One hundred twenty-one patients treated with 131I had a thyroid ultrasound to measure thyroid volume precisely. This volume measurement was used to determine the radioactive iodine dose. The average size (+/-SEM) of the thyroid glands measured in this manner was 39.7 cm3 +/- 1.9 cc. A significant correlation was found in the estimated size of the gland by the endocrinologists and the ultrasound volume. Of the 121 patients, 89 patients had the same 131I microcurie per gram of tissue factor to determine the radioactive iodine dose. This group of patients was further evaluated in this study. The average 131I dose (+/-SEM) given was 13.2 mCi +/- 0.5 mCi. The average time until hypothyroidism was achieved 2.85 +/- 0.14 months. Ultrasound provides a safe and precise way to determine actual thyroid size when calculating 131I doses.