Free thyroxine measured with equilibrium dialysis and nine immunoassays decreases in late pregnancy.

We measured free thyroxine in sera from 29 women in the ninth month of pregnancy. We used the reference method, equilibrium dialysis, and nine immunoassays. The bias between immunoassay and dialysis results was insignificant or modest. With all methods, the results were in the lower part of the non-pregnant female reference interval or below the lower limit of this interval: 10 results with the equilibrium dialysis assay and from 4 to 18 results with the nine immunoassays. Method-specific reference intervals should be determined during pregnancy to be in a position to quantify not only hypothyroidism but also hyperthyroidism and to adjust an appropriate T4 treatment.