Clinical Evaluation of the Iodide/Creatinine Ratio of Casual Urine Samples as an Index of Daily Iodide Excretion in a Population Study.

To assess the daily iodine intake in a population study, we have compared the validity and degree of reliability of the urine iodide/creatinine ratios and iodide concentrations of casual samples as a representative index of daily urine iodide excretion. The morning urine samples were obtained from apparently healthy 2,956 men and 1,182 women residing in Sapporo, Japan, and urine iodide was measured by an iodide selective electrode. The iodide/creatinine ratio was higher in women than in men, increasing more steeply with age in women than in men, due to a concomitant decrease in the urine creatinine level with age. The iodide concentration showed no age-related change. Similarly the daily urine iodide excretion measured in 22 control subjects by collecting 24-h urine specimens did not vary with age, while the iodide/creatinine ratio of these subjects increased with age. The correlation coefficient(r) of the iodide concentration with daily iodide excretion in the 95 observations of the 22 subjects was 0.832 (P<0.001), higher than that of iodide/creatinine ratio with daily iodide excretion (0.699, P<0.001). The 95% range of the iodide concentration in morning urine samples in the population (n=4,138) was 9.0-70.3 mol/L with a mean of 27.1 mol/L. These results indicate that in a population with a large difference in age, an age-associated decrease in the urine creatinine concentration should be considered for the evaluation of the iodide/creatinine ratio and the measurement of the iodide concentration in morning spot urine sample may also be useful for the estimation of daily iodide excretion, and that the Japanese subjects here have a high iodine intake, with considerable variation.