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Normalized Serum Thyroxine (T₄N)

The Test for Thyroid Function in Pregnancy

From the Department of Radiology, Division of Nuclear Medicine and the Department of Obstetrics and Gynecology, University of Miami School of Medicine, PO Box 875, Biscayne Annex, Miami, Fla 33152.

Abstract

During pregnancy and the administration of estrogen, physiologic changes make it difficult to establish the patient's thyroid status. Because it is desirable to avoid radioiodine, and in vitro tests are often misleading, serious diagnostic problems result. These physiologic factors alter thyroxine binding globulin (TBG) and are reflected in the value of serum thyroxine. Pregnancy and estrogen therapy cause thyroxine levels to rise, whereas hypoproteineimia, androgen therapy and certain drugs depress thyroxine levels. The normalized serum thyroxine (T₄N), by the dual competitive protein binding analysis, reintroduced the patient's serum into the test system at the competitive binding stage. The newly introduced TBG portion provides additional competitive binding sites that have a corrective effect on serum thyroxine values altered by TBG changes, and will normalize abnormal values that are due to physiologic changes but it will not affect normal or pathologic states. Clinical data obtained with this technic correlated better with thyroid status than did any of the earlier thyroid function tests.