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The Association of Maternal Iodine Status in Early Pregnancy with Thyroid Function in the Swedish Environmental Longitudinal, Mother and Child, Asthma and Allergy Study
Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
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Number of Authors: 92019 (English)In: Thyroid, ISSN 1050-7256, E-ISSN 1557-9077, Vol. 29, no 11, p. 1660-1668Article in journal (Refereed) Published
Abstract [en]

Background: Severe maternal iodine deficiency can impact fetal brain development through effects on maternal and/or fetal thyroid hormone availability. The effects of mild-to-moderate iodine deficiency on thyroid function are less clear. The aim was to investigate the association of maternal urinary iodine concentration corrected for creatinine (UI/Creat) with thyroid function and autoantibodies in a mild-to-moderate iodine-deficient pregnant population.

Methods: This study was embedded within the Swedish Environmental Longitudinal, Mother and child, Asthma and allergy (SELMA) study. Clinical reference ranges were determined by the 2.5th and 97.5th population-based percentile cutoffs. The associations of UI/Creat with thyrotropin (TSH), free thyroxine (fT4), free triiodothyronine (fT3), total T4 (TT4), and total T3 (TT3) were studied using multivariable linear regression in thyroid peroxidase antibody (TPOAb)-negative women. The association of UI/Creat with TPOAb and thyroglobulin antibody (TgAb) positivity was analyzed using multivariable logistic regression.

Results: Urinary iodine and thyroid function were measured at a median (95% range) gestational age of 10 (6-14) weeks in 2009 women. The median (95% range) UI/Creat was 85 mu g/g (36-386) and the UI/Creat was below 150 mu g/g in 80.1% of women. Reference ranges did not differ substantially by UI/Creat. A lower UI/Creat was associated with a lower TSH (p = 0.027), a higher TT4 (p = 0.032), and with a corresponding trend toward slightly higher fT4 (p = 0.081), fT3 (p = 0.079), and TT3 (p = 0.10). UI/Creat was not associated with the fT4/fT3 (p = 0.94) or TT4/TT3 ratios (p = 0.63). Women with a UI/Creat of 150-249 mu g/g had the lowest prevalence of TPOAb positivity (6.1%), while women with a UI/Creat of <150 mu g/g had a higher prevalence (11.0%, odds ratio [OR] confidence interval [95% CI] 1.84 [1.07-3.20], p = 0.029). Women with a UI/Creat >= 500 mu g/g showed the highest prevalence and a higher risk of TPOAb positivity, however, only a small proportion of women had such a UI/Creat (12.5%, OR, [95% CI] 2.36 [0.54-10.43], p = 0.26).

Conclusions: We could not identify any meaningful differences in thyroid function reference ranges. Lower iodine availability was associated with a slightly lower TSH and a higher TT4. Women with adequate iodine intake had the lowest risk of TPOAb positivity.

Place, publisher, year, edition, pages
2019. Vol. 29, no 11, p. 1660-1668
Keywords [en]
thyroid function tests, reference range, thyroid autoimmunity, iodine, pregnancy
National Category
Obstetrics, Gynecology and Reproductive Medicine Endocrinology and Diabetes
Identifiers
URN: urn:nbn:se:su:diva-175808DOI: 10.1089/thy.2019.0164ISI: 000489707200001PubMedID: 31524090OAI: oai:DiVA.org:su-175808DiVA, id: diva2:1371809
Available from: 2019-11-21 Created: 2019-11-21 Last updated: 2019-11-21Bibliographically approved

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