T4 (Thyroxine), Free - FRT4

Free T4 represents the unbound, biologically active form of the thyroid hormone T4 that is available to enter body tissues and regulate metabolism, heart rate, and energy.

This test is used to evaluate thyroid function, especially when there are symptoms of an overactive or underactive thyroid, and is more accurate than measuring total T4 because it excludes hormone bound to proteins.