Selenium Blood Test: A Narrow Window Between Too Little and Too Much

US reports give selenium in µg/L, the same figure as ng/mL. Intervals vary: roughly 70 to 150 µg/L at many labs and 93 to 198 at Labcorp.

The type of sample matters even more. Serum and plasma follow recent intake, while whole blood, which includes red cells, reflects longer-term status and reads higher in the same person. The UK's NHS Supraregional Assay Service puts adult serum or plasma selenium at 0.75 to 1.46 µmol/L and whole blood at 1.13 to 1.95 µmol/L, about 59 to 115 µg/L against 89 to 154. Comparing a whole-blood result with a serum range can make a normal level look high.

To convert, divide µg/L by 78.96, selenium's atomic weight: 100 µg/L is about 1.27 µmol/L. The NIH sometimes uses mcg/dL, so its sufficiency mark of 8 mcg/dL is 80 µg/L. The guide to SI and conventional units explains these conversions in general.

In North America a low selenium is unusual, because food comes from many regions and evens out poor soil. Low results turn up in medical settings: intravenous feeding (parenteral nutrition) for more than a week without added selenium, severe malnutrition, malabsorption, and long-term hemodialysis. People living with HIV run low more often, and so do people eating mostly vegetables grown in low-selenium soil, as in parts of China and some European vegan groups.

These situations rarely affect one nutrient alone, so selenium is usually checked with zinc and copper on a vitamins and nutrients panel. A typical case is someone three months into home intravenous feeding whose selenium has fallen from 95 to 60 µg/L: below the NIH mark and most lab ranges, in exactly the setting where a shortage is expected.

Low selenium rarely causes illness on its own; it leaves the body less able to cope with other stresses. Where intake is extremely low, two diseases are known. Keshan disease, a heart-muscle disease first described in low-selenium areas of China, occurred where adults averaged 10 mcg a day or less, and at least 20 mcg a day protects against it. Kashin-Beck disease, a form of osteoarthritis in children, occurs in low-selenium, low-iodine regions of China, Tibet, Siberia and North Korea. A selenium shortage also worsens iodine deficiency, raising the risk of congenital hypothyroidism.

Chronically high intake causes selenosis. Its first signs are unusually specific: a garlic smell on the breath and a metallic taste in the mouth. Hair loss and brittle or lost nails are the most common signs, followed by skin rash, nausea, diarrhea, tiredness, irritability and nerve problems. Many people search for these symptoms without connecting them to a daily supplement.

Acute poisoning is rarer and more serious. In 2008, 201 people had severe reactions to a liquid supplement that contained 200 times the selenium stated on its label.

The safe upper limit is debated. In 2023 the European Food Safety Authority set 255 mcg a day for adults, about 150 mcg below the NIH limit. Someone taking a 200 mcg capsule on top of an average US diet gets close to 300 mcg a day: under one limit and over the other. The NIH limit does not apply to people taking selenium under medical supervision.

The thyroid holds more selenium for its size than any other organ. Selenoproteins help convert the thyroid hormone T4 into its active form, T3, and protect the gland during hormone production. That is why selenium supplements are often promoted for Hashimoto's thyroiditis.

The evidence is narrower than the promotion. A 2024 meta-analysis of 35 trials, summarized by the American Thyroid Association, found supplements linked to slightly lower TSH in people not taking thyroid hormone and lower anti-TPO antibodies. Free T4, free T3, thyroglobulin antibodies and thyroid size did not change, and among trials that measured selenium at the start, half enrolled people who were already severely deficient.

Supplements do not raise selenoprotein activity in people who are not deficient, and the ATA recommends against selenium supplements in pregnant women with thyroid autoimmunity. If a thyroid problem is the concern, thyroid tests answer it; a selenium level does not.