Radioiodine has been in use for over as a treatment for hyperthyroidism. Major changes in clinical practice have led to accurate dosimetry capable of avoiding the risks of adverse effects and the optimization of the treatment. The aim of this study was to test the capability of a radiobiological model, based on normal tissue complication probability (NTCP), to predict the outcome after oral therapeutic administration. Following dosimetric study, 79 patients underwent treatment for hyperthyroidism using radioiodine and then 67 had at least a one‐year follow up. The delivered dose was calculated using the MIRD formula, taking into account the measured maximum uptake of administered iodine transferred to the thyroid, U0, and the effective clearance rate, and target mass. The dose was converted to normalized total dose delivered at per fraction . Furthermore, the method to take into account the reduction of the mass of the gland during radioiodine therapy was also applied. The clinical outcome and dosimetric parameters were analyzed in order to study the dose‐response relationship for hypothyroidism. The and parameters of the NTCP model approach were then estimated using the likelihood method. The , expressed as , resulted in (95% C.I.: ) and (95% C.I.: ) for patients affected by Graves or autonomous/multinodular disease, respectively. This supports the clinical evidence that Graves’ disease should be characterized by more radiosensitive cells compared to autonomous nodules. The parameter for all patients was 0.27 (95% C.I.: 0.22–0.36). These parameters were compared with those reported in the literature for hypothyroidism induced after external beam radiotherapy. The NTCP model correctly predicted the clinical outcome after the therapeutic administration of radioiodine in our series.