Synonyms: Free triiodothyronine
Related tests: TSH, FT4
Clinical significance
T3 is a hormone made in the epithelial cells lining the thyroid follicles from mono- and di-iodinated tyrosine residues (MIT and DIT) of thyroglobulin; once made, T3 remains bound to thyroglobulin until TSH stimulates its release into the blood.
In the blood, T3 is carried by a specific protein called TBG (thyroxine-binding globulin) and, to a lesser extent, by albumin and prealbumin, while a portion remains free (FT3), and it is this portion that is functionally active.
FT3 is certainly the most appropriate test for investigating a low TSH
Clinical indications
Suspected hyperthyroidism
Sample type
The patient must have a blood sample taken.
Preparation
You must fast for at least 8 hours; a small amount of water is allowed. You must have been standing or sitting upright for at least 30 minutes.