Article: Selenium Deficiency Symptoms: The Thyroid and Immunity Connection

Nov 23, 2025Brain health

Selenium Deficiency Symptoms: The Thyroid and Immunity Connection

Can't lose weight no matter what you try? Getting sick all the time? Feeling tired even after a full night's sleep?

You might have selenium deficiency—a nutrient problem that affects 1 billion people around the world.[1] Even though selenium is important for your thyroid, immune system, and cell health, most doctors don't test for it. That's because the symptoms look like so many other health problems.

Here's what makes selenium tricky: Your body can't make it on its own. You have to get it from food or supplements. And right now, the selenium in our soil is dropping fast, which means our food has less selenium than it used to.[2]

What is Selenium and Why Do You Need It?

Selenium is a trace mineral. That means you only need a small amount, but it's really important. Your body uses selenium to make over 25 special proteins that keep you healthy.

Here's what selenium does:

Turns your thyroid hormone from the inactive form into the active form your body can use
Helps your immune system fight off sickness
Protects your cells from damage (like rust protection for your body)
Helps men make healthy sperm
Reduces swelling and pain in your body

The problem: The amount of selenium in food depends on the soil where it grows. If the soil is low in selenium, your food will be too.

7 Warning Signs of Selenium Deficiency

1. Thyroid Problems and Slow Metabolism

What you'll notice: Weight gain you can't explain, always feeling cold, constant tiredness, dry skin, hair falling out, or trouble losing weight even when you diet and exercise.

Your thyroid gland has more selenium than any other part of your body.[3] Selenium helps change your inactive thyroid hormone (T4) into the active form (T3) that your body can actually use. Without enough selenium, this doesn't happen right. You end up with thyroid problems even if your blood tests look normal.

Watch for these signs: Weight that won't come off, cold hands and feet all the time, feeling tired no matter how much you sleep, hair getting thin, trouble thinking clearly, and constipation. If you also have muscle cramps and poor sleep, check for magnesium deficiency symptoms as well.

2. Weak Immune System and Getting Sick Often

What you'll notice: Catching every cold and flu going around, taking forever to get better from being sick, or getting the same infections over and over (like sinus infections or UTIs).

Selenium helps your immune system work right.[4] When you don't have enough, your body can't fight off germs as well. You're more likely to catch whatever sickness is going around.

The scary part: Research shows that when you're low in selenium, harmless viruses can turn into dangerous ones.[5] This makes your infections worse and last longer. Zinc deficiency symptoms can also weaken your immune system, and the two deficiencies often happen together.

3. Weak Muscles and Always Feeling Tired

What you'll notice: Muscles feeling weak for no reason, having trouble doing things that used to be easy, muscle pain even though you didn't work out, or feeling exhausted after doing very little.

Selenium protects your muscle tissue from getting damaged.[6] Without enough selenium, your muscles don't work as well and start breaking down. This causes weakness that makes daily activities hard.

Extreme cases: In parts of China where the soil has almost no selenium, people can get Keshan disease—a heart problem that can be deadly. This is rare in the US, but it shows how important selenium is for your muscles, including your heart muscle.

4. Brain Fog and Trouble Thinking

What you'll notice: Hard time focusing, forgetting things, thinking slower than normal, or feeling confused.

Your brain needs selenium to stay healthy. Selenium helps protect your brain cells from damage. When you don't have enough, your brain doesn't work as well.

Research shows that people with low selenium levels have a higher chance of getting dementia and memory problems as they age.[7] Similar brain problems can happen with B12 deficiency, which often occurs alongside selenium deficiency.

5. Fertility Problems

What you'll notice: Trouble getting pregnant, miscarriages that keep happening, poor sperm quality in men, or periods that are irregular.

Selenium plays a big role in having healthy babies.[8] For men, selenium is needed to make healthy sperm that can swim well. For women, selenium protects the developing baby from damage.

What research shows: Studies prove that taking selenium can help men's sperm work better.[9] It can also help women who keep having miscarriages.

6. Hair Loss and Weak Nails

What you'll notice: Hair getting thin, hair that breaks easy, hair that grows slowly, nails that split and peel, or white spots on your fingernails.

Your hair and nails need selenium to grow right. Without enough selenium, they become weak and grow slowly.

Important warning: Both too little selenium AND too much selenium can cause hair loss. That's why taking the right amount matters. Never take more than 400mcg per day.

7. More Swelling and Faster Aging

What you'll notice: Skin that looks older than it should, joint pain that won't go away, skin problems like eczema, or taking a long time to feel better after working out.

Selenium is the main ingredient in your body's rust protection system.[10] Without enough selenium, your cells get damaged. This causes swelling throughout your body and makes you age faster.

