Free T3 index - serum

Last updated: Wednesday, 31, March, 2004
Item Process

5 mL blood in plain tube.


Total T3: immunoassay.
T3 uptake: radiometric assay.

Calculation of free T3 index = Total T3 x T3 uptake.

Reference Interval

Depends on method.


The free T3 index has generally been replaced by Triiodothyronine (free) - serum.

Total T3 is also used when triiodothyronine (free) results are suspected, on clinical grounds, of being anomalous.

The free T3 index is an unreliable test for hypothyroidism and should not be used.


Free T3 index is increased in T3 toxicosis and thyrotoxicosis.


Kaplan MM. Med Clin N Amer 1985; 69: 863-880.