Accuracy of 123 I Na Thyroid Imaging in calculating thyroid volume.

Introduction: Hyperthyroidism is often managed with radioactive iodine therapy. The dose of 131 Iadministered to the patient is determined based on the calculated size of the thyroid gland in gram and 24 hour iodine uptake. Ultrasonography is a validated modality for determination of thyroid volume. Though necessary for assessing degree of 123 I uptake, nuclear scintigraphy also allows for the capability of estimating thyroid volume. Here we compare volume measurements calculated based on ultrasonography and nuclear scintigraphy in a cohort of hyperthyroid patients. Methods: This prospective study designed to evaluate 110 consecutive hyperthyroidism patients who were undergoing thyroid ultrasound and 123 I scintigraphy. Scintigraphy was performed after oral administration of approximately 11 MBq 123 Isodium, and uptakes at 2 and 24 hours were measured. At 24 hours, the patients underwent thyroid scan with a nuclear medicine camera with LEHR (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 RAIU as it is a planar image. Volumes calculated with these two modalities were subsequently analyzed and compared by linear regression. All patients had undergone ultrasonography with an average three months from nuclear scan. All of our patient 131 I dosages were based on the thyroid measurements obtained by thyroid scintigraphy. Results: We included 110 patients (95 females, 15 males) with age range 20-95 years and average age 56 +/- 17.4 years old. Diagnoses included 66 patients with nodular goiter, and 44 patients with Graves' Disease. There was a linear relationship between measurement of thyroid gland weight by two modalities which can be explained in the following formula: log US(g) = 0.841 + 0.649*log NM(g). Conclusion: We have validated that this method has helped obtain more accurate measurements of the thyroid gland by thyroid scintigraphy. Additionally, we have derived conversion factors that convert the estimated thyroid volume calculated from thyroid scintigraphy to the expected ultrasound value.