Expected output and significance/ expected impact. This study will be the first to use 129I as a biologic tracer in humans. Once developed, the availability of this novel method would provide a unique tool to address several key issues with regards to human iodine metabolism and thyroid function. These include the precise definition of the iodine requirement for important life stages, investigation of potential up-regulation of iodine absorption during iodine deficiency, thyroid uptake and turnover of iodine in varying iodine status, assessment of placental and breast milk iodine transfer, as well as the assessment of iodine bioavailability from different foods. Thus, this project could have significant long term impact in the field of iodine and the thyroid.