T4 FREE (FT4)
1 SST/1.0 mL serum. After 30 minutes of clotting, promptly centrifuge and refrigerate serum
Refrigerated: 48 hours
Measuring free T4 is useful when altered levels of total T4 occur due to changes in T4 binding proteins, especially thyroxine-binding globulin (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.