During radiotheranostic procedures, radioiodine-131(¹³¹I)activities deliver high internal gamma (ɣ) doses to the patient's organs. Thus, studying the internaldoses of patients is highly required to predict the stochastic or deterministiceffect of gamma radiation that may occur to patients.The objective ofthis study is to calculatethe internal doses for 11 critical organs or body tissues insideWell Differentiated Thyroid Cancer (WDTC) patients' bodiesafter receiving diagnostic and therapeutic ¹³¹I activitiesorally during 6days post administration.Internal effective doses delivered tocritical organs are estimated mathematically using Monte Carlo simulation model in which the actual geometry, volume, mass of organs and the source localization wasdesigned based onthe patients' CT images and Oak Ridge National Laboratory (ORNL) phantom.Thus, an average sized human phantom was constructed using MCNP5in whichtwo ¹³¹I sources were distributed in the abdominal and the thyroid gland sections.No significant radiation effect is expected during diagnostic procedures for the investigated organs including the thyroid itself. However,some deterministic effects or induction of thyroid cancer might be observedfor patients undergoing postoperative radiotherapy with largeprescribed activities.