We retrospectively reviewed the records of 224 patients with diffuse goiters treated with radioiodine, half of which received carbimazole. In all the cases, we carefully monitored the calculation of dosage. A lower percentage of early hypothyroidism but a higher failure rate was observed in the carbimazole subgroup. Nevertheless, after one year, a constant (4.5%) incremental rate of hypothyroidism was found regardless of carbimazole administration. Since we were able to precisely estimate the absorbed doses in our series, we evaluated by simulation the dosimetric profiles of nine methods of dose selection (MDS). These MDS were calibrated in such a way that the same threshold value of absorbed dose would always be reached at the thyroid level. We showed that the more elaborate the MDS, the more accurate the irradiation at the thyroid level and the lower the radiation dose administered. In patients not receiving carbimazole, a rapid MDS using modified early uptake measurements to predict the 24-hr actual value was found to be advisable. With patients receiving carbimazole and if a goal is to delay the occurrence of hypothyroidism, we advise MDS based on either a 48-hr uptake or on the calculation of the individual half-life.