Experimental design. The experimental design of this pilot-trial foresees the administration of a physiological oral dose of 12 µg of 129I (the tracer) in healthy adults coupled with quantitative measurement of the tracer concentration in plasma, urine, and stool samples collected prior and following tracer administration. A single dose of 12 µg 129I corresponds to a level of activity of 78.5 Bq and an exposure in adults of about 0.16 mSv for the thyroid as the target organ. Baseline plasma sample will be collected just before the administration of the tracer (-5 min) and follow-up samples will be collected 15, 30, 45, 60, and 90 min, as well as 2, 4, 8, 24, 48, 72, and 96 hours following tracer administration. Complete urine will be collected over a period of 8 days: baseline samples 1 day before the tracer dose (Day 3) and follow-up samples 7 days thereafter (until Day 10). Similarly, complete stool collection will be done on the day before the tracer dose (Day 3) as well as on the 3 following days (until Day 6). At Day 20 and 40 (follow-ups), blood sampling and spot urine collection will be repeated to verify washed out period. Baseline samples will be used for baseline-correction of post-administration samples. In order to minimize variability in the response for the purpose of this pilot-trial, the participants will consume a standardized diet during the first 6 days of the trial and will keep a food diary during the last 4 days of the trial. After suitable preparation, plasma, urine and stool samples will be analyzed for tracer quantification (determination of 129I mass on 127I mass; 127I being the naturally occurring iodine). From this data, we will construct pharmacokinetic curves of tracer appearance and clearance in blood, urine, and stool and we will derive rates and timing of human iodine absorption, retention and excretion. In addition, iodine speciation will be performed using iodine speciation based on a chromatography method coupled with ICP-MS for direct mass quantification of iodine species as plasma inorganic iodine (PII) and protein-bound iodine (PBI). This will provide an additional level of information on separate pharmacokinetics of metabolized and not metabolized 129I. Because uptake by the thyroid will be a determinant of the fraction of the tracer that is retained, the characterization of the participants' thyroid function is important to control for potential variation of calculated retention in our subjects, and thyroglobulin and thyroid hormones in whole blood will be assessed.