Free Thyroxine Index Calculation. The free thyroxine index has generally been replaced by free thyroxine in the assessment of thyroid function, but is occasionally useful when a free t4 result is suspected of being anomalous. Not recommended for routine thyroid screening.
Fti = thyroxine (t4)/thyroid binding capacity. Interassay precision (cv) is <5%. Serum separator tube or plasma separator tube.
Direct assay or laboratory measurement of free serum.
Thyroxine in blood is more tightly bound to serum proteins than is t3, consequently the free t4 (ft4) bioavailable fraction is less than free t3 (0.02% versus 0.2%, ft4 versus ft3, respectively). Some suggest that tsh levels of over 2.5 milliunits per liter (mu/l) are abnormal, while others consider levels of tsh to be too high only after they have reached 4 to 5 mu/l. Free t4 can be directly measured via equilibrium dialysis. Free thyroxine index is considered to be a reliable indicator of thyroid status in the presence of abnormalities in plasma protein binding.
