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
T4 is produced by the thyroid and converted to the active hormone T3 in the liver, gut and other tissues. Free T4 is the unbound, available fraction.
Why it matters for women
Free T4 confirms the thyroid is producing enough raw material. Low free T4 with a high TSH is classic hypothyroidism.
Optimal range
1.0–1.5 ng/dL.
Standard lab range
Lab ranges 0.8–1.8.
When it's high
- ↑Hyperthyroidism
- ↑Excess thyroid medication
When it's low
- ↓Hypothyroidism
- ↓Hashimoto's
- ↓Iodine or selenium deficiency
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.