WHAT IS THYROID TEST: THYROXINE FREE (FT4)?
Thyroxine (T4) is a hormone produced in the thyroid gland from iodide and thyroglobulin in a multistep process. Less than 0.05% of thyroxine circulates freely and is thus biologically active. Biologically active T4 stimulates the basal metabolic rate, including use of carbohydrates and lipids, protein synthesis, bone calcium release, and vitamin metabolism. In infants, T4 plays an important role in central nervous system growth and development. Circulating T4 levels affect the release of thyroid-stimulating hormone (TSH) and hypothalamic thyroid-releasing hormone (TRH) through a negative-feedback mechanism. Because of the tiny quantity of free T4 normally present, direct measurement is difficult and expensive. Equilibrium dialysis is the common standard used for measuring free T4. Radioimmunoassay is often used but is subject to the influence of the serum albumin and lipid levels. Newer testing kits are being developed to improve the accuracy and ease of direct measurement of free T4