The feasibility of using a thyroid uptake counter, normally used to measure the uptake of radioactive iodine in thyroid treatments, to assay radioactivity deposited in a persons lungs has been investigated. Variations in radioactive material distributions in the lungs, the response of the detector system to radionuclides of interest to homeland security, and the change in detection efficiency due to the varying thicknesses of intervening tissue of the victims have been simulated using the Monte Carlo N-Particle transport code (MCNP5) developed by Los Alamos National Laboratory. Point source and homogenously distributed models were created for Co-60, I-131, Cs-137, Ir-192, and Am-241 sources to simulate radiation transport between the lungs of multiple phantom models representing children and adults and the radiation detection system. To validate the simulations undertaken, the response of the counter to radiation sources in air and behind layers of Lucite have been modeled and compared to measured results.