The recommended daily amount of selenium for adults is 55 micrograms (mcg), with an upper limit of 400 mcg from all sources combined. Most people get enough from food alone, but if you’re supplementing, the form you choose, when you take it, and how much you take all matter for safety and absorption.
How Much Selenium You Need
Adults need 55 mcg of selenium per day. Pregnant women need 60 mcg, and breastfeeding women need 70 mcg. Children’s needs range from 20 mcg for toddlers up to 55 mcg by age 14. These numbers represent the recommended dietary allowance, meaning the amount that covers the needs of most healthy people.
The tolerable upper limit for adults is 400 mcg per day, and that includes everything: food, drinks, and supplements. Going above this threshold regularly is where problems start. If you already eat selenium-rich foods, a high-dose supplement can push you past this ceiling faster than you’d expect.
Choosing the Right Form
Selenium supplements come in two broad categories: organic and inorganic forms. The distinction matters because your body handles them differently.
Selenomethionine is the organic form most commonly sold in supplements. It’s the same form found naturally in foods, and your body absorbs and retains it more effectively. Research comparing organic selenium sources to inorganic sodium selenite shows that organic forms are significantly better at raising selenium levels in tissue and boosting antioxidant enzyme activity. Selenomethionine can also be stored in your muscles and other tissues, giving your body a reserve to draw from.
Sodium selenite is the inorganic form. It’s cheaper and still raises blood selenium levels, but less of it ends up deposited in your tissues. If you’re choosing a supplement, selenomethionine or selenium yeast (which contains selenomethionine) is generally the better option for long-term status.
When and How to Take It
Selenium supplements can be taken with or without food, though taking them with a meal reduces the chance of stomach upset. There’s no strong evidence that time of day matters, so consistency is more important than timing. Pick a time you’ll remember and stick with it.
If you’re also taking vitamin E or vitamin C, you can take them alongside selenium without issues. These nutrients work together as part of your body’s antioxidant system. Selenium serves as a building block for a family of enzymes called glutathione peroxidases, which neutralize harmful compounds before they can damage cells. Vitamins C and E support related pathways, so they complement rather than compete with selenium.
Food Sources Before Supplements
Before reaching for a pill, consider your diet. A single Brazil nut contains 68 to 91 mcg of selenium, which already exceeds the daily recommendation. Eating just one or two Brazil nuts a day is one of the simplest ways to maintain adequate levels without a supplement. The selenium content varies depending on the soil where the tree grew, with nuts from different regions of the Amazon containing meaningfully different amounts, but even on the low end, one nut gets you well past 55 mcg.
Other good sources include seafood (especially tuna, halibut, and sardines), organ meats, eggs, and whole grains. Most Americans already get enough selenium from food alone, which is why supplementing without a specific reason can tip you into excessive territory.
Selenium for Thyroid Conditions
One of the most common reasons people specifically seek out selenium is thyroid health, particularly Hashimoto’s thyroiditis. Your thyroid contains more selenium per gram of tissue than any other organ, and it relies on selenium-dependent enzymes to manage both hormone production and oxidative stress.
Clinical studies on Hashimoto’s have tested doses of 200 mcg per day for six months. The American Thyroid Association notes that selenium supplementation at this dose was associated with slightly reduced thyroid antibody levels and improved thyroid hormone markers. The strongest results came from the study using 200 mcg daily, though the overall effect was modest. Selenium’s antioxidant properties help reduce the inflammation that drives autoimmune thyroid damage.
If you’re supplementing for thyroid reasons, 200 mcg is the dose most studied, but it’s well above the standard recommendation of 55 mcg. This is a dose worth discussing with your provider rather than starting on your own, especially since you’re also getting selenium from food.
Interactions to Watch For
Selenium interacts with a few categories of medication. If you take immunosuppressive drugs, particularly after an organ transplant, selenium can work against them. Selenium stimulates immune activity, which is the opposite of what those medications are designed to do.
Birth control pills may slightly raise blood selenium levels on their own. The clinical significance isn’t well established, but it’s worth knowing if you’re also supplementing. The combination is unlikely to cause problems at standard supplement doses, but it’s another reason to stay within recommended ranges.
Signs You’re Taking Too Much
Selenium toxicity, called selenosis, develops from chronically exceeding the 400 mcg upper limit. The earliest signs are distinctive: brittle hair, nails that become deformed or start breaking easily, and a garlic-like odor on the breath. These symptoms were documented extensively in Chinese villages where soil selenium levels were extremely high and residents ate locally grown food for months to years.
More advanced toxicity can cause fatigue, dizziness, irritability, and in severe cases, loss of feeling and motor control in the arms and legs. There’s no reliable blood test threshold that predicts exactly when symptoms will appear, though very high blood selenium levels clearly correlate with selenosis. The key point is that the margin between a helpful dose and a harmful one is narrower with selenium than with many other nutrients. At 55 mcg you’re meeting your needs. At 400 mcg you’re at the ceiling. At 800 or 1,000 mcg you’re in dangerous territory.
This is especially relevant for Brazil nut enthusiasts. Eating five or six Brazil nuts daily could put you close to or above the upper limit all by itself. If you also take a supplement and eat selenium-rich foods, the numbers add up quickly.
A Practical Approach
For most people, the simplest strategy is eating one or two Brazil nuts a few times per week and letting the rest come from a varied diet. If you prefer a supplement, a 50 to 100 mcg selenomethionine capsule covers any dietary gaps without approaching the upper limit. Take it with food, at whatever time fits your routine.
If you’re supplementing at higher doses for a specific condition like Hashimoto’s, the 200 mcg dose used in clinical research is the most studied. At that level, keep close track of your total intake from all sources, and be aware that the window between therapeutic and excessive is relatively small.