Selenium Deficiency: Who's at Risk and How to Test

Quick answer

If you have a thyroid condition, Crohn's or celiac disease, or eat a very restricted diet, ask your doctor for a selenium blood test before supplementing. Most adults get enough selenium from food, but those at risk may need 55–200 mcg daily. Do not exceed 400 mcg per day without medical supervision.

Selenium deficiency is rare in healthy adults who eat a varied diet, but it happens in specific groups: people with thyroid disorders, inflammatory bowel disease, or those on long-term intravenous nutrition. It also shows up in regions where soil selenium is low, like parts of China, Europe, and New Zealand.

The tricky part is that signs of low selenium overlap with many other issues—fatigue, brain fog, hair thinning. That's why testing matters more than guessing. This guide walks through who actually needs a selenium blood test, what the results mean, and how to correct a shortfall without overdoing it.

Symptoms that might point to low selenium

Selenium deficiency symptoms are vague on their own. You might notice persistent fatigue, difficulty concentrating, or hair and nail changes. Some people develop a weakened immune response and get sick more often. In more severe cases—usually tied to long-term malabsorption—selenium deficiency can contribute to heart muscle weakness (Keshan disease) or thyroid dysfunction.

None of these signs are unique to selenium. Fatigue could be low iron, poor sleep, or thyroid issues. Hair thinning can come from stress, protein deficiency, or genetics. That's why symptoms alone are not a diagnosis. If you have several of these and belong to a risk group, testing is the logical next step.

• Fatigue and mental fog that don't improve with rest
• Hair thinning or brittle nails without another clear cause
• Frequent infections or slow wound healing
• Thyroid symptoms like cold intolerance or unexplained weight changes

Who actually needs a selenium blood test

A selenium blood test measures the amount of selenium in your serum or plasma. It's not a routine screening test for everyone. Doctors typically order it when someone has a condition that impairs nutrient absorption, or when they show symptoms consistent with deficiency.

The strongest reasons to test: diagnosed thyroid disease (especially Hashimoto's or Graves'), Crohn's disease, ulcerative colitis, celiac disease, or a history of bariatric surgery. Also consider testing if you've been on parenteral nutrition for months, or if you follow a very limited diet—like a strict elimination diet or one based mostly on processed foods. Vegans and vegetarians can be at risk if they don't eat selenium-rich plant foods, but this depends heavily on where their food is grown.

• Ask your doctor for a serum selenium test, not a hair or nail test—those are less reliable
• Normal serum selenium is roughly 70–150 mcg/L, but lab ranges vary
• A result below 70 mcg/L may indicate deficiency, depending on your lab's reference range

Common causes of selenium deficiency

The most direct cause is not eating enough selenium-rich foods. Selenium content in plants depends on soil levels, so produce from low-selenium regions delivers less. Animal products like fish, eggs, and meat are reliable sources because animals concentrate selenium regardless of soil.

Malabsorption is the other major driver. Conditions like Crohn's, celiac, or chronic pancreatitis reduce your ability to absorb fats and fat-soluble nutrients, including selenium. Certain medications—like some chemotherapy drugs or long-term acid reducers—can also interfere. In rare cases, genetic mutations affect selenoprotein production, but this is very uncommon.

• Brazil nuts are exceptionally high in selenium—one nut can provide a full day's requirement
• Fish, shrimp, eggs, and organ meats are dependable animal sources
• Fortified foods and multivitamins may contain selenium, but check labels

How to correct a deficiency safely

If a blood test confirms low selenium, the fix depends on severity. For mild shortfalls, adding two or three Brazil nuts daily or a serving of fish a few times a week may be enough. For diagnosed deficiency, doctors often recommend 100–200 mcg of selenium daily as selenomethionine or sodium selenite. The RDA for adults is 55 mcg, but therapeutic doses are higher.

Do not self-prescribe high doses. Selenium has a narrow safety window. The tolerable upper limit is 400 mcg per day for adults. Chronic intake above that can cause selenosis—hair loss, brittle nails, garlic breath, and nerve damage. If you take a multivitamin, check its selenium content before adding another supplement.

• Choose selenomethionine for better absorption; it's the form found in food
• Take with a meal containing fat to improve uptake
• Recheck blood levels after 8–12 weeks to confirm you're in range

When to stop testing and focus elsewhere

If your selenium blood test is normal and you still have symptoms, selenium is not your answer. Continuing to supplement won't help and could push you toward toxicity. At that point, work with your doctor to investigate other causes—iron, B12, thyroid antibodies, sleep quality, or stress.

Also stop if you've corrected the deficiency and your levels are mid-range. There's no benefit to maintaining high-normal selenium. The goal is sufficiency, not optimization. For most people, food sources are enough once levels are restored.

• Normal test + persistent symptoms = look elsewhere
• Corrected deficiency + mid-range levels = maintain with food, not high-dose supplements
• Never exceed 400 mcg daily without medical supervision

Selenium symptom checker: cause, action, and stop point

• Fatigue and brain fog: Could be selenium, iron, B12, thyroid, or sleep. If you have a risk condition (Crohn's, celiac, thyroid disease), ask for a selenium blood test. Otherwise, check iron and B12 first. Test before supplementing. If selenium is normal, look elsewhere.
• Hair thinning and brittle nails: Common in selenium deficiency but also in zinc deficiency, protein malnutrition, or thyroid imbalance. A serum selenium test plus zinc and thyroid panel gives a clearer picture. If selenium is low, correct it. If normal, investigate zinc and thyroid.
• Known malabsorption condition: Crohn's, celiac, bariatric surgery, or chronic pancreatitis increase deficiency risk. Ask your gastroenterologist to include selenium in routine nutrient panels. Annual testing is reasonable. Supplement only if levels are low.
• Taking a multivitamin already: Check the selenium content. Many multis provide 55–200 mcg. Adding a separate selenium supplement on top can push you over 400 mcg. Total daily selenium from all sources should stay under 400 mcg.

Selenium in context with other supportive nutrients

• Vitamin D3 + K2 with Magnesium Glycinate: 5000IU naturally-sourced vitamin D3, 275mg chelated magnesium glycinate, and 50mcg vitamin K2 as MK-7; supports immune health, bone health, and heart health. Selenium works alongside vitamin D and magnesium in thyroid and immune function. If you're addressing a deficiency, these nutrients often need attention too.
• Multi Collagen Complex: 1650mg collagen per serving with digestive enzymes for absorption; contains egg and fish; supports skin, hair and nail, and joint health. Hair and nail changes can stem from low selenium or other deficiencies. Collagen provides building blocks but won't fix a mineral shortfall—test first.

Selenium deficiency is uncommon but not imaginary. If you belong to a risk group or have unexplained symptoms, a simple blood test can give you a clear answer. Correcting a shortfall with food or a targeted supplement is straightforward—just don't exceed 400 mcg daily and recheck your levels. If tests come back normal, move on to other causes instead of chasing selenium.