Journal
EUROPEAN JOURNAL OF ENDOCRINOLOGY
Volume 170, Issue 2, Pages 329-339Publisher
BIOSCIENTIFICA LTD
DOI: 10.1530/EJE-13-0672
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Funding
- Musikforlaeggerne Agnes og Knut Morks Foundation
- Danish Council for Independent Research: Medical Sciences
- Ministry of Social Affairs
- Axel Muusfeldt's Foundation
- Videnskabsminister Erna Hamilton Foundation
- Director Ib Henriksen Foundation
- Snedkermester Sophus Jacobsen og hustru Astrid Jacobsens Foundation
- Faculty of Medical Science's Foundation
- Frimodt-Heineke Foundation
- Torben and Alice Frimodt's Foundation
- Augustinus Foundation
- Danish Medical Association's Foundation
- Doctor Soren Segel and Johanne Wiibroe Segel's Foundation
- Illum Foundation
- A P Moller Foundation for the Advancement of Medical Science
- Arvid Nilsson Foundation
- Gangsted Foundation
- Helsefonden
- Elsass Foundation
- Familien Hede Nielsens Foundation
- Copenhagen University Foundation
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Objectives: Correct 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. Design: Comparison 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. Methods: Different 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. Results: TSH 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. Conclusion: Even 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.
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