[en] In this work a technique used to monitor internal contamination of workers exposed to Iodine-131 incorporation from Radioisotope Production Plant was assessed. For the measurements of workers a NaI(Tl) EG and G ORTEC 905-1 were used in specific geometry, and data acquisition was controlled with Maestro^TM software. Phantom has been used for calibration of efficiency of detectors placed in the same position as the real monitored person. Moreover, background counting was evaluated by statistical analysis for determination of protocol counting. The results obtained were analyzed under two models for calculating the decision threshold (Lc), and the minimum detectable activity (MDA), finally total uncertainty was calculated. Mean of efficiency was de 0.13% (RSD 6.7%) N = 43 D 6,7%, the MDA was 224 Bq (95% NC), and total propagated uncertainty was ± 13%. We obtained an effective evaluation of thyroid monitoring system for measurement of Iodine-131 in occupationally exposed persons. (author)