[en] Internal dosimetry plays an important role in nuclear medicine dosimetry control of personnel occupationally exposed, and that in recent years there has been a large increase in the use of radionuclides both in medical diagnosis as radiotherapy. But currently, in Mexico and in many parts of the world, this internal dosimetry control is not performed. The Instituto Nacional de lnvestigaciones Nucleares de Mexico (ININ) together with the Centro Oncologico de Toluca (ISEMMYM) have developed a simple and feasible methodology for monitoring of personnel working in these facilities. It was aimed to carry out the dosimetry of the personnel, due to the incorporation of I-131, using the spectrometric devices that the hospital has, a gamma camera. The first step in this methodology was to make a thyroid phantom to meet the specifications of the ninth ANSI. This phantom is compared under controlled conditions with RMC- II phantom used for system calibration of the ININ internal dosimetry (ACCUSCAN - Ll), and with another phantom developed in Brazil with ANSI specifications, in order to determine the variations in measurements due to the density of the material of each of the phantoms and adjust to the system ACCUSCAN, already certificate. Furthermore, necessary counts were performed with the gamma camera of the phantom developed at ININ, with a standard source of ¹³³Ba which simulates the energy of ¹³¹I. With these data, were determined the counting efficiencies for a distance of 15 to 20 cm between the surface of the phantom and the the plate of the detectors. Another important aspect was to determine the lower limit of detection (LLD). In this paper we present the results obtained from the detectors calibration of the gamma camera of the hospital.