Selenium deficiency: what lab values show

A selenium deficiency caused by diet alone is rare in Europe; where one occurs, there is usually an illness behind it that impairs uptake or increases losses. Selenium status can certainly be measured — there are several blood markers, and procedures for nail and urine samples as well. Only, these values answer a narrower question than one hopes: they reliably show who is in the lower intake range. Above that they run into a plateau and barely move any further, no matter how much extra is supplied.

So both statements hold: "my selenium value is low" is not yet a deficiency, and "my selenium value is high" is not yet good provision. What lies between is not decided by the number alone, but by the material, the laboratory, the timing of the blood draw and the question of whether an inflammation is currently running.

What counts as a deficiency — and where it actually occurs

The German Nutrition Society (DGE) describes the consequences as follows: with long-term absent selenium intake, or with mutations in genes affecting selenium metabolism, the immune system is impaired and muscle function and sperm formation are disturbed.

According to the DGE, a deficiency from low intake occurs "only in certain rural areas with a low selenium content of the soil and predominant consumption of regional produce" — above all in the highlands of central Africa and Asia. In certain regions of China, deficiency diseases appeared at an intake of around 10 µg per day: Keshan disease (a disease of the heart muscle) and Kashin-Beck disease (joint changes, reduced bone growth). For comparison: EFSA estimates adult intake in several EU countries at 31 to 66 µg per day, that is three to six times as much. The reference values quoted in Europe lie between 55 and 70 µg per day, depending on the body issuing them.

In Europe, a risk of undersupply therefore exists primarily with illnesses involving reduced utilisation or increased loss of selenium — the DGE names chronic inflammatory bowel diseases, cystic fibrosis, and renal insufficiency and chronic dialysis. The same logic appears in the package leaflet of the prescription and pharmacy-only selenium medicines: the indication is "proven selenium deficiency that cannot be remedied nutritionally", with maldigestion and malabsorption states as well as malnutrition and undernutrition being named.

Why the symptoms point in both directions

The signs people search for online are hair loss, brittle nails and fatigue. That is exactly where self-assessment fails — because the same signs appear a second time at the other end of the scale. The DGE describes the later course of selenosis, that is a chronically excessive intake, as "loss of hair, disturbed nail formation"; in the first stage with neurological disturbances, fatigue, joint pain, nausea and diarrhoea. Hair loss is even the endpoint from which EFSA derived the upper limit of 255 µg per day in 2023.

A symptom that turns up at both ends cannot distinguish between them. And the signs of a genuine deficiency — impaired immune system, disturbed muscle function — are so unspecific that they fit dozens of other causes.

The markers and what they measure

In 2023, EFSA compiled which biomarkers exist and what each of them reflects. The difference between them is no subtlety: they measure different selenium pools and different time frames.

Marker	What it contains	Time window	What disturbs it
Serum or plasma selenium	non-cellular selenium: selenoproteins, albumin-bound selenomethionine, inorganic selenium	short to medium term	inflammation, illness, smoking, age, sex, chemical form of the selenium supplied
Whole-blood selenium	cellular and non-cellular selenium	medium to long term	age, smoking, illness, region of origin
Erythrocyte selenium	above all glutathione peroxidase 1 and haemoglobin-bound selenium	medium to long term	unaffected by inflammatory reactions; responds more slowly than serum
Selenoprotein P in plasma	20 to 70 % of plasma selenium, predominantly formed in the liver	responds quickly, but only in the lower range	inflammation, oxidative stress, insulin and glucose metabolism
Glutathione peroxidase activity	GPx3 in plasma corresponds to 10 to 25 % of plasma selenium	quickly	oxidative stress, gene variants, physical activity
Urinary selenium	main route of excretion	short term	kidney function, selenium form, sex
Toenail or hair selenium	deposited selenium	months	growth rate, external contamination — for instance by selenium-containing shampoos

The last row is remarkable. Nail analysis is regarded as a long-term marker, and in some studies it fits intake well — in others no association was found at all, neither with intake nor with blood selenium. EFSA also records that nail samples demand standardised collection procedures because they are easily contaminated.

Reference ranges: where the numbers on the report come from

The range most frequently quoted in Germany does not come from a nutritional recommendation but from the patient information of the sodium selenite medicines: "Treatment should continue until normalisation of selenium status (selenium in plasma 80–120 µg/l, in whole blood 100–140 µg/l). Regular checking of the selenium level at appropriate intervals is recommended."

That is a treatment target in proven deficiency, not a target value a healthy person would have to aim for. Two things follow from this for reading a report:

• Note the material. Plasma and whole blood have different ranges and cannot be converted into one another. A value of 95 µg/l means something different in plasma than in whole blood.
• Note the laboratory. Reference ranges arise from the measured values of the population examined in each case. Since the selenium content varies strongly by geography, the ranges given differ between laboratories — values from two institutions cannot be placed side by side directly.

There is a third point on top, which weighs particularly heavily with selenium: which compound is in the preparation changes the serum value independently of whether anything is happening at the selenium-dependent enzymes. Why that is so is set out in Selenium forms compared.

