Reference Intervals of Serum Iodine Concentration in Chinese Pregnant Women

The study aims to establish trimester-specific reference ranges for serum iodine (SI) in Chinese pregnant women and explore its associations with maternal and infantile thyroid function. Apparently healthy pregnant women were enrolled during their first antenatal visit. Fasting venous and spot urine samples were collected for determining serum and urinary iodine (UI) levels by a validated inductively coupled plasma mass spectrometry. Serum free triiodothyronine (FT3), free thyroxine (FT4), thyrotropin (TSH), and neonatal TSH levels were tested by electro-chemiluminescent assay. The reference ranges of SI were established by percentile method and reported as 2.5–97.5%. ROC analysis was applied to compare the discriminative ability of SI, UI, and UI to urinary creatine ratio (UI /UCr) in early pregnancy for various thyroid conditions. The trimester-specific reference ranges of SI for Chinese pregnant women were 60.91–114.53 μg/L for the first trimester (T1, n = 1029), 54.57–103.42 μg/L for the second trimester (T2, n = 379), and 52.03–110.40 μg/L for the third trimester (T3, n = 455). Maternal SI at T1 but not UI and UI/UCr was significantly correlated with FT3 (r = 0.393, P < 0.001), FT4 (r = 0.637, P < 0.001), and TSH (r = −0.299, P<0.001). Maternal SI change% from T1 to T2 (but not SI change% from T1 to T3) had marginal correlation with neonatal TSH (r=−0.106, P=0.046). ROC analysis showed that maternal SI at T1 had better predictability for several thyroid conditions than UIC and UI/UCr.