Measuring free thyroxin by using magnetic antibody-containing microcapsules.

We describe a novel magnetic-separation microencapsulated antibody displacement assay for free thyroxin. The method is rapid (results available in just over an hour), with a detection limit of 3 pmol/L, and an interassay precision of less than 10% over the range 8-75 pmol/L. Results are not influenced by changes in the concentration of thyroxin-binding globulin or albumin, nor by the presence of circulating anti-thyroxin antibodies or physiological concentrations of nonesterified (free) fatty acids. A reference interval of 8.4-18.4 pmol/L was established for a large euthyroid adult population. Patients with various nonthyroidal illnesses exhibited a similar reference interval (6.2-17.4 pmol/L). Results from pregnant women, at all stages of gestation, were within the nonpregnant euthyroid reference interval. Complete discrimination was obtained between the euthyroid reference interval and values from patients with untreated thyrotoxicosis (24-75 pmol/L). Excellent, though incomplete, discrimination was obtained for patients with untreated hypothyroidism (less than 3.0-9.0 pmol/L). Results for a group of patients defined as having compensated euthyroidism (5.0-15 pmol/L) showed considerable overlap with the euthyroid reference interval. Patients taking thyroxin, who were clinically euthyroid and whose serum concentration of thyrotropin was within its euthyroid reference interval, had free thyroxin concentrations in the range 13.4-24.2 pmol/L. We discuss the validity of this assay in clinical practice.