Introduction
Iodine is important for the formation of thyroid hormones. These are important for the growth, development and metabolism. Iodine deficiency occurs in 38% of the world population.
After the Dutch decision in 2008 to reduce iodine in salt and to add iodine to more product,the deficiency worsened. Severe or mild maternal iodine deficiency can cause different degrees of cognitive and neurological impairment in offspring. It is therefore important to monitor the iodine status in pregnant women. Iodine status is measured in spot urine samples or 24-hour urine collections and is represented by different parameters. These parameters all have strengths and limitations.This status is further researched for pregnant women in the northern part of the Netherlands by the JOZO study. 24-hour urine collection is complex and frequently collected incorrectly.
A second research described in this paper is therefore the study determining theintra-individual variationof iodineto concludethe effect of missed urine portions andincorrect time of collection duringa 24-hour urine collection.
Materials and methods
De JOZO studycollected spot-morning urine samples and 24-hour urine collections from201 pregnant women in thenorthernNetherlands. Women usingiodine containingmedicine, medication affecting the thyroid gland,medication againsthypertensionand with pregnancy complicationsuch as (pre-gestational diabetesmellitus, renal-or thyroid diseaseand hypertensionwere excluded. The women were also asked to fill in a dietary-andintake questionnaire with personal details. The iodineand creatinine levels were measuredwith respectively ICP-MS andimmunochemistry.The iodine status was represented by the urine iodine concentration (UIC), the iodine/creatinine ratio (ICR) and the 24-hour output. The cut-off values for these parameters were 150 μg/L, 150 μg/g and 225 μg/24 hours,respectively.
The Spearman correlation coefficientsfor the different parameters were calculated to determine the best parameter to represent the iodine status. In the study determiningthe intra-individual variationof iodine,20 healthy volunteers participated. Participants were excluded due tothe use of iodine containing supplements andmedication affecting the kidney. They were asked tocollect their urine during 24 hours within nine time slotsandfill in a dietary and activity diary. The iodine and creatinine levels in these samples were analyzedin the same way as the JOZO study.The intra-individual variationfor the iodine status parameters wascalculated.
Results
The JOZO populationhad median values and percentages beneaththe cut-off points of 184,95 μg/dayand 69,7% for the 24-hour iodine output, 94,30 μg/L and 88,1%for the UIC and 140,76 μg/gwith 57,7% for the ICRfor the 24-hour urine collections, respectively. The values for the spot-morning urine were 126,40 μg/L and 64,3% for the UIC and 113,49 μg/gwith a percentage of 73,4% for the ICR. Every parameter correlated with the 24-hour output. The coefficients were for the UIC (0,497), the ICR(0,762) for the 24-hour urine collection and were 0,324 for the UIC and 0,458 for the ICR of the spot-morning urine.The UIC and the ICR of the spot morning urine correlated with the 24-hour UIC. The correlation coefficients were 0,492for the UIC and 0,319 for theICR. The median values for the intra-individual variationwere50,07 % for the UIC,22,24 % for the ICR , 29,04 % for the hourly iodine output and 15,42 % for the hourly creatinine output.
Conclusion
Pregnant women in the JOZO study were iodine deficient when supplements containing 150 µg were not taken. The second best parameter to determine the iodine status besides the 24-2 hour output was the 24-hour ICR. The least accurate parameter was the 24-hour UIC. Spoturine samples cannot be used to measure iodine status in urine. When using spot-urine samples, the UIC should be measured. There was a lot of intra-individual variation found for all the iodine status parameters. It is not possible to correct for missed portions or incorrect urine collection time. The least variation is found for the ICR. The ICR is also not a stable parameter, because of the creatinine influence. Creatinine is more than just a correction factor for urine dilution. Measuring iodine status in urine thus remains difficult. Keywords Iodine, iodine status, pregnancy, 24-hour urine collection, spot-morning urine sample, intra individual variation