Introduction
Thyroxine (T4) is the main thyroid hormone secreted into the bloodstream by the thyroid gland. Together with triiodothyronine (T3) it plays a vital role in regulating the body's metabolic rate, influences the cardiovascular system, growth and bone metabolism, and is important for normal development of gonadal functions and nervous system[1]. T4 circulates in the bloodstream as an equilibrium mixture of free and serum bound hormone. Free T4 (fT4) is the unbound and biologically active form, which represents only 0.03% of the total T4. The remaining T4 is inactive and bound to serum proteins such as thyroxine binding globulin (TBG) (75%), pre-albumin (15%), and albumin (10%)[2-5]. The determination of free T4 has the advantage of being independent of changes in the concentrations and binding properties of these binding proteins; additional determination of a binding parameter (T - uptake, TBG) is therefore unnecessary. Thus free T4 is a useful tool in clinical routine diagnostics for the assessment of the thyroid status.

Advantages
Independent packaging to solve the problem of validity after opening
Simple operation and fast response

Clinical significance
✔ to help evaluate thyroid gland function
✔ to help diagnose thyroid disease
✔ to monitor effectiveness of thyroid treatment
✔ sometimes a free T4 is used to help diagnose congenital hypothyroidism in newborns

Intent use

Outpatient department, emergency department, endocrinology department, obstetrics and gynecology department, physical examination center

Reference

1. Kronenberg HM, Melmed S, Polonsky KS, et al. Williams Textbook of Endocrinology. Saunders Elsevier, Philadelphia, 12th edition, 2011, chapter 10, p. 301-311.
2. Robbins J, Rall JE. The interaction of thyroid hormones and protein in biological fluids. Recent Prog Horm Res 1957;13:161-208.
3. Oppenheimer JH. Role of plasma proteins in the binding, distribution and metabolism of the thyroid hormones. N Engl J Med 1968;278(21):1153-1162.
4. DeGroot LJ, Larsen PR, Hennemann G. Transport of thyroid hormone and cell uptake. The thyroid and its diseases. Wiley and Sons, New York, 1984:62-65.
Ekins RP. Measurement of free hormones in blood. Endocr Rev 1990;11(1):5-46.