Measuring the extent of total thyroidectomy for differentiated thyroid carcinoma using radioactive iodine imaging relationship with serum thyroglobulin and clinical outcomes

IMPORTANCE: Despite performing total thyroidectomy (TT), postoperative radioactive iodine (RAI) imaging often demonstrates the presence of residual thyroid tissue within the operative bed. OBJECTIVE: To measure the extent of TT using postoperative RAI imaging and assessing serum thyroglobulin (Tg) level for patients with differentiated thyroid carcinoma (DTC). DESIGN, SETTING, AND PARTICIPANTS: We evaluated 245 patients undergoing TT for clinically staged cT1-3N0M0 DTC, who underwent diagnostic postoperative RAI imaging. INTERVENTIONS: Total thyroidectomy. MAIN OUTCOMES AND MEASURES: On the basis of quantitative measurements, RAI uptake (RAIU) in the thyroid bed of 0.2%of administered activity was selected as the cutpoint to determine the presence or absence of thyroid remnant. RESULTS: By postoperative RAI imaging, TT in 106 patients (43%) resulted in RAIU of less than 0.2%. In the remaining 139 patients (57%), there was measurable iodine-avid thyroid tissue and/or tumor in the thyroid bed (n = 117 [84%]), the neck (n = 4 [3%]), or both (n = 18 [13%]). For the entire study population, mean 24-hour RAIU was 0.62%. Stimulated serum Tg levels were obtained in 232 of 245 patients (95%). Measurable stimulated Tg level (≥1 ng/mL) (to convert to micrograms per liter, multiply by 1) was found in 26 of 102 patients (25%) without thyroid remnant and in 87of 133 patients (65%) with thyroid remnant (P < .001). CONCLUSIONS AND RELEVANCE: A goal of postthyroidectomy RAIU of less than 0.2% helps maximize the likelihood of an unmeasurable postoperative Tg level, potentially simplifying follow-up evaluation and reducing the use of postoperative RAI in order to facilitate surveillance.