The Free TFT panel measures the biologically active, unbound forms of the main thyroid hormones and the pituitary's regulating hormone.
T4 is considered the most reliable indicator of thyroid status because it represents the hormone available to body tissues, unaffected by changes in the proteins that bind T4.

T3 is the most active thyroid hormone. T3 levels are useful in diagnosing Hyperthyroidism, especially when T3 is normal (a condition called T3 toxicosis).
Measuring the Free hormones (FT3, FT4) provides a more accurate assessment of the body's functional thyroid status because it is not affected by changes in serum binding proteins (which can be influenced by pregnancy, certain medications, or liver/kidney disease).