Clinical value of free-thyroxin measurements

Although discrepancies have been reported between free thyroxin (fT4) in serum and actual thyrometabolic status, the concentration of free thyroid hormones in blood is thought to be an important determinant of the thyrometabolic status of patients. Therefore, the diagnostic value of fT4 measurement has been emphasized. This prompted us to evaluate the diagnostic significance of direct fT4 measurement. We conclude that: (a) The different fT4 radioimmunoassays we examined did not excel the total thyroxin, fT4 index, or thyroxin/thyroxin-binding globulin ratio in the laboratory assessment of thyroid status and alterations in thyroid hormone-thyroxin-binding globulin relationships. (b) The fT4 values obtained by direct or indirect methods always shifted in parallel to total thyroxin with respect to borderline increased or decreased total thyroxin, subclinical hypothyroidism, isolated hypertriiodothyroninemia, and thyroxin substitution.