Iodine is vital for healthy thyroid function and too little can contribute to an underactive thyroid. It’s found in foods including dairy, fish, and iodized salt.
The best iodine supplement is generally recognized to be potassium iodide. However, getting the necessary amount shouldn’t require supplementation unless you’re pregnant or breastfeeding.
Some “natural” iodine supplements like kelp may deliver erratic and sometimes toxic levels of iodine.
Too much iodine can cause as many problems for thyroid health as too little.
Eating a gut-healthy diet can help improve your thyroid health.
Iodine is an essential mineral that we all need for healthy thyroid function. 70-80% of iodine in the body is contained within the thyroid gland . For iodine to get into the thyroid gland it requires us to obtain this nutrient through diet or via supplementation.
In this article we’ll consider the best iodine supplement (spoiler: it’s potassium iodide), while also noting that the best iodine supplement may be none at all.
If your thyroid is a bit sluggish and you suspect you may be deficient in this important nutrient, this is the right place for you. Read on to find out if you’re actually iodine-deficient, how to boost your levels, and other ways to improve your thyroid health.
The Role of Iodine in Thyroid Health
Iodine is needed for the production of thyroid hormones .
The two main thyroid hormones, triiodothyronine (T3) and thyroxine (T4), both contain iodine [2, 3]:
T3 contains three iodine molecules
T4 contains four iodine molecules
Iodine deficiency is the most common cause of hypothyroidism (underactive thyroid) worldwide . Insufficient iodine intake is also a risk factor for Hashimoto’s thyroiditis [5, 6, 7, 8].
Not getting enough iodine can also result in:
Goiter, which is an abnormal growth of the thyroid gland 
Miscarriage, preterm birth, and impaired infant brain development [2, 10]
The recommended intake of iodine for adults to ensure your thyroid works properly is 150 micrograms (mcg) a day. Pregnant women need 220 micrograms per day and breastfeeding women 290 micrograms per day .
Hypothyroid vs. Hashimoto’s
The two thyroid diseases that are most relevant to our topic of the best iodine supplement are Hashimoto’s thyroiditis and hypothyroidism. So what’s the difference between the two?
Hashimoto’s thyroiditis is an autoimmune condition in which the immune system starts to attack the thyroid gland. Registering elevated levels of thyroid peroxidase (TPO) or thyroglobulin (TG) antibodies in a blood test is the usual indicator of this condition .
Hypothyroidism is when your thyroid stops producing enough of the crucial thyroid hormones that keep your metabolism going. Hashimoto’s thyroiditis is a prime risk factor for developing hypothyroidism, but with the right preventive action you may stop it from occurring.
In a large thyroid study, only 10-20% of participants with elevated TPO antibodies progressed to full-blown hypothyroid disease after nine years .
Who’s Most at Risk of Low Iodine?
Those most at risk for iodine deficiency include [1, 10]:
Pregnant and breastfeeding women
Babies and young children
People who eat few or no dairy products or seafood
People who eat excessive amounts of goitrogen-rich foods (e.g. cruciferous veg, soy, and cassava) that interfere with the uptake of iodine into the thyroid.
Pregnant Women Need Iodine Supplements
Women who are trying to conceive, and women who are pregnant or breastfeeding are the only group officially recommended to supplement with iodine, at a dosage of 150 micrograms a day. They have extra iodine needs to ensure a healthy pregnancy and proper development of the infant .
However for everyone else, it’s likely better to get iodine through food to avoid negative side effects (more on that below).
Picking the Best Iodine Supplement
To ensure you do actually need iodine, you can track your dietary iodine intake with an app such as Cronometer (or see the food sources below). A 24-hour urinary iodine-creatinine ratio test is also an accurate option for testing iodine status .
If these indicate that you’re definitely not getting enough iodine, the safest and best iodine supplement to take (as recommended by the American Thyroid Association) is potassium iodide.
Potassium iodide is the form of iodine that is most well-studied, and the type that’s recommended during preconception, pregnancy, and lactation [1, 15].
Sodium iodide is another form that is safe and effective, but many people want to reduce their intake of sodium because we eat too much of it as a general rule. (Just to be clear, sodium iodide is a concentrated source of iodine used in supplements, and is distinct from iodized table salt, which is sodium chloride with iodine added).
More than 1,100 micrograms per day of iodine isn’t recommended under any normal circumstances and may result in thyroid dysfunction .
It’s wise to stick to less than 500 micrograms daily, and 150 micrograms is probably the safest place to start [15, 16].
Too Much of a Good Thing
While iodine is vital for a healthy thyroid, multiple studies have shown iodine supplements may also actually increase the overall incidence of hypothyroid conditions, including goiter and autoimmunity [17, 18, 19].
Excessive iodine is a risk factor for both hyperthyroidism (an overactive thyroid gland) and for Hashimoto‘s disease too [5, 6, 7, 8, 20].
In some cases, people with hypothyroidism or Hashimoto’s may actually respond to reducing iodine intake. For example:
In one study, a third of hypothyroid patients returned to normal thyroid function after three weeks of iodine restriction. When iodine was resumed, they became hypothyroid once more 
63% decreased their TSH values by more than 50%, to less than 10 mU/L, solely from iodine restriction .
In short, more is definitely not better and high levels of iodine can harm your thyroid health just as much as too little.
