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Aquatic Iodine, Urinary Iodine and Thyroid Function

Stig Andersen  


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Accepted by: The Faculty of Engineering and Science Aalborg University
Defended on: October 4, 2002
Official opponents: Jens Faber, Chief Physician, Copenhagen University Hospital , Per Winkel, University of Copenhagen , Asbjørn Mohr Drewes, Associate Professor, Aalborg University
Tutors: Peter Laurberg, Professor, Aalborg Hospital , Steffen B Petersen, Professor, Aalborg University , Lars Arendt-Nielsen, Professor, Aalborg University

Published in the PhD Database: April 8, 2004


English abstract
Aquatic Iodine
In study I iodine was collected from 22 waterworks in Denmark for determination of iodine content. The iodine concentration was unaltered compared with a previous study as was an east-west difference. Six sites with a relatively high iodine content in drinking water were selected for further analysis. Skagen drinking water was investigated with size exclusion chromatography before and after freeze-drying, at different pH and ionic strength, and with and without iodide preincubation. Also, emission fluorescence spectroscopy was performed on the bulk organic matter in Skagen tap water. Findings were in keeping with geological data and the technical literature on Skagen ground water showing the presence of humic substances from a marine source rock. Iodine is abundant in the marine environment and particularly rich in marine sediments. Study I indicated that iodine was present in humic substances at all sites investigated and an association was found between iodine and humic substances. This suggests that iodine in drinking water indicates concurring humic substances of marine origin. Increasingly efficient water processing at waterworks aims to reduce the content of organic matter. At Skagen waterworks this reduced the iodine content by 9%.

Urinary Iodine
Sixteen healthy men produced a blood sample and a spot urine sample monthly for twelve months. Median urinary iodine excretion was 50 g/L. Large variations and differences in variations were seen between individuals. These variations were reduced by 25% when a 24h urinary iodine excretion was estimated from age and gender specific creatinine excretions. When the estimated 24h urinary iodine excretion was computed the ability to detect an average iodine excretion level was improved by about 34%. This also reduced the number of samples necessary to obtain in order to determine the iodine excretion level in a population by 45%.

Iodine and Thyroid
Iodine may cause both increased and decreased hormone secretion from the thyroid gland. These two opposite effects exists with severe iodine deficiency and iodine sufficiency respectively. In study II the association between thyroid function and iodine excretion was described by individual correlation coefficient between TSH and estimated 24h urinary iodine excretion. With increasing average iodine excretion level a positive trend was seen in this association. Data indicated that the level of shift between these two mechanisms occurs around an iodine excretion level of 50 g/24h. This supports this level as a separator between mild and moderate iodine deficiency.

Thyroid Function
In study III large variations were seen between individuals in both level and variance of all thyroid function tests. Variations were large between individuals compared to within individuals. This caused group reference ranges to be insensitive to abnormal test results in individuals.
Study III evaluated alternative procedures for diagnosis and monitoring of thyroid disease.
Determination of individual set-points and the difference between two successive test results required to exceed random variation were calculated. Both alternative methods were based on the same general statistics and were subject to the same variation in individuals. None of the mathematical refinements improved the diagnostic performance.
Serum TSH responds heavily to minor changes in T3 and T4. Thus, TSH may leave the laboratory reference range when T3 or T4 are outside the individual reference range but still within the much wider laboratory reference range. This may provide a biochemical explanation for the clinical entity subclinical thyroid disorders.



Danish abstract
Iod I vand
Drikkevand blev indsamlet fra afgangshanen på 22 vandværker I Danmark. Iodindholdet var uændret fra en tidligere undersøgelse, med klare geografiske forskelle. Drikkevand fra 6 vandværker med relativt højt drikkevandsiod blev indsamlet til supplerende undersøgelser. Drikkevand fra Skagen blev undersøgt med gelfiltrerings kromatografi før og efter frysetørring, ved forskellig pH og ionstyrke, før og efter præinkubering med iodid, og det blev undersøgt med fluorescens spectroskopi. I overensstemmelse med geologiske data og den tekniske litteratur indeholdt drikkevand fra Skagen humus fra marine aflejringer i undergrunden. Disse vides at indeholde meget iod, og undersøgelsen viste dels at iod var tilstede i humus i drikkevand fra alle 6 steder, og dels at der var en klar association mellem koncentrationerne af iod og humus i drikkevand. Ud fra disse resultater foreslåes det at iod i drikkevand indikerer tilstedeværelse af humus fra marine aflejringer. Generelt foregår der i mange vandværker en øget rensning af råvand til drikkevand. Drikkevandsrensningen på Skagen vandværk reducerede iodindholdet i drikkevand med 9%.

Iod i urin
Seksten raske mænd gav månedlig blod- og urinprøve i 12 måneder. Den mediane iodudskillelse var 50 g/L. Der sås store forskelle i variation mellem deltagerne. Ved beregning af en anslået 24-timers iodudskillelse på basis af alders- og køns-matchede creatininudskillelser blev disse variationer 25% mindre. Denne estimerede 24-timers iodudskillelse øgede præcisionen ved bestemmelse af det gennemsnitlige iodudskillelsesniveau med 34%, og forårsagede en 45% reduktion i det antal prøver der var nødvendige for at bestemme en gruppes iodudskillelse.

Iod og skjoldbruskkirtlen
Iod kan både fremme og hæmme sekretionen af stofskiftehormoner fra skjoldbruskkirtlen, thyreoidea. Mekanismerne provokeres af øget iodindtagelse, og er fremherskende i områder hvor befolkningen har henholdsvis svær iodmangel og iodindtagelse over det anbefalede niveau. Korrelations koefficienten mellem estimeret 24-timers iodudskillelse og serum TSH blev beregnet for hver enkelt deltager. Der var en statistisk significant tendens til at stigende gennemsnitligt iodudskillelse modsvaredes af en stigende korrelations koefficient. Data antydede dermed at skiftet mellem disse to mekanismer skete ved en iodudskillelse omkring 50 g/døgn. Dette er den vedtagne grænse mellem mild og moderat iodmangel.

Stofskiftefunktion
Stofskiftefunktionen estimeres ud fra serum TSH og stofskiftehormonerne T3 og T4. Der var store forskelle i både niveau og variation i disse mellem deltagerne. De personlige reference områder var smalle i forhold til referenceområderne for gruppen. Det betød, at gruppens referenceområder var mindre følsomme for værdier der var unormale for den enkelte.
Alternative metoder til at vurdere gentagne prøvesvar blev beregnet. Således blev individuelle referenceområder og den nødvendige forskel mellem to prøvesvar for at være 95% sikker på ændring bestemt. Begge metoder var afhængig af samme grundlæggende statistik, og ingen af metoderne var bedre end anvendelse af gruppens referenceområde.
Serum TSH reagerer kraftigt på mindre udsving i T3 og T4. Det kan betyde at et serum T3 eller T4 udenfor det personlige referenceområde, men stadig inden for gruppens referenceområde, medfører at serum TSH forlader gruppens referenceområde. Denne biokemiske kombination kaldes subklinisk thyreoideasygdom, som således vil kunne tilskrives brede referenceområder for gruppen.