Urinary Iodine/Creatinine as an Indicator of Iodine Status in Lactating Women: Optimal Collection Time Between 0-8 Hours

The urinary iodine/creatinine ratio (UI/Cr) has been recognized as a stable indicator for assessing iodine nutrition status. However, data regarding UI/Cr in lactating women are relatively limited. This study aimed to explore the variation regularity of spot UI/Cr and to provide the optimal time for urine collection. Twenty-two healthy lactating women were recruited for a 7-day iodine metabolism experiment. Daily iodine intake was accurately measured and calculated; all urine samples from each participant were collected separately over the 7-day period, and urinary iodine and urinary creatinine were tested. Spot UI/Cr showed minimal variation between 00:00 and 08:00, with significant changes after 08:00. The 24-hour period was divided into six segments, with the smallest coefficient of variation observed in the 00:00 to 03:59 period. The mixed-effects model analysis revealed that spot UI/Cr during the 0:00 to 3:59 period exhibited the strongest correlation with 24-hour UI/Cr (β = 0.82, P < .001), as well as with 24-hour urinary iodine excretion (β = 0.49, P < .001). Additionally, spot UI/Cr during the 4:00 to 7:59 period demonstrated the strongest correlation with total iodine intake (β = 0.54, P < .001), while spot UI/Cr during the 20:00 to 23:59 period showed the strongest correlation with the total iodine intake of the previous day (β = 0.41, P < .001). As UI/Cr is influenced by dietary iodine and its variation increases after eating, it is recommended that urine collection time for assessing the iodine nutritional status of lactating women using spot UI/Cr be set between 0:00 and 08:00.