Free T4

CPT Code

84439

Specimen Type

Blood

Container Type

Serum Separator Tube (SST)/Yellow

Stability

Room Temperature, < 8 hours; 2-8°C, < 48 hours; -20°C, > 48 hours

Handling Instructions

Allow to clot for 30 mins, centrifuge within 2 hours

Clinical Notes

Total T4 consists of both bound and free fractions but only the free hormone is metabolically active. At any given time, most thyroxine is bound to thyroxine binding globulin (TBG), prealbumin, and albumin (>99.9%), with only a very small fraction (0.03% to 0.04%) free in the serum. Factors altering these carrier proteins will alter total T4 levels accordingly, but the free level (active fraction) is much less sensitive to changes in the serum binding proteins