Educational information from Her Form — reviewed for accuracy by our health specialists.
Not a substitute for personal medical advice. Optional 1:1 consults with Robyn, our consulting women's-health specialist, are available on request.
What it is
Free T3 is the unbound, biologically active thyroid hormone. Low T3 with normal TSH and T4 — 'low T3 syndrome' — often explains stubborn thyroid-style symptoms.
Why it matters for women
Conversion of T4 to T3 needs iron, selenium, zinc and adequate calories. Chronic dieting, stress and inflammation all suppress it.
Optimal range
3.0–4.2 pg/mL.
Standard lab range
Lab ranges 2.0–4.4.
When it's high
- ↑Hyperthyroidism
- ↑Graves' disease
When it's low
- ↓Hypothyroid symptoms with 'normal' TSH
- ↓Chronic dieting
- ↓Stress, illness, low iron
References
- American Thyroid Association (ATA) — Hypothyroidism guidelines— Full thyroid panel interpretation and treatment thresholds
- Endocrine Society — Thyroid function in women— Thyroid disorders in pregnancy and reproductive-age women
- NICE — Thyroid disease (NG145)— Assessment and management of thyroid disorders
- Jean Hailes — Thyroid health for women— Thyroid disease in perimenopause and pregnancy
References are provided for educational context only. Always interpret results with a qualified healthcare professional.
A note on interpretation. Ranges above are what our health specialists works to in healthy women. They are not a diagnosis — a single result always needs context from your symptoms, cycle phase and other markers.