{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,7,8]],"date-time":"2026-07-08T05:01:10Z","timestamp":1783486870855,"version":"3.55.0"},"reference-count":35,"publisher":"Oxford University Press (OUP)","issue":"2","content-domain":{"domain":["eje.bioscientifica.com"],"crossmark-restriction":true},"short-container-title":[],"published-print":{"date-parts":[[2014,2]]},"abstract":"<jats:sec><jats:title>Objectives<\/jats:title><jats:p>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.<\/jats:p><\/jats:sec><jats:sec><jats:title>Design<\/jats:title><jats:p>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.<\/jats:p><\/jats:sec><jats:sec><jats:title>Methods<\/jats:title><jats:p>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.<\/jats:p><\/jats:sec><jats:sec><jats:title>Results<\/jats:title><jats:p>TSH increased and free thyroxine (FT<jats:sub>4<\/jats:sub>) decreased as pregnancy progressed. Results indicated highly significant differences between cohorts in free triiodothyronine (<jats:italic>F<\/jats:italic>=21.3, <jats:italic>P<\/jats:italic>&lt;0.001) and FT<jats:sub>4<\/jats:sub> (<jats:italic>F<\/jats:italic>=941, <jats:italic>P<\/jats:italic>&lt;0.001). TSH levels were comparable (<jats:italic>P<\/jats:italic>=0.09). Up to 90.3% of the women had FT<jats:sub>4<\/jats:sub> levels outside their laboratory's nonpregnant reference range, and up to 100% outside the other cohort's gestational-age-specific reference ranges. <jats:italic>Z<\/jats:italic>-score-based reference ranges markedly improved comparison between cohorts.<\/jats:p><\/jats:sec><jats:sec><jats:title>Conclusion<\/jats:title><jats:p>Even in the same region, the use of gestational-age-specific reference ranges from different laboratories led to misclassification. Up to 100% of maternal FT<jats:sub>4<\/jats:sub> 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.<\/jats:p><\/jats:sec>","DOI":"10.1530\/eje-13-0672","type":"journal-article","created":{"date-parts":[[2013,11,26]],"date-time":"2013-11-26T03:40:50Z","timestamp":1385437250000},"page":"329-339","update-policy":"https:\/\/doi.org\/10.1530\/crossmarkpolicy-1","source":"Crossref","is-referenced-by-count":39,"title":["Gestational age-specific reference ranges from different laboratories misclassify pregnant women's thyroid status: comparison of two longitudinal prospective cohort studies"],"prefix":"10.1093","volume":"170","author":[{"given":"Sofie","family":"Bliddal","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Ulla","family":"Feldt-Rasmussen","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Malene","family":"Boas","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jens","family":"Faber","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Anders","family":"Juul","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Torben","family":"Larsen","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Dorthe Hansen","family":"Precht","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"286","reference":[{"issue":"2","key":"1_42262945","doi-asserted-by":"crossref","first-page":"235","DOI":"10.1016\/j.mcna.2012.01.004","volume":"96","author":"Yazbeck","year":"2012","journal-title":"The Medical clinics of North America","ISSN":"https:\/\/id.crossref.org\/issn\/0025-7125","issn-type":"print"},{"issue":"12","key":"2_43728323","doi-asserted-by":"publisher","first-page":"4464","DOI":"10.1210\/jc.2012-2540","volume":"97","year":"2012","journal-title":"Journal of Clinical Endocrinology &amp; 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