MSDS was designed as a comprehensive cohort trial with randomization and observation arms. Patients are enrolled at the time of the first ablative radioiodine therapy (RIT). Inclusion criteria are papillary or follicular DTC pT4 pN0/1/x M0/x, age between 18 (incl.) and 70 years (excl.) at the time of initial surgery, completion of primary surgical therapy with R0 (no tumor residues) or R1 (microscopic residues) resection, Karnofsky index of at least 70 %, freedom from distant metastases at the time of initial radioiodine therapy (RIT), and informed patient consent. Criteria for exclusion are secondary malignancy except basalioma, pregnancy, serious general disease, serious psychiatric disorder, inability to give informed consent, previous RTx and recurrence of previous DTC. From 2003, the first inclusion criterion was changed into DTC pT3/4 pN0/1/x M0/x to reflect the 2002 revision of the TNM staging system.