Most adults need 150 micrograms (mcg) of iodine per day, and the simplest way to supplement is with a potassium iodide tablet or by consistently using iodized salt. But the best approach depends on why you’re supplementing, how much you need, and what form you choose, because absorption rates vary dramatically between supplement types.
How Much Iodine You Actually Need
The recommended daily intake for adults is 150 mcg. That number jumps to 220 mcg during pregnancy and 290 mcg while breastfeeding. These increases matter because iodine is essential for fetal brain development. A meta-analysis of studies in severely iodine-deficient regions found children born to unsupplemented mothers scored an average of 8.7 IQ points lower than children whose mothers received iodine. The damage begins early: before midgestation, the developing fetal brain depends entirely on the mother’s thyroid hormones, which require iodine to produce.
The tolerable upper limit for adults is 1,100 mcg per day. Chronically exceeding that level increases the risk of thyroid disorders, including goiter and hypothyroidism. The American Thyroid Association specifically advises against supplements containing 500 mcg or more per day.
Potassium Iodide vs. Seaweed Supplements
Not all iodine supplements are absorbed equally. Potassium iodide (often labeled “KI” on supplement bottles) is the most common and most reliable form. Your body absorbs roughly 64% to 84% of the iodine from potassium iodide, depending on the dose. Seaweed-based supplements, like those made from kelp or kombu, deliver far less. In a direct comparison study, the bioavailability of iodine from kombu was only 24% to 30%, compared to 64% to 84% from potassium iodide.
The reason: only about half the iodine in seaweed is in the inorganic form your body can readily use. The rest is bound in organic compounds that your gut absorbs more slowly and less completely. Potassium iodide also hits peak absorption within a few hours, while seaweed-derived iodine trickles in over a much longer window, sometimes taking 8 hours or more to peak and not fully clearing for over 30 hours.
Seaweed supplements have a second problem: inconsistent potency. Kelp and kombu are among the most iodine-dense foods on Earth, with kombu averaging around 2,267 mcg per gram and sugar kelp reaching 4,400 mcg per gram. A single gram of kombu could deliver more than twice the tolerable upper limit. Without standardized dosing, it’s easy to take far too much or too little. If you prefer a seaweed-based supplement, look for products that list the exact iodine content per serving rather than just the weight of seaweed.
Getting Iodine From Food
Iodized salt is the most widely available dietary source. In the United States, iodized salt contains about 45 mg of iodine per kilogram, which works out to roughly 45 mcg in one gram of salt (just under a quarter teaspoon). So about 3 to 4 grams of iodized salt per day covers the adult RDA. Keep in mind that most salt used in processed and restaurant food is not iodized, so even if your overall sodium intake is high, your iodine intake may not be.
Dairy products, eggs, and seafood are other reliable sources. Cod, shrimp, and tuna provide meaningful amounts per serving. Among seaweeds, the iodine content varies enormously by species. Nori (the type wrapped around sushi) contains only about 18 mcg per gram, making it relatively safe to eat regularly. Dulse provides around 96 mcg per gram. Wakame sits at about 172 mcg per gram. At the extreme end, kombu and oarweed contain thousands of micrograms per gram and should be used sparingly.
Why Selenium Matters Alongside Iodine
Your thyroid needs selenium to actually use the iodine you give it. Selenium-containing enzymes convert the storage form of thyroid hormone (T4) into its active form (T3). Without enough selenium, supplementing iodine alone may not fully resolve thyroid issues. These same selenium-dependent enzymes also protect thyroid cells from oxidative damage that occurs naturally during hormone production. The thyroid generates hydrogen peroxide as part of its normal function, and selenium-based proteins neutralize it.
Most adults get adequate selenium from foods like Brazil nuts, seafood, and eggs. If you’re supplementing iodine for thyroid support, checking your selenium intake is a reasonable step.
How Your Thyroid Uses Iodine
Your thyroid gland actively pulls iodine from your bloodstream using a specialized transport protein in its cell membranes. This transporter moves one iodine molecule into the cell for every two sodium molecules, powered by the same energy system that keeps sodium and potassium balanced across all your cells. Once inside the thyroid, iodine is incorporated into thyroid hormones, which regulate metabolism, body temperature, and brain function throughout your life.
When you suddenly flood your body with a large dose of iodine, the thyroid temporarily shuts down hormone production as a protective mechanism. This response, called the Wolff-Chaikoff effect, typically resolves within 24 to 48 hours in a healthy thyroid. It’s one reason gradual, consistent supplementation is preferable to occasional large doses.
Who Should Be Cautious
People taking lithium should be especially careful with iodine supplements. Lithium already suppresses thyroid function, and adding supplemental iodine can amplify that effect, increasing the risk of hypothyroidism. If you take lithium, your thyroid function should be monitored before and during any iodine supplementation.
Anyone with an existing thyroid condition, whether overactive or underactive, should approach iodine supplementation carefully. In people with certain thyroid disorders, even moderate iodine increases can trigger flare-ups. Those with Hashimoto’s thyroiditis may be particularly sensitive to changes in iodine intake.
Checking Your Iodine Status
There’s no simple at-home test for iodine levels. The standard medical assessment uses a urine sample, since most iodine you consume is eventually excreted through urine. The World Health Organization classifies iodine status based on urinary concentration: below 100 mcg per liter indicates deficiency (mild at 50 to 99, moderate at 20 to 49, and severe below 20). Adequate levels fall between 100 and 199 mcg per liter. Levels above 300 mcg per liter are considered excessive and may pose health risks.
A single urine test reflects only recent intake, not long-term iodine status, so results can vary from day to day. If you suspect deficiency, a healthcare provider may test thyroid hormone levels alongside urinary iodine to get a more complete picture.
Practical Supplement Recommendations
For most people, a potassium iodide supplement providing 150 mcg per day is sufficient. Many multivitamins already include this amount, so check what you’re already taking before adding a standalone iodine supplement. Pregnant and breastfeeding women should look for a prenatal vitamin that includes 150 to 220 mcg of iodine, since dietary intake alone often falls short of the higher requirements during these periods.
If you prefer food-based approaches, consistent use of iodized salt (3 to 4 grams daily) combined with regular seafood or dairy consumption will meet most people’s needs. If you choose seaweed as a source, stick to lower-iodine varieties like nori or dulse rather than kelp or kombu, which can easily push you past safe levels. Avoid Lugol’s solution (a concentrated liquid iodine preparation containing 50 mg of iodine per milliliter) unless specifically directed to use it for a medical purpose, as even a single drop can deliver many times the daily requirement.