Reference Intervals of the Ratio of Urine Iodine to Creatinine in Pregnant Women in an Iodine-Replete Area of China

The assessment method of iodine nutrition for pregnant women lacks strong evidence-based medicine. The prevalence of iodine deficiency in pregnant women may be overestimated using urinary iodine concentration (UIC). The reference intervals of UIC-to-urinary creatinine concentration ratio (UIC/UCr) were established using a self-sequential longitudinal study of pregnant women with singleton gestation who were recruited using the criteria of the National Academy of Clinical Biochemistry in Dandong City, which is a long-term iodine-replete area. Nine thousand one hundred sixty-four pregnant women in the first trimester from Dalian City, Dandong City, and Shenyang City were included to verify our proposed reference intervals. UIC and concentrations of urinary creatinine, serum iodine, TSH, FT4, TPOAb, and TgAb were measured. The reference intervals of UIC/UCr were 38.63-489.46 μg/g for the first trimester, 58.48-644.03 μg/g for the second trimester, and 56.27-644.93 μg/g for the third trimester. The prevalence of iodine deficiency was 49.50% using UIC as the indicator ( 150 μg/L), while the prevalence was 3.28% using UIC/UCr ( 38.63 μg/g). The prevalence of iodine excess was 3.21% using UIC as the indicator ( 500 μg/L) while the prevalence was 1.45% using UIC/UCr ( 489.46 μg/g). The highest prevalence of overt hypothyroidism and positive thyroid antibodies was in the group with UIC/UCr 38.63 μg/g. In contrast to the place of residence and age, BMI was an influencing factor for UIC/UCr. The reference intervals of UIC/UCr were established. UIC/UCr may eliminate the effect of urine volume and reflect the actual prevalence of iodine deficiency in pregnant women.