This thesis concerned with iodine status, sources in human body, and measurements especially here in Canada, where iodine status for the Canadian population is not well known. With the recent re-emergence of iodine deficiency among individuals in other industrial countries, understanding the main sources of iodine to the Canadian population is necessary to ensure fortification strategies are justified and effective. Uncertinaty has arisen to the importance of iodized salt recently, along with medical warnings to reduce salt consumbtion. This conflicts give rise to improve scientific research and hone thier methods with new applications. The research question here is that: Can we benefit from the existence of long-lived radioiodine-129 in the environment and explore its potential as a tracer? To answer this question, the study was divided into an introductory chapter contains a review about the topic, then three research chapters. The second chapter was devoted to study the possibility of extracting 129I from human urine. As for third chapter of the thesis, it was about refining a method that already established, and use it to extract 129I from breastmilk using combustion, then determine the radiological dose of 129I in infants’ thyroid. While the fourth chapter was devoted to investigate the main sources of 127I and 129I in the Canadian diet based on daily food consumption and modelling the urinary iodine concentration for adults and infants through the novel application of a well-established compartment model implemented in AMBER. The path of this thesis was crowned with a set of results, which are detailed in the end of each chapter as follow: 1- The advantage of accelerator mass spectrometry (AMS) helps to measure 129I in human urine for the first time. The result for 25 participants from Ottawa ranged from 3.3 x 106 atoms/L to 884 x 106 atoms/L with a median of 108.7 x 106 atoms/L, and the 129I/127I ratio ranged from 7.38 x 10-12 to 3.97 x 10-10 with a mean of 1.3 x 10-10. 2- The concentration of 127I and 129I in Ottawa urine samples were significantly correlated and generally similar to the 129I concentrations and 129I/127I ratios from environmental samples collected around Ottawa. 3- This correlation suggests that 129I could be a potential nutritional tracer of dietary iodine. 4- In chapter 3, the 129I in breastmilk ranged from 1.26x108 atoms/L to 6.64x108 atoms/L with a median of 2.10 x108 atoms/L, and the 129I/127I ratio ranged from 1.27x10-10 to 9.9x10-10 with a median of 2.13x10-10. 5- A correlation was also observed between 127I and 129I concentrations in breastmilk. 6- The isotopic ratios in breastmilk were similar to Canadian cow’s’ milk, indicating that the milk of both cows and humans is a reflection of the 129I concentration of their local environment and the food ingested. 7- Result from chapter 3 confirms that humans are exposed to the 129I from birth through their mother breastmilk, giving them an average dose of 1.10 x10-4 Bq/year and thyroid dose rate equal to 5.92 x10-10 Sv/year. 8- In fourth chapter, the daily milk consumption was measured for 78 mother-infants’ pairs, and ranged from 275 -1202 g/day, with a mean of 731 g/day. This value agrees well with global infant milk intake which estimated at 730g/day. 9- The daily iodine intake from breastmilk ranged from 11.2 µg/day to 476.2 µg/day with a median of 127.9 µg/day. 10- The urinary iodine concentrations were estimated without urine collection using iodine biokinetic model, giving a median urinary iodine concentration (n=78) at 304.7 µg/L. The result was compared to those measured by Health Canada (median= 398.7 µg/L), showing a moderate correlation (r= 0.496). 11- A further comparison of the results was made based on gender shows that the difference between UIC in male and female infants measured by Health Canada and those estimated by AMBER is non-significant. 12- Through AMBER software, the influence of seven common diets on UICs was assessed to determine which foods play an important role in ensuring iodine adequacy. We observed that the main source of iodine in a vegan diet is grain products providing up to 70%, while in remaining diets the main source of iodine was dairy products (50-69%) when they are consumed. 13- The contribution of iodized salt to all Canadian diets was ranked second, after dairy, unless the diet is vegan or ovo-vegetarian, where dairy is not consumed, iodized salt was ranked first. 14- Among 23 scenarios for seven different diets, the urinary iodine-129 concentrations ranged from 1.4 x10-7 to 3.3 x10-7 µg/L with a median of 3.1 x10-7 µg/L, and the isotopic 129I/127I ratio ranged from 1.1 x10-9 to 1.2 x10-8 with a median of 2.8 x10-9. 15- In contrast to stable iodine, the highest isotopic ratio was observed in vegan diet, while the lowest was observed in ketogenic diet. This suggests that grain products are the main contributor of 129I to humans. 16- Despite being the primary contributors of stable iodine (127I), salt and dairy show a lower contribution of 129I. Based on this we can qualitatively predict the source of iodine 127 using isotopic ratio 129I/127I. For example, in cases where the isotopic ratio was between 10-8 and 10-9, therefore, the main sources of iodine in this person may be from grains products, vegetables, and fruits; and in cases where the isotopic ratio was between 10-10 and 10-11, therefore, the main sources of iodine in this person may be from dairy products and some contribution from salt. This study has shown the capability of 129I to be used in biomedical fields. In this thesis 129I used as a nutritional tracer where it helps to detect the sources of stable iodine in human body based on isotopic ratio. The extraction method invented in Chapter 2 can be used to evaluate 129I exposure directly in the human body for those who live nearby nuclear fuel reprocessing plants. An additional application for this method can be in assessing 129I in human to investigate 131I uptake in the event of a nuclear emergency using 129I in urine as a proxy. Moreover, the extraction technique used Chapter 3, can be extended to other biological samples such as thyroid or brain. Furthermore, Chapter 4 shows that with the right estimation of daily iodine intake and urine volume, a biokinetic model of iodine, built in the AMBER software, can predict urinary iodine concentration with a high degree of accuracy without collecting urine samples.