Introduction: Iodine is an essential element for normal growth and function, and is also a component of thyroid hormones. Dietary iodine intake influences the prevalence of thyroid disorders. Therefore, for sufficient iodine intake consumption of iodized salt is important. Measurement of urinary iodine concentration (UIC) is recommended method for assessment of iodine intake, but it has certain drawbacks. Due to the excretion of iodine to saliva, it is reasonable to determine whether the measurement of salivary iodine concentration (SLIC) could replace UIC measurements. In order to assess the content and distribution of iodine in the body, information on the serum iodine concentration (SERI) would be useful. The aim of our work was to introduce a reliable and simple method for measuring SLIC and SERI using a modified Sandell-Kolthof (S-K) reaction and to validate this method with the gold standard method for measuring iodine in various liquids, inductively coupled plasma mass spectrometry (ICP-MS). In addition, we wanted to define the influence of the time and type of sampling on iodine concentration and to determine whether the collection of a urine sample for the measurement of UIC could be standardized. We were interested in whether saliva or a combination of samples (urine/saliva/serum) would be more reliable for iodine intake assessment of iodine content in a healthy individual than urine.
Materials and methods: We invited 145 volunteers, which gave blood sample to test thyroid function, an ultrasound of thyroid was made, and later at home they provided urine and saliva samples strictly following the protocol. With self-prepared chemicals we validated the possibility of measuring SLIC and SERI with the S-K reaction. For the implementation and evaluation of both methods, we followed the recommendations of the Clinical and Laboratory Standards Institute (CLSI) and tested the influence of the matrix, determined the lower limit of quantification, reproducibility and repeatability, linearity, the influence of interferences, the suitability of using different concentrations of the used solutions. Creatinine in urine was measured on a Dimension Vista® 1500 Intelligent Lab System analyzer (Siemens Healthineers, Germany), measurements on ICP-MS were done using an Agilent 7900ce analyzer (Agilent Technologies, USA). Statistics was calculated using MedCalc® (MedCalc Software Ltd, version 22.021, Belgium), statistically important value was P<0.05.
Results: Healthy 104 volunteers with complete collection of all biological samples were included to the study. Importantly, none took dietary supplements or iodine medications, and all of them were without thyroid disease. We partially confirmed the first hypothesis, the S-K reaction can reliably determine SLIC, as the values are in excellent agreement with those measured by ICP-MS. Passing-Bablok regression was Y(ICP-MS)=0.89(95 % CI: 0.86–0.92)*X+6.2(95 % CI: 3.3–8.6). Cusum's linearity test showed no significant difference from linearity, P=0.89. Positive bias between both methods was determined, in average 5.9 % (95 % CI: 4.1–7.7). The introduced SLIC measurement enabled measurement of both, UIC and SLIC, using the same protocol. However, SERI determination by S-K reaction was not reliable, the values were not comparable to those measured by ICP-MS. Passing-Bablok regression: Y(ICP-MS)=1.90(95 % CI: 1.34–2.99)*X-48.8(95 % CI: -128.6–(-8.1)). Cusum's test of linearity showed a significant difference from linearity, P=0.07. A negative bias was observed between both methods, in average 30.8 % (95 % CI: -88.026.4), and was increasing throughout the entire concentration range (P<0.001). The methods were not comparable. The second hypothesis was not confirmed, the ratios between UIC, SLIC, and SERI are not comparable between healthy individuals. To the calculation of the ratio only the UIC values that corresponded to the measured UIC in the 24-hour urine and the SLIC that corresponded to the amount of excreted iodine in one day, i.e. the UIC value of the second morning urine and the SLIC values 60 and 120 min after first and second meals, were included. We did not observe a statistically significant difference between second morning urine UIC values and SERI, P=0.583. However, those values did not correlate with the amount of iodine excreted in the 24-h urine. Additionally, we observed a very good correlation of SLIC values 60 and 120 min after the first and second meal with the amount of iodine excreted in the 24-hour urine (P>0.05). On the other hand, there was no comparison between SLIC and SERI and between second morning urine UIC and SLIC, for all samples P<0.001.
Conclusions: The measurement of SLIC by the S-K reaction represented an important contribution to the science, as the method was comparable to the gold standard method ICP-MS and can be performed in any laboratory worldwide. With this method, it is possible to measure UIC and SLIC simultaneously and by the same protocol. SERI measurements by S-K reaction were not comparable to ICP-MS. The ratios between SERI and SLIC and between UIC and SLIC are not comparable. From the results, we can conclude that it is important to focus on the measurement of SLIC and UIC, which, unlike serum, are matrices of greater dynamics of iodine excretion and reflect iodine intake, which is also a dynamic process. An important scientific contribution of the research was very good agreement of SLIC values 60 and 120 min after the first and second meal with the amount of iodine excreted in the 24-hour urine. Therefore, the SLIC measurement could replace the measurement of the amount of iodine excreted in a 24-hour urine, the collection of which is much more demanding than the collection of saliva. We have not managed to find a proper assessment of iodine content and distribution in healthy individual.
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