Day-to-day and within-day variation in urinary iodine excretion

Objective: To examine the day-to-day and within-day variation in urinary iodine excretion and the day-to-day variation in iodine intake. Design: Collection of consecutive 24-h urine samples and casual urine samples over 24 h. Setting: The study population consisted of highly motivated subjects from our Institute. Subjects: Study 1: Ten healthy subjects (seven females and three males) aged 30-46 y. Study 2. Twenty-two healthy subjects (9 males and 13 females) aged 30-55 y. Methods: Study 1: 24-h urine samples were collected for four consecutive days. Study 2. Each urine voided over 24 h was collected into separate containers. In both studies dietary records were kept. Main outcome measures: Twenty-four-hour urinary iodine excretion, 24-h urinary iodine excretion estimated as I/Cr*24 h Cr and as a concentration in casual urine samples. Results: Study 1: Both iodine excreted in 24-h urine and iodine intake varied from day-to-day. Iodine excretion correlated with iodine intake (r = 0.46, P = 0.01). Iodine intake (mean 89 +/- 6.5 mu g/d) was not significantly different from iodine excretion (mean 95 +/- 5.3 mu g/d). Study 2. Twenty-four hour iodine excretion estimated as I/Cr*24 h Cr from the morning urine sample was significantly lower than actual 24-h iodine excretion, whereas 24-h iodine excretion estimated as I/Cr*24 h Cr from the first sample after the morning sample and the last sample before the subjects went to bed was not significantly different from actual 24-h iodine excretion. Twenty-four-hour urine excretion estimated as a concentration was lower than actual 24-h iodine excretion in casual urine taken at any time of the day. Conclusions: For determination of iodine status in an individual, more than one 24-h urine sample must be used. The use of the I/Cr ratio in casual urine samples is a usable measure of iodine status if corrected for the age- and sex-adjusted 24-h creatinine excretion. Further, the study suggests that fasting morning urine samples would underestimate iodine status in this population.