Your thyroid needs iodine to make its hormones and selenium to help activate them and protect the gland from damage. Most adults can meet both needs with ordinary foods: dairy, eggs, seafood and modest amounts of iodized salt for iodine, and fish, eggs, meat, whole grains and the occasional Brazil nut for selenium. The main caution is that more is not better. High-dose supplements of either one can cause harm, so food first, and supplements only with medical advice.
How iodine and selenium work together
Iodine is a building block of the thyroid hormones T4 and T3, which help regulate metabolism, energy use and body temperature. Without enough iodine, the thyroid cannot make enough hormone.
Selenium is part of the enzymes that convert T4 into its active form, T3, and of antioxidant enzymes that protect the thyroid as it works. A review in the International Journal of Endocrinology describes how selenium-containing deiodinase enzymes control thyroid hormone turnover, including the conversion of T4 to active T3.
That is why the two are often discussed together: iodine supplies the raw material, and selenium helps put it to use and keeps the gland protected.
How much you need
Health Canada’s dietary reference intakes for adults 19 to 50 are:
| Nutrient | Adults (RDA) | Pregnancy | Breastfeeding | Upper limit (all adults) |
|---|---|---|---|---|
| Iodine | 150 µg/day | 220 µg/day | 290 µg/day | 1,100 µg/day |
| Selenium | 55 µg/day | 60 µg/day | 70 µg/day | 400 µg/day |
Source: Health Canada, Dietary Reference Intakes, reference values for elements. The upper limit is the most you can take daily without likely risk of harm, from all sources combined.
Best food sources
Iodine
The NHS lists cows’ milk and dairy products, eggs, sea fish and shellfish as good sources. Plant foods contain some iodine, but levels depend on the soil they were grown in. In Canada, table salt is iodized, but sea salt, kosher salt and specialty salts often are not; check the label. You don’t need to add extra salt to get iodine. A small amount of iodized salt in home cooking, plus dairy or seafood, is usually enough.
Selenium
Good sources include Brazil nuts, fish, meat and eggs, plus whole grains and legumes. Brazil nuts are unusually concentrated, so think of them as one or two at a time, not a handful every day.
| Food | Iodine | Selenium |
|---|---|---|
| Fish and shellfish (cod, tuna, shrimp, salmon) | Good | Good |
| Eggs | Good | Good |
| Milk, yogurt, cheese | Good | Some |
| Brazil nuts | Little | Very high |
| Poultry and meat | Some | Good |
| Whole grains, legumes | Varies | Varies with soil |
| Iodized salt | Good | None |
Cautions: when more isn’t better
- Too much iodine. The NHS warns that high doses taken for long periods can change the way the thyroid works, causing symptoms such as weight gain. Kelp and seaweed supplements can vary widely in iodine content; check with your doctor before using them.
- Too much selenium. Excess selenium can cause selenosis. The thyroid review notes it generally arises when intake exceeds 400 µg per day, and the NHS notes that in its mildest form it can cause hair and nail loss. Watch for stacking: a multivitamin plus a selenium supplement plus Brazil nuts can add up.
- Vegan diets. Without dairy, eggs or fish, iodine can run low. The NHS suggests fortified foods, such as some fortified plant beverages, or a supplement after speaking with a professional.
Goitrogens, soy and gluten
- Cruciferous vegetables (broccoli, cabbage, kale, cauliflower) contain compounds that can interfere with iodine use in very large amounts. For most people who get enough iodine, normal servings, especially cooked, are not a concern, and these vegetables bring fibre and other nutrients.
- Soy foods are generally fine in usual amounts for people with adequate iodine. If you take thyroid medication, ask your pharmacist about spacing it from soy, calcium and iron, which can affect absorption.
- Gluten matters for people with coeliac disease, which is more common in people with autoimmune thyroid conditions. If you suspect coeliac disease, get tested before cutting gluten, since removing it first can affect the test.
For a wider look, including iron, see thyroid-friendly eating: selenium, iodine and iron. If you are cutting back on salt for blood pressure, salt sense explains how to keep iodine in mind while reducing sodium.
Sample menus
These days include dependable iodine and selenium sources without supplements.
Day 1
- Breakfast: Scrambled eggs on whole-grain toast, a glass of milk, orange slices
- Lunch: Tuna and white bean salad with greens and a lemon dressing
- Snack: Plain yogurt with berries and one Brazil nut, chopped
- Dinner: Baked cod, brown rice and roasted carrots, seasoned with a pinch of iodized salt
Day 2
- Breakfast: Oatmeal cooked in milk with sunflower seeds and banana
- Lunch: Egg salad sandwich on whole-grain bread, cucumber and cherry tomatoes
- Snack: Cheese and whole-grain crackers
- Dinner: Shrimp and vegetable stir-fry over quinoa
Day 3 (vegetarian)
- Breakfast: Greek yogurt parfait with oats and fruit
- Lunch: Lentil soup, whole-grain roll, a glass of milk
- Snack: Hard-boiled egg and an apple
- Dinner: Cheese and spinach omelette with roasted potatoes and steamed broccoli
FAQ
Should I take selenium for my thyroid?
Not without medical advice. Some studies have looked at selenium in specific autoimmune thyroid conditions, but results are mixed and the safe range is narrow. Your doctor can tell you whether it makes sense for you.
I use sea salt. Am I missing iodine?
Possibly, if you also eat little dairy, eggs or seafood. Look for the word “iodized” on the package. You can keep using sea salt for finishing and use a small amount of iodized salt in cooking.
Can diet fix an underactive thyroid?
Food can prevent iodine or selenium shortfalls, but it cannot treat hypothyroidism caused by an autoimmune condition or other causes. Follow your treatment plan and use diet to support overall health.
When to check with a professional
See your doctor if you notice ongoing tiredness, feeling cold, unexplained weight changes, a swelling at the front of your neck, a racing heart, or changes in your periods. These can have many causes, and thyroid conditions are diagnosed with blood tests, not diet. If you have hypothyroidism, hyperthyroidism or Hashimoto’s thyroiditis, or are pregnant, don’t take iodine or selenium supplements without medical advice; some thyroid conditions are worsened by extra iodine. A registered dietitian can help tailor your meals.
Key takeaways
- Iodine builds thyroid hormones; selenium helps activate and protect them.
- Adults need about 150 µg of iodine and 55 µg of selenium a day.
- Dairy, eggs, seafood and iodized salt cover iodine; fish, eggs, meat and a Brazil nut cover selenium.
- Avoid high-dose supplements and kelp products unless your doctor recommends them.
- Thyroid symptoms need a blood test, not a diet experiment.