Free T4

Performed in Chemistry

FT4

Green Top LiHep, Plasma
BLMC and WyMCC - SST

Gold/SST or Red Top

Sample stabilized with azide
Multiple freeze/thaw cycles
Collected in an outdated/expired tube
Hemolyzed, icteric or lipemic
Contaminated

Daily, Sunday through Saturday

Less than 4 hours

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. 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 (75%), pre-albumin (15%), and albumin (10%). The determination of free T4 has the advantage of being independent of changes in the concentrations and binding properties of the binding proteins; additional determination of a binder parameter (T-uptake, TBG) is therefore unnecessary. Therefore free T4 is a useful tool in clinical routine diagnostics for the assessment of the thyroid status. It should be measured together with TSH if thyroid disorders are suspected and is also suitable for monitoring thyrosuppressive therapy. A variety of methods are available for estimating the free thyroid hormone levels. The direct measurement of fT4 and fT3 via equilibrium dialysis or ultrafiltration is mainly used as a reference method for standardizing the immunological procedures generally used for routine diagnostic purposes. In the Elecsys FT4 II assay a specific anti-T4 antibody labeled with sulfonylruthenium complexa) is used to determine the free thyroxine. a) Tris(2,2’-bipyridyl)ruthenium(II)-complex (Ru(bpy)2+3