Methods of analysis. The tracer concentration in plasma, urine and stool will be measured by a high resolution multi collector ICP-MS with isotope dilution analysis (IDA) using a tellurium internal standard following sample digestion according to a method recently developed in our laboratory [Dold et al, Thyroid 2015]. For the additional speciation of 129I in plasma, analysis will be performed by using ion chromatography ICP-MS analysis (IC-ICP-MS) at the Federal Food Safety and Veterinary Office, Swizterland. Urinary iodine concentration will be measured by means of the Sandell-Kolthoff reaction [Pino et al, Clin Chem 1996, 42:239-243] in our laboratory. Thyroglobulin will be assessed using an ELISA dried blood spot method recently developed in our laboratory [Stinca et al, Thyroid 2015, 25:1297-1305]. Thyroid hormones will be measured by the University Hospital in Zurich, Switzerland, using a DELFIA (dissociation-enhanced lanthanide fluorescent immunoassay) method. Iodine content in the standardized diet will be will be measured in our laboratory by ICP-MS analysis and data from the food diaries will be evaluated by using the nutrition software EBIS-Pro (University of Hohenheim).