Free T4 immunoassays are flawed during pregnancy
Abstract
Objective The purpose of this study was to evaluate the diagnostic accuracies of 2 free thyroxine immunoassays during pregnancy.Study design Serum was collected from healthy, thyroid peroxidase antibody-negative women during each trimester and nonpregnant controls. Thyrotropin, total T4 (TT4), free T4 index (FT4I), and 2 different FT4 immunoassays were studied.Results As expected, TT4 was elevated in all 3 trimesters compared to controls (P < .001). FT4I was elevated in the 1st trimester as compared with controls (P < .05) and returned to the nonpregnant range in the 2nd and 3rd trimesters. In contrast, 1st trimester FT4 immunoassay values were either comparable or lower than controls and by the 2nd and 3rd trimesters had decreased to approximately 65% of controls.Conclusion Neither FT4 immunoassay accurately reflects established free T4 changes during pregnancy. TT4 and the FT4I retained an appropriate inverse relationship with TSH throughout pregnancy and appear to provide a more reliable free T4 estimate.
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With some automated free T4 assays, nonpregnancy free T4 is much higher than late pregnancy free T4.
“However, with some automated free T 4 assays nonpregnancy free T 4 is much higher than late pregnancy free T 4 (446,492).”
From 2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis (2016) · cites this paper as [446]
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- Authors
- Lee, R. H., Spencer, C. A., Mestman, J. H., Miller, E. A., Petrovic, I., Braverman, L. E., & Goodwin, T. M.
- Affiliations
- University of Southern California (US)
- Boston University (US)
- Venue
- American Journal of Obstetrics and Gynecology
- Publisher
- Elsevier BV
- Published
- 2009-03
- Volume
- 200
- Issue
- 3
- Pages
- 260.e1-260.e6
- ISSN
- 0002-9378 (ISSN-L)
- OpenAlex
- W2011956011
- Cited by
- 281
- References
- 17
- Citations by year
- 2026: 3 · 2025: 6 · 2024: 7 · 2023: 9 · 2022: 22 · 2021: 9 · 2020: 13 · 2019: 18 · 2018: 20 · 2017: 18 · 2016: 18 · 2015: 17 · 2014: 19 · 2013: 26 · 2012: 22
- FWCI
- 12.48
- Type
- journal article
- Language
- en
- Open access
- closed
- Licence
- Locations
- American Journal of Obstetrics and Gynecology (publishedVersion)
- PubMed (publishedVersion)
- Topics
- Health Sciences › Medicine › Endocrinology, Diabetes and Metabolism › Thyroid Disorders and Treatments
- Health Sciences › Medicine › Physiology › Clinical Laboratory Practices and Quality Control
- Health Sciences › Medicine › Endocrinology, Diabetes and Metabolism › Thyroid Cancer Diagnosis and Treatment
- Keywords
- free thyroxine, pregnancy, free T4 index, total thyroxine, thyroid-stimulating hormone, thyroid function in pregnancy, diagnostic accuracy, pregnancy trimesters
- MeSH terms
- Chemistry, Clinical, Female, Humans, Immunoassay, Pregnancy, Reference Values, Thyroid Gland, Thyrotropin, Thyroxine, Reproducibility of Results
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