Total activities due to the ingestion of 131 I were evaluated in different compartments of the human body of patients. It has been shown that the 131 I activity in urine of patients increases when the 131 I uptake decreases which could represent a source of radiation for their relatives when they leave hospitals. A new dosimetric model based on the specific betadose concept was developed for evaluating committed equivalent doses to thyroid due to 131 I uptake by different age groups of patients. Data obtained are in good agreement with those obtained by using the ICRP model for iodine. Committed equivalent dose to the thyroid gland is influenced by the mass of thyroid, 131 I uptake and energy of the emitted beta particles. In addition, 131 I uptake was measured by using a gamma camera and committed equivalent doses to the thyroid gland of female patients from the ingestion of 131 I for the treatment of hyperthyroidism diseases were evaluated. Data obtained by using our model and the ICRP ingestion dose coefficients are in good agreement with each other.