Laboratory Medicine Program

Thyroxine, free (Free T4)
(T4, free, FT4)

Clinical Decription:
Thyroxine (3,5,3?,5?-tetraiodothronine,L-thyroxine or T4) is a hormone synthesized and secreted by the thyroid gland and plays an important role in regulating metabolism. Secretion into the circulation is in response to the pituitary hormone TSH (thyroid stimulating hormone) and is regulated by a negative feedback mechanism involving the thyroid gland, pituitary gland and hypothalamus. In the circulation, 99.95% of the T4 is reversibly bound to transport proteins, primarily thyroxine-binding globulin (TBG) and to a lesser extent albumin and thyroxine-binding prealbumin (TBPA). The remaining T4 is not bound to transport proteins, but is free in the circulation. This unbound fraction, or free T4 (FT4) is both metabolically active and a precursor to triiodothyronine (T3). Free T4 levels correlate with T4 secretion and metabolism. In hypothyroidism and hyperthyroidism, FT4 levels paralleled changes in total T4 levels. Measuring free T4 is useful when altered levels of total T4 occur due to changed in FT4 binding proteins, especially TBG. TBG levels remain relatively constant in healthy individuals, but certain conditions, such as normal pregnancy and steroid therapy, can alter these levels. In these conditions, free T4 levels are unchanged, while total T4 levels parallel the changes in TBG.
<br/><em>Bayer Centaur Manufacturer's Manual</em>

Method: Chemiluminometric technology; i2000

Component Tests Used: n/a

Reference Ranges Used:
9- 19 pmol/L

Specimen Type: serum
Volume: 2.5 mL (minimum: 1 mL)

Shipping: room temperature or 4C

Special Instructions: Centrifuge and aliquot sample.

Testing Schedule(s): Daily.

Turnaround Time: 1 day

For more information, call 416.340.5227 or 1.866.865.5227