Free thyroxine measurement by analogue immunoassay and equilibrium dialysis in dogs with non-thyroidal illness

•Free T4 concentration as measured by an analogue method is affected by the severity of nonthyroidal illnesses•Free T4 concentration as measured by an analogue method and by equilibrium dialysis cannot be used interchangeably•Free T4 as measured by an analogue method provides limited information over total T4 in dogs with nonthyroidal illnesses