Hematopoiesis plays a significant role in mammals' life. Hematopoietic damage due to radiations of different arrays, such as ionizing and non-ionizing radiations, could lead to external infections, internal infections, and anemia. Different cancers' treatment using radioactive sources affects the blood parameters (i.e., mature cells, immature cells, and stem cells). In the present paper, the effect of intravascular injection of Iodine-131 (131I) on the blood parameters for thyroid cancer treatment has been investigated utilizing the MCNPX code and Runge-Kutta fourth-order method. The simulation results showed that the accumulated absorbed dose in adult phantom (ORNL whole body phantom) due to normalized intravascular injection of 1 mCi (mCi) of 131I was 1.97 × 10−2 Sv. Based on the accumulated absorbed dose, the blood parameters show a function of injected 131I activity, and their variation in the activity range of 15–300 mCi was calculated using the fourth-order Runge-Kutta method. It shows that injecting 131I with an activity of less than 150 mCi does not have much effect on blood parameters in the period of two to eight days. But, the injection of 131I with an activity greater than 150 mCi and up to a maximum value of 300 mCi, reduces blood parameters by a maximum of 21.21% in a period of two to eight days. Based on the smaller variation (in percent) of blood parameters due to the injection of low activity 131I (<150 mCi), it can be mentioned that thyroid cancer treatment has an insignificant effect on the blood parameters. Besides, it is recommended to integrate the MCNPX code and Runge-Kutta fourth-order method to study the effect of radiation exposure on the blood parameters.