We prospectively investigated urinary iodine concentration (UIC) in pregnant women and in female, non-pregnant controls in the canton of Berne, Switzerland, in 1992. Mean UIC of pregnant women [205 +/- 151 microg iodine/g creatinine (microg l/g Cr); no. = 153] steadily decreased from the first (236 +/- 180 microg l/g Cr; no. = 31) to the third trimester (183 +/- 111 microg l/g Cr, p < 0.0001; no. = 66) and differed significantly from that of the control group (91 +/- 37 microg l/g Cr, p < 0.0001; no. = 119). UIC increased 2.6-fold from levels indicating mild iodine deficiency in controls to the first trimester, demonstrating that high UIC during early gestation does not necessarily reflect a sufficient iodine supply to the overall population. Pregnancy is accompanied by important alterations in the regulation of thyroid function and iodine metabolism. Increased renal iodine clearance during pregnancy may explain increased UIC during early gestation, whereas increased thyroidal iodine clearance as well as the iodine shift from the maternal circulation to the growing fetal-placental unit, which both tend to lower the circulating serum levels of inorganic iodide, probably are the causes of the continuous decrease of UIC over the course of pregnancy. Mean UIC in our control group, as well as in one parallel and several consecutive investigations in the same region in the 1990s, was found to be below the actually recommended threshold, indicating a new tendency towards mild to moderate iodine deficiency. As salt is the main source of dietary iodine in Switzerland, its iodine concentration was therefore increased nationwide in 1998 for the fourth time, following increases in 1922, 1965 and 1980.

Objective: Because of known significant seasonal fluctuations of iodine concentration in cow's milk (ICM) in Switzerland (winter=summer (w=s) ICM ratio averages 5.6), we looked for seasonal variations and familial aspects in urinary iodine and sodium concentrations (UIC, UNaC). Design: Prospective sequential cohort investigation. Setting: University hospital. Subjects: Thirteen children (six girls, seven boys) aged 3 -10 y and their parents (n ¼ 9) aged 30 -47 y. Interventions: The volunteers collected 1729 spot urine samples (5.1996 -5.1998). UIC data from winter (UIC w : DecemberFebruary) and summer seasons (UIC s : July -September) were compared with UNaC and average milk consumption. Iodine intakes from milk and salt were modelized. Results: Highest seasonal fluctuations of UIC were found in six pre-school children (w=s UIC group average ¼ 1.56, significant), followed by seven school children (w=s UIC group average ¼ 1.24, N.S.); none existed in adults. UIC=UNaC showed corresponding seasonal fluctuations in children, but not in adults. Winter milk was an important iodine source for children, as proven indirectly by similar seasonal fluctuations of ICM, UIC, UIC=UNaC and an important part within UIC due to milk. Conclusions: Contribution to UIC from milk intake during winter was high in children (40 -50%) and lower in adults (about 20%). Compared with children, dietary habits of adults are more complex and their iodine supply depends mainly on iodized salt and not on milk, so the effect of seasonal ICM variations on UIC is less marked. Because of significant seasonal UIC fluctuations in consumers of fresh milk products in Switzerland, results of future UIC studies conducted during summer and winter seasons should be compared cautiously, especially in young children. Furthermore, consumption of milk is to be promoted, since, besides calcium and vitamins, it is an essential source of iodine. UNaC determination should be included in epidemiologic studies if the dietary source of iodine is questioned. Sponsorship: This work was supported by grants from the University Hospital in Bern, the Swiss Federal Office of Public Health, the 'Swiss National Foundation for Scientific Research' (32-49424.96), the 'Fondation Genevoise de Bienfaisance V Rossi di Montelera', the 'Schweizerische Lebensversicherungs-und Rentenanstalt' and the 'Schüpbach Foundation of the University of Bern'. European Journal of Clinical Nutrition (2003) 57, 636 -646. doi:10.1038=sj.ejcn.1601590 Keywords: iodine; season; milk; child; diet; salt *Correspondence: C Als, Division of Clinical Chemistry, Inselspital, University of Bern, CH-3010 Bern, Switzerland. E-mail: claudine.als@insel.ch Guarantors: C Als, B Zimmerli and M Haldimann. Contributors: CA had the initial idea, wrote and coordinated the project over several years. CA and BZ wrote the manuscript. MH and BZ did the calculations and the statistical evaluations. MH did the analyses of urinary iodine in his laboratory. All contributors helped in planning the study, evaluating the results a...

An inductively coupled mass spectrometric method was developed for the direct determination of iodine in urine. The application of isotope dilution analysis with added 129I offers new possibilities for automatic and accurate determinations. The sample preparation consists of dilution with an ammonia solution containing 129I. The validation was made by comparison with the results obtained in another laboratory by a spectrophotometric method based on the Sandell–Kolthoff reaction. Different regression models, including maximum likelihood estimation, were used to compare the methods. None of the models revealed analytical bias between the two methods. The urine samples analyzed for validation were from three persons previously exposed to an iodine bath and covered a concentration range of 0.2 to 2.8 μmol/L. A detection limit of 0.02 μmol/L, a within-run CV of 2.5%, and a between-run CV of 11.9% were estimated for the proposed method.