Abstract
Introduction: The aim of the study was to compare the pre-ablative thyroid scintigraphy and serum thyroglobulin (TG) positivity with a therapeutic I-131 scan for the detection of residual tissue.
Materials and Methods: Patients included in the study were those with neck residual tissue only and no residual or metastatic tissue in the ablative radioactive iodine (RAI) scan anywhere in the body. Pre-ablative thyroid scintigraphy for the residual tissue in the neck was obtained from all patients. Ablative TSH, TG and anti-TG levels were measured on the same day or one day before the RAI therapy. The evaluation of neck residual tissue detected with thyroid scintigraphy and TG measurement were made in this study accepting I-131 scan as the gold standard.
Results: The I-131 scan was positive for residual neck tissue in 67 (93.1%) patients and negative in 5 (6.9%) patients. The sensitivity, specificity, accuracy, positive predictive value, and negative predictive values of thyroid scintigraphy were 70.1%, 80%, 70.8%, 97.9%, and 16.7%, respectively. For the high positive predictive value, the kappa value was 0.182, which showed a slight agreement between Tc-99m thyroid scintigraphy and I-131 scan. The sensitivity, specificity, accuracy, positive predictive value, and negative predictive values of TG measurement were 92.5%, 60%, 90.3%, 96.9%, and 37.5%. There was moderate agreement between the TG measurement and I-131 scan.
Conclusion: Negative thyroid scintigraphy should not be noteworthy because of the very low negative predictive value and kappa value. A measurable TG level seems to be more accurate and useful for the detection of residual tissue.

Key words: Thyroid cancer, scintigraphy, thyroglobulin