The plateau: why a high value says little

The functional markers have an upper bound beyond which they simply stop responding. EFSA puts the figures like this:

Marker	Moves up to …
Glutathione peroxidases	reach their maximum activity at an intake of 40 to 60 µg per day
Selenoprotein P in plasma	plateau at an intake of 60 to 70 µg per day; correlates with plasma selenium only up to about 80 to 90 µg/l
Glutathione peroxidase in red blood cells and platelets	plateau above a plasma selenium of 100 µg/l

And where does a normal intake put you? In 2023 EFSA modelled the relationship between intake and plasma concentration across several studies. At a mean intake of 70 µg per day the model predicts a mean plasma concentration of 111 µg/l (95 % confidence interval 102 to 119; 99 µg/l in the simpler linear model). That is exactly the range in which the functional markers already come to a halt.

In practice this means: someone who is already supplied can push their serum value further up through more intake — the enzymes at issue do not follow. A higher lab value is then not a better status, only a higher lab value. EFSA additionally points out that these proteins are upregulated by oxidative stress and their concentration can therefore rise "even without any change in selenium intake".

What pushes a value downwards falsely

The most important confounder is an ongoing inflammation. Serum and plasma selenium as well as selenoprotein P fall as part of the acute-phase reaction, without anything necessarily having changed in the body's stores. In an investigation of 141 healthy people and patient groups, serum selenium concentrations in the group with high CRP values were significantly lower than in the healthy; in an animal model, selenium moved out of the liver after an immune stimulus and returned to the baseline level only around 80 hours later, with the subsiding of the acute-phase reaction (Maehira et al., Clinical Chimica Acta 2002; PubMed 11750284).

For practice it follows: a value drawn a few days after an infection, an operation or during an acute phase of illness is of no use for judging selenium status. A CRP determined at the same time shows whether such a reaction is running at all. Erythrocyte selenium, by contrast, is unaffected by inflammatory reactions — it just responds slowly.

Small but real are the remaining influences EFSA lists: age, sex, smoking status, gene variants and region of residence. With nail selenium, contamination from outside is added.

What this means in practice

Selenium is in no standard blood panel. It is requested specifically, and anyone having the value determined on their own initiative should know beforehand what they want to do with it. The measurement makes sense where one of the constellations named applies — an illness with disturbed uptake, dialysis, a severely restricted diet. Then it belongs in medical hands, together with the question of the cause. Without such an occasion, a single value mainly answers the question of where one lies within a broad population range.

Anyone who would rather start with intake will find the solid figures for that in Selenium in foods — the considerably more reliable quantity, because it cannot be shifted by a cold.

Our Selenium 100 µg supplies 100 µg of selenium per tablet and thus 182 % of the nutrient reference value of 55 µg (Regulation (EU) No 1169/2011, Annex XIII); the recommended intake is one tablet daily after a meal. Which strengths and forms are otherwise available is shown by the Selenium category, and the remaining trace elements are next to it under Minerals.

The following claims are authorised for selenium (Regulation (EU) No 432/2012): Selenium contributes to the normal function of the immune system. Selenium contributes to normal thyroid function. Selenium contributes to the protection of cells from oxidative stress. Selenium contributes to the maintenance of normal hair and nails. Food supplements are not a substitute for a balanced and varied diet and a healthy lifestyle.

Frequently asked questions

Is a selenium test worth it without a specific reason? For the question "am I well supplied", it delivers less than the price suggests. In the lower range the measurement is informative, in the middle and upper range it runs into the plateau. An occasion — an illness with disturbed uptake, dialysis, a very one-sided diet — makes the difference between a number and a piece of information.

Serum or whole blood — which is better? It depends on the question. Serum or plasma reflects the more recent intake and responds quickly, but is susceptible to inflammation. Whole blood and erythrocyte selenium reflect a longer period and respond more sluggishly; erythrocyte selenium remains untouched by inflammatory reactions. The numbers cannot be compared: each material has its own reference range.

I have hair loss — is that a selenium deficiency? From the sign alone that cannot be deduced, and with selenium it leads particularly easily astray: hair loss and disturbed nail formation are also the signs of selenosis, that is of a chronically excessive intake. Anyone already taking a selenium preparation should therefore first add up their own total amount before increasing it.

Do I have to have a follow-up measurement after starting to take one? With food supplements in usual amounts, nobody provides for that. For the medicines something else applies: there, regular checking of the selenium level is expressly recommended, because they address a proven deficiency and treatment is to run until normalisation. When and with what a preparation is best taken is set out in How to take selenium.

Sources: DGE, selected questions and answers on selenium (March 2021) · EFSA, Scientific opinion on the tolerable upper intake level for selenium, EFSA Journal 2023;21(1):7704, including Annex A on the biomarkers · EFSA, Scientific Opinion on Dietary Reference Values for selenium, EFSA Journal 2014;12(10):3846 · patient information selenase® 50 peroral (as of October 2020) · Maehira F et al., Alterations of serum selenium concentrations in the acute phase of pathological conditions, Clinical Chimica Acta 2002;316(1–2):137–146 · Regulation (EU) No 1169/2011, Annex XIII · Regulation (EU) No 432/2012.