7 Signs of Low Selenium (and What Your Thyroid Is Trying to Tell You)

The most common signs of low selenium are ongoing fatigue, thinning or shedding hair, brittle nails, getting sick more often than usual, brain fog, muscle weakness, and thyroid changes. Because these overlap with so many other things, low selenium is easy to miss and easy to be told is "nothing." If several of the signs below sound like you, it is worth a simple blood test rather than a guess. Here are seven signs to know, why selenium sits behind them, and what to do next.

In this article

• 1. Fatigue that rest does not fix
• 2. Hair thinning or shedding
• 3. Brittle nails with white spots
• 4. Catching every bug going around
• 5. Brain fog and low mood
• 6. Muscle weakness or aching
• 7. Thyroid symptoms that will not settle
• Frequently asked questions

1. Fatigue that rest does not fix

Selenium helps build the antioxidant enzymes your cells use to manage everyday oxidative stress, and it supports the thyroid's role in energy. When selenium runs low, a heavy, persistent tiredness is often the first thing people notice, the kind that a good night's sleep does not touch. On its own, fatigue has a hundred causes, so treat it as a prompt to look closer rather than a diagnosis.

2. Hair thinning or shedding

Selenium plays a part in healthy hair growth, and both too little and too much can show up in your hair. With low selenium, people often describe more strands in the brush, a thinner ponytail, or hair that feels dull. Because hair changes slowly, this one tends to creep up over months.

3. Brittle nails, sometimes with white spots

Nails are a quiet status report on your nutrition. With low selenium, people report nails that split, peel, or break more easily, and sometimes small white flecks. As with hair, nails grow slowly, so improvement takes patience once you address the cause.

4. Catching every bug going around

Selenium supports normal immune function, so a run of back-to-back colds or feeling like your defenses are down can be a signal worth noting. Plenty of things dent immunity, but if frequent illness lines up with other signs on this list, selenium is worth checking.

5. Brain fog and low mood

That fuzzy, hard-to-focus feeling and a flatter mood can travel with low selenium, partly through its link to thyroid function. It is easy to write this off as stress or age. You are not imagining it, and it is fair to ask for a proper look rather than being brushed off.

6. Muscle weakness or aching

Selenium-dependent enzymes help protect muscle tissue from oxidative stress, and low levels can show up as weakness or a general achiness that does not match your activity. If your muscles feel more tired than your day warrants, add it to the picture.

7. Thyroid symptoms that will not settle

Your thyroid holds more selenium per gram than any other organ, and it relies on the mineral to do its daily work. When selenium is low, thyroid-related symptoms like fatigue, weight changes, and feeling cold can be harder to settle. If your thyroid is already being watched, this is a conversation to have with your practitioner. For the full picture on dosing, see our guide on how much selenium to take for thyroid support.

What to do if these signs sound familiar

Two calm steps. First, ask for a blood test rather than guessing, because the signs above overlap with many other things and a test gives you a real answer. Second, look at your intake. Selenium comes from foods like Brazil nuts, seafood, eggs, and whole grains, and a measured daily supplement is an easy way to keep a steady, known amount. The adult baseline is 55 mcg a day, many people choosing thyroid support use 100 to 200 mcg, and the safe ceiling from all sources is 400 mcg a day. Wondering whether nuts or a capsule is the better route? See our comparison of Brazil nuts vs a selenium supplement.

Frequently asked questions

How do I know if I am low in selenium?

The only reliable way is a blood test ordered by your practitioner. The signs, like fatigue, hair and nail changes, and frequent illness, are suggestive but not proof, because they overlap with many other conditions.

What causes low selenium?

Common causes include a diet low in selenium-rich foods, living where the soil is low in selenium, certain digestive conditions that reduce absorption, and kidney dialysis. Restrictive diets can also play a part.

How long does it take to correct low selenium?

Blood levels can respond within weeks of steady intake, but visible signs like hair and nails follow the slow pace of growth, so give them a few months. Consistency matters more than any single large dose.

Can low selenium affect the thyroid?

Selenium supports normal thyroid function, and the thyroid concentrates more selenium than any other organ. Low selenium is one of several factors that can make thyroid symptoms harder to settle, which is why it is worth checking alongside a proper thyroid workup.

What foods are highest in selenium?

Brazil nuts are by far the richest source, followed by seafood, eggs, poultry, and whole grains. Brazil nuts vary a lot in how much selenium they contain, so a measured supplement is a steadier option if you want a known daily amount.