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Heleen I Jansen Department of Clinical Chemistry, Endocrine Laboratory, Amsterdam UMC location Vrije Universiteit Amsterdam, Amsterdam, The Netherlands
Amsterdam Gastroenterology, Endocrinology & Metabolism, Amsterdam, The Netherlands
Department of Clinical Chemistry, Endocrine Laboratory, Amsterdam UMC location University of Amsterdam, Amsterdam, The Netherlands

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Antonius E van Herwaarden Radboud University Medical Center, Department of Laboratory Medicine, Nijmegen, The Netherlands

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Henk J Huijgen Department of Clinical Chemistry, Red Cross Hospital, Beverwijk, The Netherlands

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Rebecca C Painter Department of Obstetrics and Gynaecology, Amsterdam UMC Vrije Universiteit Amsterdam, Amsterdam, The Netherlands
Amsterdam Reproduction & Development Research Institute, Amsterdam, The Netherlands

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Jacquelien J Hillebrand Amsterdam Gastroenterology, Endocrinology & Metabolism, Amsterdam, The Netherlands
Department of Clinical Chemistry, Endocrine Laboratory, Amsterdam UMC location University of Amsterdam, Amsterdam, The Netherlands

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Anita Boelen Amsterdam Gastroenterology, Endocrinology & Metabolism, Amsterdam, The Netherlands
Department of Clinical Chemistry, Endocrine Laboratory, Amsterdam UMC location University of Amsterdam, Amsterdam, The Netherlands
Amsterdam Reproduction & Development Research Institute, Amsterdam, The Netherlands

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Annemieke C Heijboer Department of Clinical Chemistry, Endocrine Laboratory, Amsterdam UMC location Vrije Universiteit Amsterdam, Amsterdam, The Netherlands
Amsterdam Gastroenterology, Endocrinology & Metabolism, Amsterdam, The Netherlands
Department of Clinical Chemistry, Endocrine Laboratory, Amsterdam UMC location University of Amsterdam, Amsterdam, The Netherlands
Amsterdam Reproduction & Development Research Institute, Amsterdam, The Netherlands

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Objective

Thyroid hormone measurements are often performed in pregnant women, as hypo- and hyperthyroidism during pregnancy can severely affect the fetus. Serum free thyroxine (fT4) measurements are well known for their analytical challenges, due to low serum concentrations and the subtle equilibrium between free and bound T4 (to thyroid-binding globulin (TBG), transthyretin and albumin). Pregnant women have high TBG concentrations due to an increase in human chorionic gonadotropin (hCG) and estrogen and lower albumin concentrations which change the equilibrium and may affect the validity of fT4 measurements in their samples. As accurate serum fT4 measurements in pregnant women are important for the long-term health of the fetus, we aimed to evaluate the accuracy of several fT4 immunoassays in the serum of pregnant women.

Methods

FT4 was measured in healthy controls and pregnant women using a candidate-reference method (LC-MS/MS) and five commercially available automated immunoassays (Alinity (Abbott), Atellica (Siemens), Cobas (Roche), Lumipulse (Fujirebio) and UniCel DXI (Beckman Coulter)). Method comparisons (Bland Altman plots and Passing and Bablok analyses) were performed.

Results

Serum samples from both healthy controls (n  = 30) and pregnant women (n  = 30; mean gestational age, 24.8 weeks) were collected. The fT4 immunoassays deviated +7 to +29% more from the LC-MS/MS in serum samples of pregnant women than healthy controls (falsely high).

Conclusions

Our results indicate that immunoassays overestimate fT4 in pregnant women, which might lead to an overestimation of thyroid status. Physicians and laboratory specialists should be aware of this phenomenon to avoid drawing false conclusions about thyroid function in pregnant women.

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