Abstract: The relationship between ¹³¹I activity in the thyroid and radiation detection results is related to the thyroid geometry, detection distance and other factors, which is a key parameter for estimating the ¹³¹I content in the thyroid and its possible irradiation damage. We developed an application for gamma radiation measurement simulation of NaI detectors based on the MCPT radiation transport numerical simulation algorithm library. Then several physical models with different volumes of thyroid containers and different detection distances were established. Finally, the detection efficiency of detectors were obtained under different measurement states by Monte Carlo numerical calculations. The results of the numerical simulation are consistent with that of the theoretical calculation when the distance between the thyroid-like container and the detector is large, which proves that the application could be used to quantitatively analyze the detection efficiency of NaI detectors. It is indicated that the detection efficiency for the detector with small distances is significantly influenced by the size of the thyroid-like container and the distance. The detection efficiency table given by the simulations provides a basis for an in-depth experimental study.