What Is Free T4?
Free thyroxine (FT4) measures the unbound, biologically active fraction of thyroxine in the blood. Thyroxine is the most abundant thyroid hormone, produced exclusively by the thyroid gland, and serves primarily as a prohormone — it is converted to the more active T3 in peripheral tissues by deiodinase enzymes.
Approximately 99.97% of circulating T4 is bound to proteins (thyroxine-binding globulin, albumin, and transthyretin). Only the free fraction (0.03%) is available to enter cells. Measuring free T4 rather than total T4 avoids the confounding effects of binding protein changes (pregnancy, oral contraceptives, liver disease).
Why Is Free T4 Tested?
- Confirming thyroid dysfunction — when TSH is abnormal, FT4 determines severity
- Distinguishing subclinical from overt disease — abnormal TSH with normal FT4 = subclinical
- Monitoring thyroid medication — FT4 helps optimise levothyroxine dosing
- Hyperthyroidism assessment — FT4 may be elevated before FT3 (T4 thyrotoxicosis)
- Pregnancy thyroid monitoring — FT4 ensures adequate thyroid hormone for foetal brain development
Normal Ranges
| Group | Normal Range (pmol/L) |
|---|---|
| Adults | 12.0–22.0 |
| First trimester pregnancy | 12.0–19.6 (lower reference limit) |
What Do High Free T4 Levels Mean?
- Hyperthyroidism — Graves' disease, toxic nodular goitre, thyroiditis
- Over-replacement with levothyroxine
- Thyroiditis (acute phase) — thyroid hormone leaks from inflamed gland
- Amiodarone-induced thyrotoxicosis
- Factitious thyrotoxicosis — excessive thyroid hormone ingestion
- hCG-mediated (pregnancy) — mildly elevated FT4 in first trimester is normal
What Do Low Free T4 Levels Mean?
- Hypothyroidism — Hashimoto's thyroiditis, post-thyroidectomy, radioiodine ablation
- Under-replacement with levothyroxine
- Central hypothyroidism — pituitary or hypothalamic failure (low FT4 with low/normal TSH)
- Severe non-thyroidal illness — euthyroid sick syndrome in critically ill patients
- Iodine deficiency — the thyroid cannot produce adequate T4 without iodine
How to Optimise Thyroid Function
- Take levothyroxine properly — same time daily, empty stomach, 30 min before food or coffee
- Avoid interfering supplements — calcium, iron, and biotin supplements should be taken 4 hours apart from thyroid medication
- Ensure adequate iodine — 150 µg/day from dairy, fish, or iodised salt
- Support T4-to-T3 conversion — selenium, zinc, and iron are cofactors for deiodinase enzymes
- Regular monitoring — recheck 6–8 weeks after any dose change, then every 6–12 months
When Should You Get Tested?
- Whenever TSH is abnormal
- Monitoring thyroid medication
- Symptoms of thyroid dysfunction with normal TSH (rare central hypothyroidism)
- During pregnancy, particularly the first trimester
Which Lola Health Tests Include Free T4?
FT4 is included in Core Health, Vital Check, and Peak Insights. Available as an add-on with any blood test.
Check Your Free T4 (Thyroxine) Levels at Home
The Core Health 45 includes Free T4 (Thyroxine) testing along with 44 other biomarkers. Results in 2 working days with a free at-home phlebotomist visit.
View Core Health 45 →Check Your Free T4 Levels
Get a comprehensive blood test from Lola Health with GP-certified results and personalised recommendations. All tests use venous blood draws for medical-grade accuracy.
At-Home Blood Testing
Check your levels from home
Professional phlebotomist visit. Doctor-reviewed results in 2-5 days. Track your health with comprehensive blood panels.
→45-70 biomarkers tested · Venous blood draw · From £130