Selenium, Blood
A trace mineral essential for thyroid hormone conversion (deiodinase enzymes), glutathione peroxidase antioxidant activity, and immune function.
Required to convert T4 into active T3 — deficiency is a genuine and correctable cause of poor thyroid conversion. Also the best-evidenced supplement for reducing thyroid antibodies in Hashimoto's.
The plan of attack
In this order. Most people start at step four, which is why they change five things at once and learn nothing.
- Confirm the number is real
Trace-element tube. Trace-element analytes are sensitive to tube contamination, and the wrong tube produces falsely high results. Confirm a royal-blue trace-element tube was used. - Read it with its partner
One of the few supplements where the toxic dose is genuinely within reach — respect the ceiling. Draw it alongside: Free T3 (Triiodothyronine), Free T4 (Thyroxine), Thyroid Antibodies (TPO + TgAb). - Work out which direction is yours
If it's high — Genuinely toxic in excess. Don't megadose.
If it's low — Impaired T4→T3 conversion, higher thyroid antibodies, reduced antioxidant capacity, weakened immunity. - Fix it in this order
Nutrition. 2 Brazil nuts per day covers most people's needs — but content varies enormously by soil, so don't eat a handful. Also seafood, eggs, organ meats.
Lifestyle. No specific lifestyle lever beyond diet.
Supplements. Selenomethionine. RCT evidence supports for lowering TPO antibodies in Hashimoto's. Stay under total.
Hormones. Supports thyroid function; particularly relevant with autoimmune thyroid disease.
Compounds. Check if Free T3 is low despite adequate Free T4 — selenium deficiency is a classic conversion blocker.
Work down the list, not across it. Adding a compound on top of an unfixed diet is why generic protocols fail. - Retest
12 weeks after supplementation. Change one thing at a time, or the retest can't tell you which thing worked.
How to fix it
📚 Toulis KA et al., Thyroid 2010 — selenium and TPO antibodies meta-analysis.
🩸 Test your Selenium, Blood
Order directly through Marek Diagnostics — no doctor's visit needed, drawn at any Quest location in the US. Code CAMERON applies 10% off automatically.
Order this test — 10% off → Browse all 102 markers →What Selenium, Blood is usually tested alongside
On its own, one marker is a data point. These panels include Selenium, Blood plus the markers that make it interpretable — each names every test, why it is on the list, and loads the whole set into your cart in one click at 10% off.
includes this + 7 more markers — Fatigue, cold intolerance, weight change, hair loss, constipation, low mood, or a 'normal TSH' that never explained how you feel. Women are 5–8x more likely to be affected.
includes this + 8 more markers — Abnormal TSH, a goitre, a family history of thyroid disease, postpartum thyroid changes, or thyroid symptoms with 'normal' basic labs.
What moves your Selenium, Blood
1 supplement in the Vault have a documented effect on this marker, or are a reason to have measured it first:
Browse all 237 compounds & 350 supplements →
Would you feel it? Symptoms Selenium, Blood helps explain
People rarely search for a marker — they search for how they feel. These are the complaints where this marker is worth checking, and whether it is a first-line test or a follow-up once the obvious causes are ruled out.
Why your Selenium, Blood might be wrong
Most abnormal results are interference, not disease. These are the things that measurably move this specific marker — check them before you change anything. Each one says whether the number is wrong and should be repeated, badly timed and should be redrawn, or real with a cause worth explaining.
Selenium is carried on selenoprotein P, and circulating levels fall during inflammation independently of your intake or stores.
Read against CRP — a low selenium in an inflamed person may not be a deficiency.
Trace-element analytes are sensitive to tube contamination, and the wrong tube produces falsely high results.
Confirm a royal-blue trace-element tube was used.
A handful of brazil nuts can contain several times the daily requirement and shifts the result.
Note recent intake; avoid a large dose in the days before.
What Selenium, Blood means in combination
A single marker tells you a little. Combinations tell you the story — these are the named patterns this marker takes part in, and what each one points at.
Converting T4 into active T3 needs selenium, iron, zinc and adequate energy. When those are short — or under stress and illness — you make inactive reverse T3 instead.
Adding more T4 to a conversion problem does not work. Correct ferritin and selenium first, address energy availability and stress, and retest in 8–12 weeks before changing any thyroid medication.
What to test next
Markers rarely answer alone. These are the ones that put Selenium, Blood in context — each with its own full breakdown.
Frequently asked questions
70–150 µg/L (assay dependent) Ranges vary by laboratory and assay — always compare to the range printed on your own report.
Mid-to-upper range — there's a narrow therapeutic window; more is not better.
Genuinely toxic in excess. Don't megadose.
Impaired T4→T3 conversion, higher thyroid antibodies, reduced antioxidant capacity, weakened immunity.
You can order Selenium, Blood directly through Marek Diagnostics without a doctor's visit — drawn at any Quest Diagnostics location in the US. Code CAMERON applies 10% off automatically.