Free T4 (Free Thyroxine) Blood Test
Free T4 is the active form of your main thyroid hormone — the natural follow-up when TSH looks unusual.
What Free T4 measures
Thyroxine (T4) is the main hormone produced by your thyroid gland. Most of it circulates bound to proteins, which makes it inactive. Free T4 is the small unbound fraction that's biologically active — the part that actually does the work in your tissues. Once it reaches your cells, T4 is converted into the more potent hormone T3, so free T4 reflects the raw material your body has to work with. Free T4 is almost always read alongside TSH rather than on its own: TSH is the pituitary's signal to the thyroid, while free T4 shows how the thyroid is responding. Reading the two together is what lets a doctor distinguish an underactive thyroid from an overactive one, or a thyroid problem from a pituitary one. Discuss your results with your doctor for an interpretation that fits your symptoms and history.
Who should consider checking Free T4
Have you noticed any of these:
- TSH that came back high or low and you want a fuller picture
- fatigue, weight changes or temperature sensitivity
- thinning hair, brittle nails or dry skin
- irregular periods or fertility questions
- starting or adjusting thyroid medication
- tracking your thyroid alongside TSH
How the test works
Single blood draw, no fasting required. Morning testing recommended. If you take thyroid medication, most people test before their morning dose. Results back same day.
What "normal" can look like
Lab reference ranges for free T4 vary slightly between providers but typically span roughly 12–22 pmol/L in Australia. As with TSH, what's optimal for feeling well can be tighter than the lab range, and interpretation depends on TSH, antibodies, symptoms, and life stage. Always discuss results with your doctor.
Where to get a Free T4 test
Free T4 is included in these panels:
- Thyroid + Antibodies Panel — The full thyroid picture — both how your thyroid is working and whether your immune system is involved. A$169
Frequently asked questions
Why test free T4 instead of total T4?
Total T4 is mostly bound to proteins and biologically unavailable. Free T4 is the active fraction your tissues actually use.
I'm on thyroid medication — when should I test?
Most people test before their morning dose for consistency. Talk to your doctor about timing.
What does low free T4 mean?
Low free T4 with high TSH suggests an underactive thyroid (hypothyroidism). Low free T4 with low TSH points to a different picture (central / pituitary cause). Interpretation requires both numbers — and a doctor.
What's the difference between free T4 and free T3?
Free T4 is the main hormone your thyroid releases; your body then converts it into free T3, the more active form. T4 reflects supply, T3 reflects what's actually working in your tissues. They're often tested together for a fuller picture, and your doctor can advise which is relevant for you.