Concentration of free thyroxin in serum of a patient with euthyroid hyperthyroxinemia secondary to increased thyroxin-binding prealbumin: results by various methods compared.

We used various kits to measure free thyroxin (T4) in serum from a patient with the very rare syndrome of euthyroid hyperthyroxinemia secondary to increased thyroxin-binding prealbumin in serum. This patient's free T4 by equilibrium dialysis was 13 ng/L (normal 9-21), whereas the free-T4 index was 18.6 (normal 5.5-11.5). Using six different commercial methods, the last three listed being T4-analog methods, we measured the serum free T4 in this patient. The mean +/- SD (and normal range) free-T4 concentrations (ng/L) measured were: GammaCoat, Two-Step, 19.2 +/- 4.5 (6.9-26.9); Liquisol, 17.8 +/- 3.0 (12.4-20.8); Immo Phase, Two-Step 26.8 +/- 0.3 (12.4-21.2); GammaCoat, Single-Step, 19.7 +/- 5.7 (10.3-21.0); Immo Phase, Single-Step, 27.7 +/- 0.9 (8.6-25.0); and Amerlex, 14.1 +/- 0.6 (5.3-14.1). In four patients with familial dysalbuminemic hyperthyroidism, in whom the serum T4 by equilibrium dialysis was normal, all three T4-analog methods gave results far exceeding normal (greater than 50 ng/L).