Determination of Reference Intervals of Ratios of Concentrations of Urinary Iodine to Creatinine and Thyroid Hormone Concentrations in Pregnant Women Consuming Adequate Iodine in Harbin, Heilongjiang Province

The objectives of this study were to explore reference intervals of ratios of concentrations of urinary iodine to creatinine (UIC/UCr) in pregnant women, to determine specific reference intervals for thyroid hormone concentrations (FT3, FT4, TSH) in the first trimester of pregnancy, and provide a comprehensive evaluation of iodine nutritional status and thyroid function. Cross-sectional data analysis, including questionnaires and thyroid ultrasonograms, were used to exclude subjects who had a history of thyroid disease, a family history of thyroid disease, or thyroid morphological abnormalities. Tests of thyroid hormone were evaluated together with urinary iodine concentration (UIC), urinary creatinine concentration (UCr), and salt iodine concentration in pregnant women. For the sample of pregnant women eligible for inclusion, the 95% reference intervals of the bilateral limits of UIC/UCr and thyroid hormone concentrations in pregnant women were determined by the percentile method. Pregnant women were recruited in Harbin, Heilongjiang Province, a particular geographical area of China, where iodine nutrition is adequate. The median UIC was 141.47 mu g/L, while the median UIC/UCr was 141.12 mu g/g. The reference intervals of thyroid hormone concentrations in the first trimester were FT3 3.63-6.12 pmol/L, FT4 11.89-22.91 pmol/L, and TSH 0.013-3.814 mIU/L. The reference intervals of UIC/UCr were 52.47 to 532.09 mu g/g (first trimester 47.25-353.91 mu g/g, second trimester 57.95-639.08 mu g/g, third trimester 46.98-494.73 mu g/g).The established UIC/UCr medical reference intervals and thyroid hormone concentration reference intervals may be used for iodine nutritional assessment and thyroid disease screening in pregnant women.