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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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© 1993 Elsevier Inc. Published by Elsevier Inc. All rights reserved.