New method ("SPAC ET") for free thyroxin in serum evaluated.

We analytically and clinically evaluated a new commercial method ("SPAC ET") for simultaneously determining total and free thyroxin in serum. We found the method to be rapid, simple, and precise. Results significantly correlated with those by "symmetric" dialysis and also agreed with the clinical data for patients with extremely low or high concentrations of thyroxin-binding globulin. On contrast, in those patients the most commonly used thyroid indexes (free thyroxin index or thyroxin:thyroxin-binding globulin ratio) failed to correctly reflect the true functional status of the thyroid.