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Urine iodine-to-creatinine ratio more accurate test for deficiency in pregnancy
In pregnant women, the urine iodine-to-creatinine ratio is a more accurate indicator of iodine levels than spot urinary iodine concentration, according to recent findings.
Weiping Teng, MD, professor in the department of endocrinology and metabolism at the First Hospital of China Medical University in Shenyang, and colleagues evaluated 143 pregnant women (mean age, 27.9 years; mean BMI, 21.7 kg/m2) before 8 weeks gestation and 506 reproductive-age women (mean age, 28 years; mean BMI, 21.7 kg/m2) recruited from the Liaoning province of northeastern China, which is an iodine-adequate region. The researchers collected first-morning spot urine and fasting blood samples from all participants, and these were assayed for urinary iodine and creatinine, as well as blood concentrations of thyroglobulin. Total circulating iodine levels were measured at weeks 8, 20 and 36 of gestation and at 6 months postpartum. Among the pregnant women, follow-up visits occurred at 8, 12, 16, 28 and 36 weeks of gestation, and at 3 and 6 months after delivery.
Researchers found a reduction in the median urinary iodine concentration from 183.6 µg/L to 104.2 µg/L during pregnancy. Changes in blood concentration of iodine were similar to the urine iodine-to-creatinine ratio (UI/Cr). Mean serum iodine concentration was lowest at week 8 of gestation (60.5 µg/L), increased significantly until week 20 (106.5 µg/L), and then decreased to 84.7 µg/L at 36 weeks. The area under the receiver-operator characteristic (ROC) curve for UI/Cr was 0.92 for diagnoses of iodine deficiency and 0.78 for serum iodine. For diagnoses of excessive iodine, the area under the ROC curve was 0.82 for serum iodine and 0.75 for UI/Cr (P < .001). For urinary iodine, the area under the ROC curve was 0.61 (P = .11) for iodine-deficient diagnoses and 0.65 (P = .08) for iodine excess. Moreover, the area under the ROC curve values for thyroglobulin were 0.54 (P = .53) for deficiency and 0.53 (P = .74) for excess.
“The single time spot [urinary iodine concentration] level decreased gradually as the gestational weeks increased ... however, the opposite was observed with the single time spot UI/Cr levels, which gradually increased as pregnancy progressed. ...The [urinary iodine concentration] and UI/Cr bell-shaped distribution curves were almost identical in the nonpregnant women, suggesting that these two indexes could be used interchangeably,” the researchers wrote. “However, in the pregnant women, the two distribution curves were separate from each other, suggesting that these two urinary iodine indexes were not interchangeable.” – by Jennifer Byrne
Disclosure: The researchers report no relevant financial disclosures.