The best iodine supplement for most people is a potassium iodide tablet with a dose close to 150 mcg, the recommended daily amount for adults. Potassium iodide is the most studied, most stable, and most predictable form available. But the right choice depends on why you’re taking it, whether you’re pregnant, and whether you actually need one at all.
Why Potassium Iodide Is the Standard
All major forms of iodine are broken down into iodide in your gut, where it’s almost fully absorbed in the stomach and upper small intestine. From there, your thyroid gland and kidneys pull it from your bloodstream. Since the body converts everything to iodide anyway, the simplest approach is to start with a stable iodide compound.
Potassium iodide (often abbreviated KI) is the form used in most clinical research and in iodized salt programs worldwide. It’s available as tablets in precise doses, which makes it easy to stay within safe limits. One important caveat: potassium iodide in tightly bound inorganic tablet form has an estimated assimilation rate of around 20%, meaning your body uses a fraction of the total dose. Manufacturers account for this when setting the labeled amount, so a tablet listing 150 mcg is formulated to deliver a meaningful dose.
Nascent iodine, a liquid form that carries an electromagnetic charge, is marketed as more easily recognized and absorbed by the body. Some users prefer it for that reason. However, it lacks the extensive clinical track record of potassium iodide, and dosing can be less precise since you’re counting drops rather than taking a standardized tablet.
Why Kelp Supplements Are Risky
Seaweed-based supplements, especially kelp, are the most popular “natural” iodine source. They’re also the least reliable. A study analyzing 14 commercially available kelp and seaweed supplements found iodine content ranged from 5 to 5,600 mcg per dose. That’s a more than 1,000-fold difference between products sitting on the same shelf.
The labels weren’t much help either. For nearly every supplement that declared an iodine amount, the actual measured content was lower than what the label claimed. In some cases the deviation was significant. Researchers concluded that the declared values on macroalgae supplements “are not reliable” and that such products “may pose a health risk” due to this unpredictability. If you’re trying to hit a specific daily target, a kelp capsule makes that nearly impossible.
Liquid Iodine: Lugol’s Solution and Drops
Lugol’s solution is a liquid blend of elemental iodine (5%) and potassium iodide (10%) in distilled water. It’s been around since 1829 and is still used in certain clinical settings, particularly for thyroid-related procedures. For everyday supplementation, it has some practical downsides: it tastes bitter, can be corrosive to tissues, and is typically mixed into juice to mask the flavor. Dosing requires careful measurement with a dropper.
Other liquid options include saturated potassium iodide solutions and various branded iodine drops (often marketed as nascent or colloidal iodine). These can work fine if you’re comfortable measuring drops and want flexible dosing. For most people, though, a tablet is simpler and harder to get wrong.
How Much You Actually Need
The recommended daily intake for adults is 150 mcg. Pregnant women need 220 to 250 mcg, and breastfeeding women need 250 to 290 mcg. The American Thyroid Association specifically recommends that pregnant and lactating women take a supplement providing 150 mcg of iodine daily, on top of dietary sources, and that all prenatal vitamins contain at least that amount.
The tolerable upper limit for adults, including pregnant women, is 1,100 mcg per day. Going above that raises the risk of thyroid dysfunction. Many iodine supplements on the market contain doses well above the RDA, sometimes 500 mcg or more per capsule. Higher is not better here. Excess iodine can actually cause hypothyroidism or hyperthyroidism, particularly in people with underlying thyroid conditions.
Foods That Interfere With Iodine
Certain foods contain compounds called goitrogens that block your thyroid from using iodine effectively. Cruciferous vegetables like broccoli, cauliflower, kale, and cabbage contain substances that inhibit iodine transport into the thyroid. Cassava and sweet potatoes contain a different class of goitrogens with similar effects. This doesn’t mean you need to avoid these foods, but if your diet is heavy on cruciferous vegetables and light on iodine-rich foods like dairy, eggs, and seafood, your effective iodine intake may be lower than you think.
Research from New Caledonia found that women who ate a lot of cruciferous vegetables and had low iodine intake had nearly double the risk of thyroid problems compared to those with adequate iodine. The takeaway: goitrogens matter most when your iodine status is already marginal.
Do You Actually Need a Supplement?
Most people in developed countries get enough iodine from iodized salt, dairy products, seafood, and eggs. A single half-teaspoon of iodized salt contains roughly 150 mcg of iodine. If you eat a varied diet and use iodized salt, supplementation is often unnecessary.
Supplementation makes the most sense for people who avoid dairy, don’t eat seafood, use non-iodized salt (sea salt, Himalayan salt, kosher salt), or follow a vegan diet. Pregnant and breastfeeding women have a strong case for supplementing, given the higher demands and the critical role iodine plays in fetal brain development.
If you already have a thyroid condition, particularly Hashimoto’s thyroiditis or other autoimmune thyroid disease, iodine supplements can make things worse. The Mayo Clinic advises against taking iodine supplements for hypothyroidism unless specifically instructed by a doctor, noting that excess iodine “could cause hypothyroidism or make it worse” in people whose condition isn’t caused by deficiency.
How to Test Your Iodine Levels
The gold standard is a 24-hour urine iodine test, which measures how much iodine your body excretes over a full day. A more involved version, the iodine loading test, has you take 50 mg of iodine and then collect urine for 24 hours. If you excrete 90% or more, your stores are adequate. If you excrete less, say 75%, it suggests your body is holding onto iodine because it needs more.
You may have seen the iodine skin patch test promoted online, where you paint iodine tincture on your forearm and see how fast it disappears. This test is unreliable and should not be used to determine your iodine status. The rate at which the stain fades depends on skin temperature, humidity, and evaporation, not your internal iodine levels.
Choosing the Right Supplement
For most people who’ve confirmed they need supplementation, a potassium iodide tablet in the 150 mcg range checks every box: well-studied, precisely dosed, stable, and inexpensive. Look for a product that lists potassium iodide (not kelp or “iodine complex”) as the iodine source, and check that the dose per serving doesn’t wildly exceed 150 mcg unless you have a specific reason to take more.
If you’re pregnant, verify that your prenatal vitamin contains iodine. Many do not. The American Thyroid Association has flagged this as a gap, recommending 150 mcg of supplemental iodine during pregnancy and lactation regardless of dietary intake. Potassium iodide is the preferred form in prenatal supplements.
Avoid supplements that combine iodine with large doses of other thyroid-targeted ingredients like selenium, tyrosine, or ashwagandha unless you have a clear reason to take them. These “thyroid support” blends can complicate dosing and introduce variables you don’t need. Simple, single-ingredient potassium iodide gives you the most control.