Free T4 Test
The free T4 (FT4) test measures thyroxine — the main hormone the thyroid produces — that is not bound to proteins and is therefore biologically active.
It is typically ordered alongside TSH when thyroid dysfunction is suspected or confirmed.
Last medically reviewed: 1 November 2025
Key points
- Free T4 measures the unbound active thyroid hormone.
- It is interpreted alongside TSH.
- Low free T4 with high TSH indicates overt hypothyroidism.
- Levels can be affected by pregnancy, illness and medication.
What it measures
The thyroid produces mostly T4 (thyroxine); tissues convert some to the more active T3. Most T4 in blood is bound to carrier proteins; "free" T4 is the small fraction available to cells.
FT4 reflects actual hormone availability better than total T4, which can be affected by protein levels (pregnancy, liver disease, certain drugs).
Interpreting results
High TSH + low FT4 = overt hypothyroidism. High TSH + normal FT4 = subclinical hypothyroidism. Low TSH + high FT4 = overt hyperthyroidism. Low TSH + normal FT4 = subclinical hyperthyroidism.
FT4 is essential for distinguishing subclinical from overt disease — TSH alone cannot tell severity.
Preparation and limitations
No special preparation is usually needed. Results can be affected by severe illness, certain medications, and pregnancy — interpretation requires clinical context.
FT4 alone without TSH is less informative — the pair together gives the clearest picture.
Frequently asked questions
What is the difference between T4 and free T4?
Total T4 measures all thyroxine including protein-bound; free T4 measures only the active unbound fraction, which better reflects thyroid status.
Can free T4 be normal when TSH is abnormal?
Yes — that combination defines subclinical disease: the pituitary is compensating but hormone levels are still normal.
Why not just test T4?
Free T4 is preferred because protein-binding variations can make total T4 misleading in pregnancy, liver disease, or with certain medications.
References & further reading
This page is based on established medical knowledge and guidelines from authoritative sources. Always consult a qualified clinician for personal medical advice.
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