Background: The thyroid gland absorbs nearly all the iodine in the blood, independently of its isotopic composition. When a large enough activity of radioactive iodine like ¹³¹I, ¹³²I, ¹³³I is taken into the body, it can destroy the healthy thyroid gland cells as well as the cancer’s cells. In fact, as it is well known, some isotopes of iodine are used not only for acquiring thyroid images but also for curing thyroid cancer or hyperthyroidism due to Graves’ disease. Moreover, some of them are released in nuclear accidents. The aim of this work is to evaluate the absorbed fraction of beta rays from different iodine radioisotopes in thyroid lobe, using Monte Carlo method. Materials and Methods: We have applied the MCNPX code to calculate the beta absorbed fractions for the most important iodine radioisotopes in the thyroid lobes, supposed to have an ellipsoidal shape, with the volume varying from 1 ml to 25 ml and the material composition suggested by ICRP. The beta rays spectra of iodine isotopes have been taken from the LBNL website. Results: The result showed that the volume lobe variation had a significant effect on the absorbed fraction for beta rays in thyroid gland, up to 25% for 124I. The absorbed fractions of beta rays were decreasing from ¹²³I to ¹³¹I, ¹³⁰I, ¹²⁶I, ¹³⁵I, ¹³³I and ¹²⁴I respectively. Conclusions: Decreasing of the absorbed fraction might be related to the beta rays energy spectra of the isotopes. Moreover, for ¹³¹I results obtained for beta absorbed fraction in spheres was in agreement with previously published results.