Selenium: what it is, what it tells you and when it’s tested

Selenium is a mineral your body uses in small amounts. It goes into proteins that protect your cells, support your immune defences and activate thyroid hormone. You get it from fish, meat, eggs and cereals. How much food contains depends on the soil it grows in.

When it’s tested

Doctors order it when they suspect a shortage, for example with long-term artificial nutrition, a condition that reduces absorption, or dialysis. They also ask for it if you take selenium supplements and show signs of too much.

It is not a routine test for people who feel well. With selenium, balance is what counts: too little and too much are both harmful, and taking supplements when you already have enough can make things worse. A single value, without a reason, rarely changes what to do.

That is why Rosa does not test it. Instead it looks at TSH, for your thyroid, and blood sugar and HbA1c, because too much selenium goes with a higher risk of diabetes.

What it tells you, and what it does not

If it is low, you may be getting little from food, or absorbing little. If it is high, you almost always take a supplement or often eat very rich foods.

On its own it does not say whether you have a thyroid problem, or whether you need a supplement. Your doctor decides that with your other tests.

That is why it is read together with other tests:

• TSH: It shows how your thyroid is working
• Anti-TPO antibodies: They show autoimmune thyroiditis
• Blood sugar: Too much selenium is linked to a higher diabetes risk
• Zinc: Another mineral your doctor may order alongside

Normal selenium levels

The lab decides what is normal, and here it varies more than for other tests. Some labs measure selenium in serum, others in whole blood, with different numbers. And typical levels depend on how much selenium local food carries.

So it is not worth comparing your number with one found online. The range printed next to your result is what counts.

A value slightly outside the limits, without symptoms, is usually not a problem. Your doctor reads it with what you eat and the supplements you take.

If your selenium is low or high

• Low selenium: Often little selenium in your food, sometimes an absorption problem. A severe shortage is rare.
• High selenium: Almost always a supplement, or a lot of Brazil nuts. Too much selenium is harmful.

What causes low or high selenium

It depends mostly on what you eat and what you take.

• Low: little selenium in food: A diet low in fish, meat and eggs, or food grown in selenium-poor soil.
• Low: reduced absorption: With some bowel conditions or after surgery that reduces absorption.
• Low: dialysis or artificial nutrition: People on dialysis or fed through a vein for a long time can run low. Their doctor checks it.
• High: supplements: The most common cause. Besides selenium-only products, many multivitamins contain it too.
• High: Brazil nuts: They are extremely rich in selenium. Eating them often can bring too much.

Symptoms of too much or too little selenium

The most typical sign of too much shows in your hair and skin:

• Hair loss, usually the first sign of too much selenium
• Skin rashes
• With too little selenium: tiredness and weaker defences, signs that are not specific

If you take selenium supplements and are losing hair, talk to your doctor soon and bring the pack with you.

Selenium, thyroid and cancer

Your thyroid uses selenium to activate its hormone. That is why it is much discussed in thyroid conditions, such as Hashimoto’s thyroiditis. Whether you need a supplement, and for how long, is decided by your doctor or endocrinologist, case by case.

For years selenium was thought to protect against cancer. The strongest studies say no: supplements do not reduce cancer. In some trials, people taking them had more type 2 diabetes, more hair loss and more skin rashes.

With selenium, balance is what counts: a supplement can help people who have too little and harm people who already have enough.

What can skew the result

A few things change the number or how to read it.

• Supplements: If you take selenium, even inside a multivitamin, the value rises. Tell your doctor.
• The type of sample: Serum and whole blood give different numbers. To compare two results, use the same lab and the same sample.
• Where you live: Local food can be richer or poorer in selenium. That is why typical levels vary between countries.

When to worry

Talk to your doctor if:

• Your selenium is outside your report’s limits.
• You take selenium supplements and are losing hair or have skin rashes.
• You have a thyroid condition and are thinking of taking selenium: decide it with your doctor.
• You are on dialysis, have a bowel condition or have been fed through a vein for a long time.
• You took a large amount of a supplement by mistake and feel unwell, with vomiting or diarrhoea: call 112 or go to the emergency department.

Frequently asked questions

What are normal selenium levels? It depends a lot on the lab, the type of sample and where you live. The range printed next to your result is what counts.

Selenium and thyroid: what is the link? Your thyroid uses selenium to activate its hormone. In thyroid conditions your doctor may consider whether a supplement helps, but it does not apply to everyone.

What are the symptoms of selenium deficiency? A severe shortage is rare and gives signs that are not specific, such as tiredness and weaker defences. It is suspected mostly with dialysis, feeding through a vein or bowel conditions.

Is high selenium dangerous? Too much selenium over time is harmful: the first sign is usually hair loss. It almost always comes from supplements or a lot of Brazil nuts. Talk to your doctor.

Does Rosa test selenium? No. Rosa measures TSH, for your thyroid, and blood sugar and HbA1c. If your doctor wants selenium, they order it separately.

Sources

• Rayman, selenium and human health. The Lancet, 2012. PMID 22381456
• Vinceti and colleagues, selenium for preventing cancer. Cochrane Database of Systematic Reviews, 2018. PMID 29376219
• EFSA Panel on Nutrition, Novel Foods and Food Allergens, scientific opinion on the tolerable upper intake level for selenium. EFSA Journal, 2023. PMID 36698500