# Free T4 Test

URL: https://thethyroidologist.com/tests/free-t4-test/
Section: Tests
Last medically reviewed: 2025-11-01

> The free T4 test measures the active, unbound thyroid hormone. When it is ordered, how to interpret results, and its role alongside TSH.

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.

## 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.

## Disclaimer

The content on this website is for general educational purposes only and does not constitute medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional about your specific condition.

---
Part of The Thyroidologist — https://thethyroidologist.com