What does medical code 84439 stand for?

Cpt 84439 tests the amount of free thyroxine in your body chemistry. This test may be ordered as a FT4, free T4, FTI or FT4 index. The specimen is serum, requiring special handling. Methods may include radioimmunoassay and equilibrium dialysis for reference method. Free thyroxine is a minimal amount of the total T4 level (approximately 1%). This test is not influenced by thyroid-binding abnormalities and perhaps correlates more closely with the true hormonal status. It may be effective in the diagnosis of hyperthyroidism and hypothyroidism.