The administration of (131)I for the treatment of benign thyroid disease is widely used in clinical practice. The appropriate dose of (131)I, so as the gland could receive the specified absorbed dose, is determined by various methods. The mostly used is the one based on the 24 h uptake. In the present study we examined the time to measure (131)I uptake which better represents the total accumulated activity in the thyroid gland and consequently is more reliable for dose calculation. Fourteen patients, who were referred to the Nuclear Medicine Department of AHEPA University Hospital, were included in the study. 1.85 MBq of (131)I were administered and the uptake at 24, 48, 72 and 192 h was measured. From the curve of the activity vs time we calculated the area under it, which represents the total accumulated thyroid activity. We compared the uptakes of every individual with the total area and we found that the 192 h uptake was best correlated with it (r=0.996). The absorbed dose to the thyroid was calculated in the following ways: a) was based on the 24 h uptake and b) was based on the total accumulated activity on the 192 h uptake. We found differences from -19.9% to +33.7%. In conclusion, the 192 h uptake consists the most representative and reliable parameter for the estimated activity of (131)I given to the thyroid for the treatment of hyperthyroidism.