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Brain & nervous system
Selenium test
Selenium status — important for thyroid function and antioxidant defense.
At a glance
In about 5 days
Most results are ready in about 5 days. Some specialty markers can take up to 21 days.
Check before your draw
This marker has no preparation tag, but it can still need preparation. Contact us before your draw if you’re not sure.
Walk in at Quest
Your lab order arrives by email. Most Quest locations take walk-ins, so bring it with a photo ID.
What it measures
Selenium is a trace mineral that your thyroid, the gland in your neck that helps set your energy level, needs to work well. It also helps protect your cells from damage.
Why it matters for women
For its size, your thyroid holds more selenium than any other organ. Thyroid conditions, including Hashimoto’s, an autoimmune condition that slows the thyroid, are much more common in women. How much selenium helps with these conditions is still being studied.
You might check it if…
- a doctor mentioned Hashimoto’s or another thyroid condition
- a gut condition affects how you absorb food
- you eat Brazil nuts often, which are very high in selenium
What your result may mean
High Selenium: what it may mean
High selenium may mean supplement use, or a diet rich in Brazil nuts, which can contain very large amounts. At very high levels, selenium toxicity (selenosis) can cause hair and nail loss and stomach or gut issues. Some people may also notice a garlic smell on the breath, a metallic taste, tiredness or irritability. Thyroid-support blends can contain selenium, so a high level can appear in women managing Hashimoto's. Amounts from several supplements can stack up faster than you'd expect.
Low Selenium: what it may mean
Low selenium may slow the step that turns thyroid hormone T4 into T3, and it can weaken the body's antioxidant defense. It can happen with a poor diet or in regions with selenium-depleted soil, and gut conditions, dialysis or recent illness can lower it too. Selenium deficiency may worsen Hashimoto's thyroiditis, which can affect women far more than men, although how much selenium helps is still being studied. Some people may notice tiredness or muscle weakness, and thyroid tests can help check how the gland is doing.
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These notes provide context and do not diagnose a condition. Not yet available in NY, NJ, RI, HI, ND, SD or WV.