Free T3 (Triiodothyronine)
The active thyroid hormone that actually drives metabolic rate at the cellular level. T4 is largely a storage form requiring conversion to T3.
The most informative thyroid marker for energy, metabolism and body composition — and the one most likely to be suppressed in someone dieting hard or over-training, even with a normal TSH.
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
Order with Reverse T3; the FT3:rT3 ratio is what optimization practitioners actually track. Draw it alongside: TSH (Thyroid-Stimulating Hormone), Free T4 (Thyroxine), Reverse T3. - Work out which direction is yours
If it's high — Hyperthyroidism or excess T3 supplementation — anxiety, tachycardia, heat intolerance, muscle loss.
If it's low — Cold intolerance, fatigue, stalled fat loss, low mood, constipation, hair thinning. - Fix it in this order
Nutrition. Restore calories and carbohydrate — this is the number-one fix for low T3 in dieters. Adequate iodine, selenium, zinc, iron. Extended very-low-carb or aggressive deficits reliably suppress T3.
Lifestyle. Deload from training, sleep, reduce stress (cortisol blocks conversion and pushes T4 toward reverse T3), and take diet breaks/refeeds during long cuts.
Supplements. Selenium (required cofactor for deiodinase enzymes), zinc, iron if ferritin is low. Correcting ferritin alone often restores conversion.
Hormones. T3-containing therapy (liothyronine or NDT) is sometimes used for poor converters — physician territory, with cardiac and bone risks if over-dosed.
Compounds. Anyone deep in a cut — including on GLP-1s — who stalls with fatigue and cold hands should check Free T3 with Reverse T3.
Work down the list, not across it. Adding a compound on top of an unfixed diet is why generic protocols fail. - Retest
6–8 weeks after intervention. Change one thing at a time, or the retest can't tell you which thing worked.
How to fix it
📚 American Thyroid Association guidelines. Douyon L, Schteingart DE, Endocrinol Metab Clin North Am 2002 — caloric restriction and thyroid.
🩸 Test your Free T3 (Triiodothyronine)
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 Free T3 (Triiodothyronine) is usually tested alongside
On its own, one marker is a data point. These panels include Free T3 (Triiodothyronine) 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 + 11 more markers — Persistently exhausted despite adequate sleep, and either haven't been tested or were told everything is normal.
includes this + 9 more markers · built for women — Training hard with absent or irregular periods, recurrent stress fractures, persistent fatigue, or unexplained performance decline. Also anyone with a history of restrictive eating.
What moves your Free T3 (Triiodothyronine)
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 Free T3 (Triiodothyronine) 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 Free T3 (Triiodothyronine) 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.
Falls first and furthest when calories are short — this is an adaptation to under-eating, not thyroid disease, and it happens before TSH moves at all.
Interpret against your actual intake and training load, not against the reference range alone.
This is an acute-phase reactant — it moves substantially during any infection, injury or inflammatory episode, and the change has nothing to do with your baseline.
Wait two weeks after any illness, vaccination or injury before drawing or interpreting.
What Free T3 (Triiodothyronine) 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.
Low energy availability shuts the reproductive axis down to save fuel. It is not a badge of being lean — it costs bone density in a way that does not fully come back.
Eat more. That is the treatment, and no supplement substitutes for it. Get bone density assessed if it has been going on for a year or more. The RED-S panel exists for exactly this picture.
Female pattern shedding is very often nutritional and thyroidal before it is hormonal. Each of these alone can shed hair; together they reliably do.
Ferritin above 50 is the usual threshold for regrowth — normal is not the same as sufficient here. Fix all three and give it 3–6 months; the hair cycle is slow and nothing shows in weeks.
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.
The gland is being pushed harder to produce the same output. Whether to treat is genuinely contested; whether it explains your symptoms is a separate question worth taking seriously.
Check TPO — antibodies change the calculus considerably. Confirm on a second draw (TSH swings across the day and is highest early morning). Fix ferritin, selenium, iodine and energy intake before concluding the gland is the problem.
What to test next
Markers rarely answer alone. These are the ones that put Free T3 (Triiodothyronine) in context — each with its own full breakdown.
Frequently asked questions
2.3–4.2 pg/mL Ranges vary by laboratory and assay — always compare to the range printed on your own report.
Upper half of the range, provided TSH and symptoms agree.
Hyperthyroidism or excess T3 supplementation — anxiety, tachycardia, heat intolerance, muscle loss.
Cold intolerance, fatigue, stalled fat loss, low mood, constipation, hair thinning.
You can order Free T3 (Triiodothyronine) directly through Marek Diagnostics without a doctor's visit — drawn at any Quest Diagnostics location in the US. Code CAMERON applies 10% off automatically.