Free T4 (Free Thyroxine)
Free T4 measures the unbound, biologically active portion of the main thyroid hormone. Together with TSH it pins down whether the thyroid is over- or under-active.
What it measures
The thyroid secretes thyroxine (T4); >99% travels bound to plasma proteins and is biologically inactive. Free T4 — the small unbound fraction — is what tissues actually use, which is why it correlates better with thyroid status than total T4. TSH plus Free T4 form the standard diagnostic pair: TSH high + Free T4 low = overt hypothyroidism; TSH low + Free T4 high = overt hyperthyroidism; discordant patterns flag subclinical disease or pituitary problems.
What a high value can mean
- Hyperthyroidism — Graves' disease, toxic nodule or multinodular goitre; TSH typically suppressed.
- Thyroiditis (transient phase) — early Hashimoto's or subacute thyroiditis.
- Excess thyroid hormone replacement — over-replaced patients.
- Amiodarone, iodine contrast — drug-induced.
What a low value can mean
- Overt hypothyroidism — TSH typically elevated; consider Hashimoto's.
- Pituitary failure (central hypothyroidism) — TSH inappropriately normal or low.
- Severe illness, starvation — non-thyroidal illness syndrome.
When to discuss with a doctor
Free T4 outside the reference range alongside a clear TSH signal warrants an endocrinology referral or initiation of replacement / suppression. Subclinical patterns (TSH abnormal, Free T4 normal) are commonly followed before treating. Mediora.AI surfaces the pair together; thyroid dosing belongs with a clinician.