The “S” value, representing the energy deposited per unit mass per particle, is essential for internal dose calculations. In this study, GEANT4 and MCNP6.1 are used to compute “S” values in selected organs using male and female MRCP phantoms. A comparison of results from both tools provides reference data for assessing I-131 exposure. Generally, female phantoms yield higher “S” values than males in most organs, except for the colon and small intestine. Differences are also more evident in organs located farther from the thyroid or composed of multiple anatomical parts, suggesting that organ location and structural complexity significantly affect the calculated dose.