Reverse T3
An inactive mirror-image of T3. Under stress the body shunts T4 into reverse T3 instead of active T3 — effectively applying the metabolic brakes.
Explains the 'my thyroid labs are normal but I feel hypothyroid' pattern common in hard dieters, over-trainers, and people under chronic stress or recovering from illness.
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
Biotin (B7). Biotin interferes with the streptavidin-biotin immunoassays most hormone tests run on. Depending on the assay design it pushes the result falsely HIGH or falsely LOW — there is no single direction to correct for. Hair, skin and nail products are the usual source, and doses in those are far above the RDA. Stop biotin 48–72 hours before the draw, and check every supplement label rather than just the one labelled biotin. - Read it with its partner
Always order alongside Free T3 — the ratio carries the information, not the single value. Draw it alongside: Free T3 (Triiodothyronine), TSH (Thyroid-Stimulating Hormone), Cortisol (AM). - Work out which direction is yours
If it's high — Metabolic down-regulation — fatigue, cold, stalled progress despite adherence.
If it's low — Rarely a concern in isolation. - Fix it in this order
Nutrition. Restore calories and carbohydrate; end the extended deficit. This is the primary lever.
Lifestyle. Deload, sleep, reduce total stress load, treat underlying illness or inflammation.
Supplements. Selenium, zinc, iron (if ferritin low) support proper deiodination toward active T3.
Hormones. Addressing the driver is preferred over adding hormone. Discuss persistent elevation with a provider.
Compounds. A useful marker to run when someone on a long GLP-1 cut reports the metabolism feeling 'stuck.'
Work down the list, not across it. Adding a compound on top of an unfixed diet is why generic protocols fail. - Retest
8–12 weeks after restoring energy availability. Change one thing at a time, or the retest can't tell you which thing worked.
How to fix it
📚 Peeters RP et al., J Clin Endocrinol Metab 2005 — reverse T3 in nonthyroidal illness.
🩸 Test your Reverse T3
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 Reverse T3 is usually tested alongside
On its own, one marker is a data point. These panels include Reverse T3 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.
Would you feel it? Symptoms Reverse T3 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 Reverse T3 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.
Biotin interferes with the streptavidin-biotin immunoassays most hormone tests run on. Depending on the assay design it pushes the result falsely HIGH or falsely LOW — there is no single direction to correct for. Hair, skin and nail products are the usual source, and doses in those are far above the RDA.
Stop biotin 48–72 hours before the draw, and check every supplement label rather than just the one labelled biotin.
Reverse T3 rises specifically as a conservation response to stress of any kind. A raised value in someone recovering from illness or dieting hard is the expected physiology, not a finding.
Read it as a ratio against free T3, and only when you are well and eating normally.
All inhibit the deiodinase that converts T4 to T3, pushing more down the reverse T3 path.
Note any of these — the raised rT3 is the drug, not your thyroid.
What Reverse T3 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.
Metabolic adaptation to sustained energy deficit. The body shunts T4 into inactive reverse T3 and down-regulates the whole endocrine axis. TSH stays normal, so a basic thyroid panel misses it entirely.
Restore calories and carbohydrate, take a genuine diet break, deload, sleep. Check ferritin and selenium — both are required for conversion. This is physiology, not a broken thyroid.
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 Reverse T3 in context — each with its own full breakdown.
Frequently asked questions
8–25 ng/dL Ranges vary by laboratory and assay — always compare to the range printed on your own report.
Lower within range; the Free T3 : Reverse T3 ratio is the practical metric.
Metabolic down-regulation — fatigue, cold, stalled progress despite adherence.
Rarely a concern in isolation.
You can order Reverse T3 directly through Marek Diagnostics without a doctor's visit — drawn at any Quest Diagnostics location in the US. Code CAMERON applies 10% off automatically.