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Free thyroxine (T4) by direct equilibrium dialysis accurately separates euthyroid patients with significantly altered serum T4 binding from those with overt thyroid hormone excess or deficiency. Thus, with either normal or altered pituitary-thyroid hormone axis, free T4 by direct equilibrium dialysis can be effectively used as an initial screening test, with a sensitive thyrotropin determination as confirmation in patients with abnormal values. Other free T4 methods frequently provide inappropriate values in sera with significantly altered T4 binding, such as occurs with severe nonthyroidal illnesses, extreme increases or decreases of thyroid hormone—binding globulin, increased transthyretin binding, familial dysalbuminemia, or T4 autoantibodies, but appropriate values in sera with mild alterations of serum T4 binding.
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