Positive predictive value of detectable stimulated Tg during the first year after therapy of thyroid cancer and the value of comparison with Tg-ablation and Tg measured after 24 months

This study evaluated the positive predictive value (PPV) of detectable stimulated thyroglobulin during the first year after treatment of thyroid carcinoma (Tg-1) and the value of comparison with Tg-ablation and measured after 24 months (Tg-2). Forty-two consecutive patients undergoing total thyroidectomy and ablation with detectable Tg-1 (>1 ng/mL) were selected. The patients had well-differentiated tumors, which were completely resected, and there was no ectopic uptake on whole body scan after 3.7-5.5 GBq I-131. Imaging methods during follow-up revealed metastases in 10 patients (24%) (15% if Tg-1 <= 10 ng/mL and 55% if Tg-1 >10 ng/mL). Tg-ablation (cutoff of 10 ng/mL) presented a negative predictive value (NPV) of 91% and PPV of 42%. Comparing Tg-ablation with Tg-1, the PPV of an increase was 100%, whereas the NPV of a decrease was 88%. Thirty-six patients presented negative imaging results upon first assessment and Tg-1 was compared to Tg-2. Metastases were detected in all patients who presented an increase in Tg (n = 4), whereas patients without variation (n = 4) or with a decrease (n = 28) showed no apparent disease. Among disease-free patients (n = 32), 50% presented undetectable Tg and 40% showed a >50% decrease after 2 years. In conclusion, most patients with detectable stimulated Tg during the first year after therapy had no metastases, and evaluation of the slope of Tg helped discriminate cases with apparent disease.