RAPID COMMUNICATION: A COMPARISON OF FREE T4 AND THE RATIO OF TOTAL T4 TO T4-BINDING GLOBULIN IN SERUM THROUGH PREGNANCY

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Abstract
Serum free T4 [thyroxine] concentrations have been variously reported as either constant or falling pregnancy. Here, 122 serum samples from apparently normal [human] pregnancies were used to derive euthyroid ranges throughout pregnancy for T4 and free T4 and for T4-binding globulin (TBG). The mean free T4 concentration fell with increasing gestational age, the range of values in the 3rd trimester being narrower than the nonpregnant normal range. Evidence is reviewed which shows that the Amerlex free T4 assay is unaffected by the elevation of TBbG in nonpregnant subjects. The decline in mean free T4 as pregnancy proceeds is in good agreement with the change in free T4 predicted by the falling T4/TBG ratio.