Free T4

Keywords: T4, Free thyroxine, Thyroxine, FT4, Thyroxine (free)


Free T4 measurement may be useful in the following situations:

  • Thyroxine therapy
  • Suspected pituitary disease
  • Interferring medications (eg, Amiodarone, Lithium, Steroids)
  • Structural thyroid disease

5 mL blood in plain tube.



Reference Interval:

Method dependent.

10-25 pmol/L (method dependent). Value may be higher in patients on 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.


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. Primary hypothyroidism cannot be excluded by Free T4 alone.

In hyperthyroidism TSH is suppressed and often Free T4 is elevated. If Free T4 is not elevated consider the measurement of Free T3.

In hypothyroidism TSH is typically elevated with low Free T4. However in central or pituitary based hypothyroidism TSH may be normal or low with a low Free T4.

In non-thyroidal illness (sick euthyroid syndrome) Free T4 may be normal or elevated usually with normal TSH.


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

de Groot LJ, Jameson JL. Endocrinology. 5th Ed 2006. Elsevier.

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