Hyperthyroid and Thyroid Carcinoma patients treated with radioactive 131I (RAI) are a potential source of external and internal exposure to members of the public, to medical staff and especially to family members, who are in close contact with these patients. The relationship between radiation dose rates and various clinical parameters, including gender, age, thyroid size and weight and iodine uptake, was assessed. Dose rates were measured on 8 patients with Thyroid Carcinoma after total or subtotal thyroidectomy and on 6 patients with Hyperthyroidism. All measurements were taken at 1 meter from the patient on two levels – anterior to the neck, and body center. Dose rates were measured at three or four times – at the time of RAI administration, and after 24 h, 48 h and 168 h. Based on these measurements, retention curves were obtained. The effective half-life was derived by fitting an exponential equation and was estimated to be 12.8 h.