Thyroglobulin increment after thyroid hormone discontinuation, a more reliable indicator for detection of significant remnants or metastasis from differentiated thyroid carcinoma

Introduction: Serum thyroglobulin (Tg) measurement has a pivotal role in management of differentiated thyroid cancer (DTC). Different threshoulds for serum Tg level were considered as indicators of tumor remnant or metastasis during suppression or after discontinuation of thyroid hormones. Serum Tg increment after recombinant TSH administration is also suggested as a better predictor of tumor recurrence, however with respect to economic considerations in developing countries, this approach is not cost effective mainly due to high cost of recombinant TSH (about 10005 per patient). This study tries to evaluate the value of different levels of Tg increment after thyroid hormone discontinuation. Method and patients: From 343 patients with DTC who referred to nuclear medicine department of Ghaem hospital, 55 patients completed the study. All patients had a previous history of I-131 ablation. Serum Tg and TSH were measured in all patients (Using high sensitive IRMA assay) during suppression(OnT4Tg) and after discontinuation of thyroid hormones(OffT4Tg). No treatment was used for patients between the tests. Whole body iodine scan and chest X-ray were performed in all patients and neck ultrasonography or neck CT scanning were done in patients with negative whole body iodine scan and CXR and elevated thyroglobulin levels. All patients were followed for at least one year. Results: Fifty five patients studied, 46 female,9 male with mean age of 41.40 yrs (SD=18.81)and age range of 8-80 years. Papillary cancer was the most frequent type (90.9%) with fbllicular (5.5%) and Hurtle cell consisting of 3.6% of cases. The mean time interval between the two laboratory tests was 110.29 days(SD=53.43) and less than 180 days in all. Delta Tg (ATg) was considered as difference between offr4Tg and OnT4Tg levels. Serum Tg level was increased equal or more than 1 ng/ml in 25 patients after discontinuation of thyroid hormones. From these patients 17 patients had disease or thyroid remnant. In 16 patients with unchanged Tg (-I<ATg<I) , 3 patients had thyroid or disease remnant. In 14 patients with Tg decrement(A<-l) , 2 patients had thyroid remnant. The positive predictive value(PPV) for Tg increment of more than 1, 2 and 5 ng/ml were 68%, 77.2% and 88.9% respectively, while for onT4Tg levels the numbers were 37.5%, 52.3% and 57.1% respectively. The PPV for offT4Tg levels were 54%, 67.8% and 80% respectively. All patients with ΔTg of more than 7 ng/ml had residual disease or metastases. If ATg was unchanged or decreased, the negative predictive value was 83.3%. The sensitivity of diagnostic whole body iodine scan was 63.6% for detection of remnant or metastases. Conclusion: Our study showed that in follow up of patients with differentiated thyroid cancer, ΔTg is more reliable-indicator of remnant disease than OnT4Tg or OffT4Tg levels. (authors)