Free T3 index - serum

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

5 mL blood in plain tube.

Method

Total T3: immunoassay.
T3 uptake: radiometric assay.

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

Reference Interval

Depends on method.

Application

The free T3 index has generally been replaced by Triiodothyronine (free) - serum.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

Interpretation

Free T3 index is increased in T3 toxicosis and thyrotoxicosis.

Reference

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