Free T3 low, TSH normal — the conversion problem
A low free T3 with a normal TSH indicates that your pituitary is signalling appropriately and your thyroid is making enough T4 — but the conversion of T4 (the storage hormone) to T3 (the active hormone) is impaired. This pattern is sometimes called "low T3 syndrome" or "non-thyroidal illness" depending on context, and it appears in research literature alongside chronic illness, prolonged caloric restriction, low selenium status, and elevated cortisol. A standard GP thyroid panel often only includes TSH (and sometimes free T4), missing this pattern entirely.
Reference ranges
| Free T3 standard range | 3.1 – 6.8 pmol/L |
| Free T4 standard range | 12.0 – 22.0 pmol/L |
| TSH standard range | 0.4 – 4.0 mIU/L |
| Reverse T3 (rT3) range | < 24 ng/dL |
What this marker measures
Free T3 (triiodothyronine) is the biologically active thyroid hormone — it binds nuclear receptors to drive metabolic rate, mitochondrial biogenesis, and tissue thermogenesis. The thyroid produces ~80% T4 and ~20% T3 directly; the rest of the active T3 is generated peripherally by deiodinase enzymes (DIO1, DIO2, DIO3) converting T4 to T3 in the liver, kidney, and muscle. When this conversion is impaired, free T3 falls while TSH and free T4 stay in range — because the pituitary feedback loop responds primarily to T4 status.
Why might it be low?
- ·Chronic illness ("euthyroid sick syndrome" / non-thyroidal illness)
- ·Prolonged caloric restriction or aggressive dieting
- ·Selenium deficiency (selenium is a cofactor for DIO1/DIO2)
- ·Elevated cortisol — chronic stress or HPA-axis dysregulation
- ·Heavy training without adequate fuelling (overtrained athletes)
- ·Liver dysfunction (~60% of T4-to-T3 conversion happens in liver)
- ·Genetic DIO2 polymorphisms
Why might it be elevated?
- ·T3 supplementation (Cytomel / liothyronine)
- ·Hyperthyroidism with T3-predominant pattern (early Graves' disease)
Compounds whose research literature has investigated this area
These are research-grade compounds in our catalogue whose published study literature touches the same biology. Listed for research context — not as recommendations for self-administration.
MOTS-c is a 16-residue peptide encoded in the mitochondrial 12S rRNA region. Described by the Cohen lab in 2015 — studied in metabolic and exercise biology.
Tesamorelin is an approved GHRH analogue. The research-grade form is used widely in GH-axis, lipid-metabolism, and hepatic-fat research.
FAQ
Why didn't my doctor catch this?+
Most NHS GP and US PCP thyroid screens are TSH-only. A normal TSH closes the case for most clinicians. Free T3 has to be specifically requested — Medichecks, Thriva, and most US functional-medicine labs include it; standard panels don't.
Is reverse T3 worth checking too?+
In research-context: yes, when T3 is low. Reverse T3 is the inactive isomer produced when DIO3 converts T4 to rT3 instead of T3. An elevated rT3 with low T3 is the textbook fingerprint of conversion-problem patterns.
How do I improve conversion?+
Research literature points to: ensuring adequate selenium (Brazil nuts, supplementation), zinc, and iron status; reducing chronic caloric restriction; addressing chronic stress / cortisol; and treating any underlying liver or gut dysfunction. The pharmacological route is direct T3 supplementation, which requires a clinician.
This page describes biomarker research and reference ranges for self-tracking and research-context discussion only. It is not medical advice, not a diagnosis, and not a substitute for a qualified physician. Take any concerns about your health to a clinician.
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