Thyroxine (free) - serum

Last updated: Tuesday, 30, March, 2004

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5 mL blood in plain tube.


Immunoassay, using various techniques to distinguish free from protein-bound hormone.

Reference Interval

10-25 pmol/L. Values up to 35 pmol/L may be acceptable for patients on full replacement therapy with thyroxine.


Investigation of thyroid function, as an adjunct to TSH, which is the preferred test for assessment of thyroid status.

Monitoring patients on thyroid replacement therapy.

Free thyroxine has largely replaced the free thyroxine index.


The results should always be interpreted in association with TSH.

Free T4 is elevated in hyperthyroidism. Levels are usually depressed in hypothyroidism, but TSH elevation is a more sensitive indicator.
A patient may be hypothyroid, with TSH levels greater than twice the reference interval but free T4 within the reference interval.
Total T4 is occasionally measured (as part of the free thyroxine index) when free T4 results are suspected, on clinical grounds, of being anomalous.
Primary hypothyroidism cannot be excluded by free thyroxine alone.

Free T4 may be reduced or increased in non-thyroidal illness (sick euthyroid syndrome), but TSH levels are not abnormal.


Surks MI et al. 2004; 291(2): 228-238.