Equations are derived and the results of numerical calculations shown that illustrate the effect of counting system deadtime on measured thyroid uptake of radioiodine. It is predicted that the observed uptake is higher than the true uptake due to system deadtime. This is shown for both paralyzing and nonparalyzing deadtime. The effect of increasing the administered activity is shown to increase the measured uptake, in a manner predicted by the paralyzable and nonparalyzable deadtime models.