Synonyms: Free thyroxine
Related tests: TSH, FT3, thyroglobulin antibodies, thyroid peroxidase antibodies
Clinical significance
T4 is a hormone made in the epithelial cells lining the thyroid follicles from 2 di-iodinated tyrosine residues (DIT) of thyroglobulin; once made, T4 remains bound to thyroglobulin until TSH stimulates its release into the blood. In the blood, T4 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 (FT4) and can reach the peripheral tissues, where T4 is converted into T3.
FT4 is certainly the most appropriate test for investigating a raised TSH.
Clinical indications
Suspected hypothyroidism
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.