[en] The isodosis curves generated by the ¹³¹I in the surrounding regions of the thyroid patient can be simulated in mathematical phantoms appropriate, using some materials whose response to radiation is similar to human tissue. It is necessary to conduct a study of the characteristics shown in front of radiation materials frequently used in the construction of physical thyroid phantoms. For this is constructed the mathematical thyroid phantom with the characteristics of the thyroid gland of an adult standard, this will allow us to calculate curves appropriate dose in surrounding regions equal to the thyroid inside and outside the patient's neck. This application of radioactive iodine I-131 in the simulated patient is using the MCNP-4B code, considering the thyroid gland located inside a cylinder which represents the neck with diameter of 12.8 cm and 12 cm of high. To determine the dose used thermoluminescent dosimeters type chip LiF with dimensions 3 x 3 x 1 mm³ distributed in a plane perpendicular to the neck and in a plane parallel to the axis of the neck. For the lifting of curves equal doses in regions inside and outside the phantom use different materials such as water, acrylic and adipose tissue, which simulated the patient neck. Results in the simulation described similar curves dose phantoms of different materials: water, acrylic and adipose tissue. This tells us that curves dose phantoms of water, acrylic and fatty tissue. (author)