Clinical evaluation of nine free thyroxine assays: persistent problems in particular populations.

Over the past decade, numerous papers have addressed the various methodological problems encountered with free thyroxine (FT4) assays. We evaluated the clinical performance of nine FT4 assays in five centres, using a panel of 310 sera: 156 from euthyroid controls; 27 from hyperthyroid patients; 34 from untreated hypothyroidism; 22 from patients with renal failure; 30 from women in the last trimester of pregnancy; 23 from patients on thyroid substitutive therapy; and 18 from patients treated with amiodarone. Only three methods showed a Gaussian distribution of FT4 concentrations.