This swelling contributes to almost every disease that comes with aging—heart disease, diabetes, arthritis, and even cancer.

Why Selenium Deficiency is Becoming More Common

Most people think if they eat healthy, they're getting all the nutrients they need. But here's the problem: even healthy food is losing its selenium.

The soil crisis:

For the last 50 years, farmers have been growing more food on the same land. Modern farming pulls nutrients out of the soil faster than nature can put them back. Selenium is one of the minerals disappearing from our farm soil.

Scientists studied this problem. They found something worrying: by the year 2099, our farm soil will have even less selenium than today.[2] Climate change is making it worse. Areas that used to have enough selenium are now becoming low in it.

Where the problem is worst:

Most of Europe (especially Eastern Europe)
Parts of China
New Zealand (until they started adding selenium to fertilizer in the 1980s)
Some areas of the United States (Pacific Northwest, Great Lakes region)
Large parts of Africa

Why processed foods make it worse:

When food companies process grains (like turning wheat into white flour), they remove the parts that have the most selenium. So even if the wheat started with some selenium, it's gone by the time it gets to your plate.

What modern farming does:

The fertilizers farmers use today give plants three main nutrients: nitrogen, phosphorus, and potassium. These help plants grow big and fast. But they don't replace trace minerals like selenium. Over time, the soil loses more and more selenium.

Some countries fixed this:

Finland saw this problem coming. In 1984, they started adding selenium to their fertilizer.[11] Before that, most people in Finland didn't get enough selenium. Now they do. But most countries haven't done this yet.

The bottom line: Even if you eat vegetables and whole grains, you might not get enough selenium. It's not your fault—the selenium just isn't in the food anymore.

Best Food Sources of Selenium

Top selenium foods:

Brazil nuts: 68-91mcg per nut (just 2-3 nuts gives you what you need for the day)
Tuna: 92mcg per 3-ounce serving
Sardines: 45mcg per 3-ounce serving
Beef: 33mcg per 3-ounce serving
Turkey: 31mcg per 3-ounce serving
Eggs: 20mcg per large egg

Don't go over: 400mcg per day

Remember: The amount of selenium in food depends on where it was grown. In places with low-selenium soil, it's almost impossible to get enough selenium from food alone.

Best Forms of Selenium Supplements

Not all selenium supplements work the same way:

Best forms:

L-selenomethionine: Your body absorbs over 90% of this form.[12] It's the same form found in food.
Selenomethionine: Your body absorbs 85-90%. It stores selenium for later use.
Selenium yeast: Natural form with several types of selenium.

Forms that don't work as well:

Sodium selenite: Your body only absorbs about 50-60%.[13] Your body has to change it before it can use it.
Sodium selenate: Similar to selenite.

Why selenomethionine is better: It's the form of selenium that's naturally in food. Your body can use it right away and store extra for when you need it later.

How Much Selenium Do You Need?

Many people would benefit from taking 100-200mcg daily of L-selenomethionine to stay healthy.

How long before you feel better:

Thyroid working better: 3-6 months
Immune system stronger: 6-12 weeks
Thinking clearer: 3-6 months
Hair and nails healthier: 3-6 months

Important fact: Selenium and iodine work together for your thyroid. If you take selenium without enough iodine (or the other way around), your thyroid problems can get worse.

Get the Selenium You Need with THRIVE

THRIVE has 100mcg of L-selenomethionine—the form your body absorbs best. It also has 200mcg of iodine and other nutrients that work together to help your thyroid and immune system.

Most multivitamins use sodium selenite (the cheap form your body doesn't absorb well) or don't give you enough. THRIVE gives you the right amount of the best form, plus everything else you need to make it work.

What's in THRIVE:

100mcg L-selenomethionine (the premium form)
200mcg potassium iodide (works with selenium for your thyroid)
500mg vitamin C (helps selenium protect your cells)
20mg zinc picolinate and 2mg copper glycinate (for immune balance)
Full B-complex vitamins (helps your body use the selenium)

THRIVE gives you complete protection with the right minerals in the right balance. This helps with selenium deficiency while supporting your thyroid, immune system, and cell protection.

Learn more about THRIVE Premium Multivitamin →

Note: This article is for educational purposes only and does not constitute medical advice. Always consult a physician before taking any supplement. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

References:

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† THESE STATEMENTS HAVE NOT BEEN EVALUATED BY THE FOOD AND DRUG ADMINISTRATION. THIS PRODUCT IS NOT INTENDED TO DIAGNOSE, TREAT, CURE, OR PREVENT ANY DISEASE.