I¹³¹ is a famous radio-iodine isotope in use for diagnosis and treatment of hyper functioning and cancerous thyroid gland. It is a nuclear reactor product; however nuclear reactor may be unavailable in some areas. Replacement by I-124 may be possible, another iodine isotope producible by cyclotron; a system more available than reactor. Here absorbed fraction of Gamma and Beta rays of I-124 throughout thyroid and neck soft tissues are measured and compared with I-131. Employing I-124 as substitute for I-131 is also discussed. First of all, the input file for MCNPX code has been prepared to calculate F6 and F8 tallies. Then the code has been run for F6 and F8 tallies for variation of lobe volume from 1 to 25 milliliters. From the output file of tally F6, the gamma absorbed fraction in thyroid and neck tissue for the volume lobe variation from 1 ml to 25 ml have been derived. As well as, form the output of F8 tally the absorbed energy of beta in thyroid and soft tissue of neck is obtained and then absorbed fraction of beta has been calculated. Finding indicated that for constant activity in thyroid and soft tissue the gamma absorbed fraction for I-124 is less than I-131. For the same situation, the beta absorbed fraction of I-124 in thyroid is less than I-131 and in soft tissue is more than I-131. If I-131 is non-available it may be replaced by I-124for treatment of cancerous or hyper functional gland