The effect of temperature and shielding on the lower detection limit of a thyroid 131I monitoring system was investigated in an anthropomorphic thyroid-neck phantom fitted with an imaging plate. The phantom was loaded with an 131I aqueous solution and monitored with the imaging plate for 10 min. After exposure, the plates were incubated with or without the shield at 0, 10, 20, 25, 30, or 40°C. The latent image was read out at 0 min to 7 d after exposure. The thyroid equivalent doses corresponding to the detection limit were calculated in six age categories, using the inhalation equivalent dose coefficients recommended by the International Commission on Radiological Protection. The detection limit was distributed between 0.13 and 4.2 kBq, and depended on the age of subjects, elapsed time, temperature, and shielding provision. The maximum detection limit of 4.2 kBq was below the emergency screening level of 30 kBq in Japan. The thyroid equivalent dose corresponding to the detection limit ranged from 0.17 to 46 mSv. From the maximum equivalent dose of 46 mSv, the effective dose was estimated as 1.8 mSv, lower than the annual effective dose limit of 20 mSv for radiation workers. At 2 d after exposure, the measured dose was below the annual effective dose limit of 1 mSv for the public, regardless of age, temperature, and shielding provision. The imaging plate system effectively monitors the 131I thyroid levels in emergency situations.