ObjectivesCorrect interpretation of thyroid status during pregnancy is vital to secure fetal development. Pregnancy-related changes in maternal thyroid status necessitate the use of gestational age-specific reference ranges. In this study, we investigated between-laboratory reproducibility of thyroid reference ranges in pregnant women.DesignComparison of two longitudinal prospective cohort studies including 255 (cohort 1) and 101 (cohort 2) healthy antibody-negative Danish pregnant women attending prenatal care at Copenhagen University Hospital.MethodsDifferent immunoassays were used to measure thyroid hormone levels in the two cohorts. Thyroid hormone reference ranges were established for every 5 weeks of gestation. Differences between cohorts were explored through mixed-model repeated measures regression analyses. By applying reference ranges from one cohort to the other, the proportion of women who would be misclassified by doing so was investigated.ResultsTSH increased and free thyroxine (FT₄) decreased as pregnancy progressed. Results indicated highly significant differences between cohorts in free triiodothyronine (F=21.3, P<0.001) and FT₄ (F=941, P<0.001). TSH levels were comparable (P=0.09). Up to 90.3% of the women had FT₄ levels outside their laboratory's nonpregnant reference range, and up to 100% outside the other cohort's gestational-age-specific reference ranges. Z-score-based reference ranges markedly improved comparison between cohorts.ConclusionEven in the same region, the use of gestational-age-specific reference ranges from different laboratories led to misclassification. Up to 100% of maternal FT₄ levels fell outside the other cohort's reference range despite similar TSH levels. In clinical practice, thyroid testing of pregnant women without adding method specificity to gestational age-dependent reference ranges will compromise patient safety.