Free T3

Keywords: T3, Free triiodothyronine, Triiodothyronine, FT3, Triiodothyronine (free)


5 mL blood in plain tube.



Reference Interval:

4.0-8.0 pmol/L (method dependent).


Thyroid stimulating hormone is the preferred initial test in the assessment of thyroid function.

The primary indication for Free T3 is for the diagnosis of T3 toxicosis (that is, suppressed TSH and normal Free T4).

Although occasionally used as an early or confirmatory test for thyrotoxicosis (Free T3 is often elevated before Free T4), it is not required to establish this diagnosis.

It is an unreliable test for hypothyroidism, and should not be used. For monitoring of patients on T3 therapy.


Increased levels of Free T3 are found in T3 toxicosis and in thyrotoxicosis. Free T3 may be elevated earlier than Free T4 in thyrotoxicosis.


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