Accuracy of ¹²³I-Sodium Thyroid Imaging in Calculating Thyroid Volume

Hyperthyroidism is often managed with radioactive iodine therapy. The dose of ¹³¹I administered to the patient is based on the calculated size of the thyroid gland in grams and 24-h iodine uptake. Ultrasonography is a validated modality for determination of thyroid volume. Though necessary for assessing the degree of ¹²³I uptake, nuclear scintigraphy also allows for estimating thyroid volume. Here we compare volume measurements calculated on the basis of ultrasonography and nuclear scintigraphy in a cohort of hyperthyroid patients. Methods: This prospective study was designed to evaluate 110 consecutive hyperthyroidism patients who were undergoing thyroid ultrasonography and ¹²³I scintigraphy. Scintigraphy was performed after oral administration of approximately 11 MBq of ¹²³I-sodium, and uptake at 2 and 24 h was measured. At 24 h, thyroid scintigraphy was performed using a nuclear medicine camera with a low-energy high-resolution collimator next to the patient's chin. Thyroid measurements were calculated via the formula for determining a prolate ellipsoid. The formula was modified for radioactive iodine uptake because it is a planar image. Volumes calculated with these 2 modalities were subsequently analyzed and compared by linear regression. All patients had undergone ultrasonography at an average of 3 mo from nuclear scanning. All our patients' ¹³¹I dosages were based on the thyroid measurements obtained by thyroid scintigraphy. Results: We included 110 patients (95 women, 15 men) with an age range of 20-95 y and an average age of 56 ± 17.4 y. Diagnoses included 66 cases of nodular goiter and 44 of Graves disease. There was a linear relationship between measurement of thyroid gland weight by the 2 modalities, which can be explained in the following formula: log US (g) = 0.84 + [0.65 × log NM (g)], where NM is thyroid scintigraphy and US is ultrasonography. Conclusion: We have validated that this method has helped obtain more accurate measurements of the thyroid gland by thyroid scintigraphy. Additionally, we have derived factors that convert the estimated thyroid volume calculated from thyroid scintigraphy to the expected ultrasonography value.