Free T4 measures the unbound, biologically active portion of thyroxine, providing the most accurate assessment of available thyroid hormone. Unlike total T4, free T4 is not affected by variations in thyroid hormone-binding proteins, which can be altered by pregnancy, estrogen therapy, liver disease, and other factors.
This test is part of standard thyroid evaluation, particularly in pregnancy, liver or kidney disease, and any setting where binding proteins may be abnormal. It is also used to monitor patients on thyroid hormone replacement. Sample type is serum.
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