eat a variety of foods, reasonable amounts are not a concern [1].
Can I take iodine if I am on thyroid medication?
Talk to your doctor first. People with existing thyroid conditions are more sensitive to extra iodine, and iodine can add to the effect of antithyroid drugs [1][4]. Your doctor can tell you whether any added iodine makes sense for you.
Sources
- NIH Office of Dietary Supplements. Iodine: Fact Sheet for Health Professionals. Updated November 5, 2024.
- NIH Office of Dietary Supplements. Selenium: Fact Sheet for Health Professionals. Updated September 4, 2025.
- Zimmermann MB. Iodine deficiency. Endocrine Reviews. 2009.
- Leung AM, Braverman LE. Consequences of excess iodine. Nature Reviews Endocrinology. 2014.
- Teas J, Pino S, Critchley A, Braverman LE. Variability of iodine content in common commercially available edible seaweeds. Thyroid. 2004.
- Aakre I, Solli DD, Markhus MW, et al. Commercially available kelp and seaweed products: valuable iodine source or risk of excess intake? Food & Nutrition Research. 2021.
- Winther KH, Bonnema SJ, Cold F, et al. Does selenium supplementation affect thyroid function? Results from a randomized, controlled, double-blinded trial in a Danish population. European Journal of Endocrinology. 2015.
Disclosure: OPA Nutrition sells supplements, including products mentioned in this article. This content is for education only and is not medical advice. Statements about dietary supplements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease. Talk to your healthcare provider before using any supplement, especially if you take medication or have a medical condition.
Written and reviewed by Michael Gonzales, Founder, OPA Nutrition · Last reviewed: · How OPA researches and writes
