Evaluation of thyroid status by measurement of free thyroid hormone concentrations seems particularly helpful in conditions with altered serum binding proteins. In pregnancy, a condition of increased thyroxine binding globulin, serum free thyroxine (FT4) and free triiodothyronine (FT3) concentrations have been reported to be normal, increased or decreased. In the present study we have measured serum total and free thyroid hormone concentrations in pregnant women, their newborns and nonpregnant women. Serum FT4 and FT3 concentrations have been measured with 10 different commercially available kits and the results obtained have been compared. Serum total thyroid hormone concentrations in pregnant women were significantly higher than in their newborns and in nonpregnant women. Maternal serum FT4 concentrations measured with the different kits were always significantly lower than values in nonpregnant women. Furthermore, with one kit, the mean maternal serum FT4 concentration was below the normal range and with many kits, a large number of maternal serum samples had serum FT4 concentrations below the normal range. With all kits, except two, neonatal serum FT4 concentrations were higher than values in their respective mothers and, in general, lower than values in nonpregnant women. Serum FT3 concentrations in nonpregnant women were in the normal range, except with one kit, in which the mean serum FT3 concentration was below the normal range. Serum FT3 concentrations in newborns resulted markedly lower than in parturient and in non pregnant women. With almost all kits, serum FT3 values were below the normal range in many maternal samples. With one kit, maternal serum FT3 concentrations resulted higher than in nonpregnant women, whereas with the other kits serum FT3 concentrations were lower.(ABSTRACT TRUNCATED AT 250 WORDS)

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