Free T4, serum

The free fraction of the circulating thyroxine (T4) is considered to exert the main influence on metabolic control. Consequently, the FT4 concentration is believed to be the most direct indicator of an individual’s thyroid status. FT4 concentrations are generally depressed in hypothyroidism and raised in hyperthyroidism. Measurement of FT4 thus provides an aid to the differential diagnosis of thyroid disease. FT4 concentrations are independent of the concentration of thyroid hormone binding proteins  and may therefore be measured in patients with elevated or reduced binding protein concentrations without the need for additional tests of binding capacity. In borderline cases of suspected thyroid malfunction, additional tests such as free T3 or TSH may be necessary