If you’re interested in optimizing your thyroid function with iodine, but are concerned about overdoing it, food sources (or a low-dose iodine supplement) are probably the best way to go.
Food Sources of Iodine
Below are some of the best sources of iodine and the amount of the mineral they contain as a percentage of the daily recommendation (150 mcg) .
By far the best sources are dairy (especially milk), seafood (especially white fish), and iodized salt. However, you’ll also get iodine from bread that’s been made with potassium iodate or calcium iodate as a dough conditioner (this won’t be apparent from the label).
mcg of Iodine per Serving
% of Daily Recommended Amount
Cod, baked, 3 ounces
Oysters, cooked, 3 ounces
Milk, nonfat, 1 cup
Iodized table salt, ¼ teaspoon
Fish sticks, cooked, 3 ounces
Pasta, enriched, boiled in water with iodized salt, 1 cup
Ice cream, chocolate, ⅔ cup
Egg, hard boiled, 1 large
Cheddar cheese, 1 ounce
Liver, beef, cooked, 3 ounces
Shrimp, cooked, 3 ounces
Tuna, canned in water, drained, 3 ounces
Fish sauce, 1 tablespoon
Beef, chuck, roasted, 3 ounces
Chicken breast, roasted, 3 ounces
Take Care With Kelp
Organic kelp and other forms of concentrated seaweed are often promoted as an alternative natural iodine supplement, but should be treated with caution because iodine levels in kelp can be very unpredictable. One study found that the iodine content of seaweed and kelp supplements ranged between 5 and a whopping 5,600 micrograms . By comparison, potassium iodide always delivers a reliable iodine dose .
Focus on an Anti-Inflammatory Diet
Rather than getting hung up on what might be the best iodine supplement, it’s a good idea to focus more generally on the broader dietary changes that can promote a healthy thyroid.
In fact, I can count on the fingers of one hand how many times a patient of mine with thyroid issues such as Hashimoto’s or hypothyroidism has genuinely needed to take iodine in the past year.
What actually works for most people is targeting the underlying inflammation and autoimmunity that underlies most cases of thyroid dysfunction. Remember, Hashimoto’s thyroiditis, which proceeds many cases of hypothyroidism, happens because the body’s immune system starts to attack the thyroid gland.
Targeting inflammation so you can improve thyroid health starts with:
Eating a healthy, unprocessed diet full of non-starchy veggies, quality proteins, and healthy fats
Balancing blood sugar
Working out which foods you might have sensitivity to
Try a Paleo Diet
When eliminating potentially troublesome foods that could be contributing to thyroid issues, a gradual approach is best.
For many people with thyroid issues, a fully or mostly gluten-free, Paleo-style diet works well. A Paleo diet is along the lines of what our ancestors ate and minimizes common inflammatory foods. It still allows you to eat a wide variety of healthy foods, but is often sufficient to ease symptoms such as fatigue, brain fog, and weight gain (which frequently accompany hypothyroidism).
If this approach doesn‘t make you feel sufficiently better, you can work with a low FODMAP diet instead. A little more restrictive than standard Paleo, low FODMAP temporarily eliminates foods that feed overgrown bacteria in the gut and is often the most effective approach for thyroid issues in people who also have stubborn GI symptoms.
The Gut-Thyroid-Probiotic Connection
Getting your underlying gut health sorted is often key to addressing your thyroid health too, as there’s a link between the two and the symptoms of both can be very similar.
Increased gut permeability (a leaky gut) has shown to occur more commonly in those with a dysfunctional thyroid, and is linked with more thyroid symptoms . It’s therefore logical that taking probiotics, which can improve intestinal permeability , may also help improve thyroid symptoms. Though proper research in this area is still lacking, the benefits of probiotics on thyroid health is something we observe in the clinic all the time.
One randomized controlled trial found that through taking probiotics, people with hypothyroidism decreased their levels of TSH or thyroid stimulating hormone (indicating improved thyroid function), had improved fatigue, and reduced the required dose of medication .
Other Dietary Supplements for Thyroid Problems
We’ve covered what is the best iodine supplement, but bear in mind that there may be more useful supplements to support your thyroid health. Some of the key ones that could help include (after you’ve made the relevant diet changes) are:
Ginger: A 2022 randomized, placebo-controlled clinical trial found that ginger improved hypothyroid symptoms and metabolic markers for those who have normal TSH levels but persistent thyroid symptoms .
Selenium and inositol: A 2020 study published in the Endocrine journal found that a combination of these two nutrients led to lower TSH levels when used for up to 12 months .
Vitamin D: Deficiency of vitamin D has shown in research to be associated with the severity of hypothyroidism , and those with hypothyroid disease. A meta-analysis concluded that vitamin D supplementation improves thyroid antibody levels .
Iodine Is Only One Part of Thyroid Health
Improving your thyroid health isn’t just about searching for the best iodine supplement. Indeed it’s usually safer to work on increasing dietary sources of iodine, as overdoing iodine intake through supplements can actually create more of the thyroid hormone imbalances that you’re trying to solve.
Most people will benefit from more fundamental diet improvements for thyroid issues like Hashimoto’s or hypothyroid. For example, a Paleo-style diet, backed up with probiotics, works for many people to reduce inflammation and autoimmune symptoms.
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