Urinary iodine concentration follows a circadian rhythm: A study with 3023spot urine samples in adults and children

Our overall aim is to monitor iodine supply in a prospective study before a nd after the September 1998 increase of salt iodide content in Switzerland. Because iodide is supplied by alimentation, we moreover wondered whether u rinary iodine concentration (UI) is governed by circadian rhythmicity. Forty-two subjects (18 males and 24 females, including 13 children) collect ed 3023 urine spots between May 1996 and May 1998, at a rate of three to fi ve samples per month, at any time of the day. The results show that circadi an rhythmicity of UI in adults and children was found independent of the in dividual subject, age, gender, and season. Lowest UI levels were found betw een 8-11 h. A curve increasing progressively between 12 and 24 h was obtain ed. UT returned to base-line levels between 21 and 22 h in children only. U I peaks occurred 4-5 h after main meals; children's peaks occurred later th an that of adults. Although the existence of a circadian rhythm of UI is probably universal, i ts profile, however, depends on alimentation. Because nadir of UI is repres ented by morning spots, this might seem an appropriate collecting period. I n view of the significant circadian rhythmicity of UI, studies with restric tion of sampling time to morning hours, for example, cannot be directly com pared with studies in which urine is sampled all over the day.