One or several radioiodine uptake measurements are usually made to calculate the activity necessary to give the prescribed absorbed dose to the thyroid of patients treated for hyperthyroidism. If the initial uptake, estimated at time zero, and the effective half-time are to be determined, more than one uptake measurement is needed. The first measurement is often made 24 h after administration of radioiodine. In this work we show that one uptake measurement taken at an appropriate time postadministration is sufficient to determine the product of the initial uptake and the effective half-time, and thus give an estimate of the absorbed dose. A total of 515 patients (14-92 years) who received radioiodine treatment for hyperthyroidism at Malmö University Hospital were analyzed. Individual effective half-times were determined from three uptake measurements, 24 h, 48 h, and a third measurement between 3 and 8 days after the intake of the test activity and were used for the absorbed dose calculations. We found a significant linear relationship between the uptake at 4 days or longer after administration and the product of initial uptake and effective half-time. The relationship was independent on diagnosis, age, and sex of the patient. Therefore a single uptake measurement is sufficient for an accurate absorbed dose determination, provided that the measurement is made at least 4 days after administration.