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 lg/g (36–386) and the UI/Creat was below 150 lg/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 lg/g had the lowest prevalence of TPOAb positivity (6.1%), while women with a UI/Creat of <150 lg/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 lg/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.

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Keywords thyroid function tests, reference range, thyroid autoimmunity, iodine, pregnancy
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Journal Thyroid
Levie, D, Derakhshan, A., Shu, H., Broeren, M.A.C., de Poortere, R.A., Peeters, R.P, … Korevaar, T.I.M. (2019). The Association of Maternal Iodine Status in Early Pregnancy with Thyroid Function in the Swedish Environmental Longitudinal, Mother and Child, Asthma and Allergy Study. Thyroid. doi:10.1089/thy.2019.0164