Selenium

What is selenium?

Selenium is a trace element, a mineral you need only very little of, that your body builds into a group of about twenty-five proteins. The best known are the enzymes that capture the oxygen radicals that arise during combustion in your cells, and the enzymes that in your liver, kidneys and muscles convert the thyroid hormone T4 into the active T3. Without enough selenium your antioxidants work less and the thyroid becomes sluggish, even if it makes enough T4 itself. You take in selenium through food, and how much is in it depends on the soil the food grew on. North American soil is rich in it, European soil poor, and that is why a Dutch person on average sits at the low end of the band. Fish, meat, eggs, whole grains and dairy deliver the most; Brazil nuts are a case apart, because one nut contains roughly the daily amount, and a handful a day is too much. The blood value reflects your intake of the past weeks. Your report shows selenium in µmol/L or in µg/L, and that matters: three labs report in molar units and one in mass, and the numbers cannot be set side by side. Normal from 16 years is about 0.8 to 1.8 µmol/L, where the labs reporting in molar units overlap. Above 2.5 µmol/L it becomes toxic, and you only reach that with supplements.

Why is selenium relevant?

Selenium is the value you read with the thyroid and with the question of recovery. Anyone with an underactive thyroid, or a free T4 that is normal while free T3 sits low, looks at selenium because the enzyme that converts T4 into T3 needs it. In autoimmune thyroid disease selenium dampens the inflammation in the thyroid a little, and that is the reason it is often topped up in Hashimoto's, though the gain is modest and only shown in anyone who takes in too little. For anyone who does sport or works hard, selenium is one of the minerals behind the antioxidants that clear the damage of a hard effort. A deficiency gives no clear symptoms, but goes with less muscle strength, a weaker immune system and, in the regions where the soil is really poor in it, with a heart muscle that weakens. In the Netherlands a real deficiency is rare in anyone who eats a varied diet, and more common in anyone who eats one-sidedly for long, drinks a lot of alcohol, has a bowel disease or is fed through a tube. The other side is the pitfall. Selenium is toxic in an amount not so far above the need: anyone who takes a daily supplement and eats Brazil nuts alongside can pass the upper limit, and then you lose hair, your nails break and your breath smells of garlic. More is not better here, and a supplement without a measured deficiency is not needed with a varied diet.

Selenium reference ranges

Look at the unit first here and only then at the number: three labs report selenium in molar units, in µmol/L, and one in mass, in µg/L, and those two scales do not differ by a power of ten, so the numbers cannot be set side by side. This page works in µmol/L. A low value is almost always diet; the upper limit is real, because selenium is toxic in an amount not far above the need.

Selenium high or low: what it means

A low value means too little comes in or too little is absorbed. The most common explanation first: food from selenium-poor soil, because European soil is poor in it and anyone who eats little fish, meat, eggs and whole grains sits at the low end; poor absorption in the gut with coeliac disease, Crohn's disease or after stomach surgery; and inflammation, because with an infection or an illness that has been running a while the blood value falls temporarily without the store being smaller. Anyone who eats plant-based, drinks a lot of alcohol or is fed through a tube for long sits low more often. A high value almost always comes from outside. Supplements are the most common reason, certainly in combination with Brazil nuts, and a handful of Brazil nuts a day does it even without a supplement. Above 2.5 µmol/L it becomes toxic, with hair loss, breaking nails, a garlic breath, nausea and tingling, and then you stop everything that contains selenium and measure again after a few months. What you do about it: with a low value you eat fish two to three times a week, and otherwise eggs, meat, whole grains and dairy, and take two to three Brazil nuts a week, because that is enough to get into the band; a supplement of fifty to a hundred micrograms a day is the way with a real deficiency or a bowel disease, and more than that makes no sense. In Hashimoto's topping up can dampen the inflammation a little, but only if you take in too little. With a high value you stop supplements and Brazil nuts. Read selenium beside free T4 and free T3, because the enzyme that does the conversion needs selenium, beside CRP, because inflammation lowers the value temporarily, and with the unit on your report alongside, because µmol/L and µg/L cannot be set side by side.

How does a selenium blood test work?

Referral

Not needed. You can have selenium measured yourself, though with a varied diet it is rarely needed; it counts with thyroid symptoms, a bowel disease or one-sided eating.

Fasting

Not needed; the value reflects your intake of the past weeks and does not move with a meal.

When

Not during or just after an infection, because inflammation lowers the value temporarily. Skip a supplement for a few days if you want to know where your diet gets you.

The draw

One tube of blood from a vein in your arm, in a special tube without traces of metal; the person drawing avoids sweat on the skin. Free T4, free T3 and CRP come from the same draw. Look at the unit on the report.

Repeating

Three months after adjusting your diet or starting a supplement, and after that no more often than once a year. With a high value three months after stopping.

Where to draw

You can draw at 250+ locations near you, no referral needed. See the locations

Educational information only, not medical advice. Consult a healthcare professional for clinical decisions.