Current iodine requirements defined for pregnancy and lactation are rough factorial estimates extrapolated from older studies in adults that used radioactive iodine tracers. To ensure optimal thyroid function in these vulnerable groups, a tracer method that could be safely used to accurately define iodine requirements would be valuable. Iodine-129 (129I), a long-lived semi-stable isotope with no health risks, could be used as a tracer, but the analytic challenges are formidable. However, we have developed an ICP-MS method to measure pp-billion (10-9) to pp-trillion (10-12) quantities of 129I in biological samples. In this project we will perform a study in which iodine-replete adult subjects will consume an oral dose of 129I. We will quantify 129I kinetic patterns in plasma, urine and stools after the oral dose, and use these data to derive tracer absorption, retention and excretion rates. This trial will allow us to define optimized procedures for the routine application of this method to assess iodine metabolism in humans. The use of 129I may prove to be a breakthrough technique to safely assess iodine metabolism and requirements in pregnant/lactating women in order to ensure healthy thyroid function in these age groups.