Prognostic value and optimal threshold of first thyroglobulin in low/intermediate risk DTC.

The aim of this study was to define prognostic value and optimal threshold of first thyroglobulin (fTg) measured after thyroidectomy and just before radio-iodine therapy (RIT), in low/intermediate risk patients with differentiated thyroid cancer (DTC). This is a retrospective study in 383 patients with DTC who were treated with surgery followed by RIT. Response to treatment was assessed 1 and 2 years after RIT. Odds ratio of different risk factors like age, sex, TNM stage, fTg and Anti-Tg Ab were compared between patients with and without incomplete response 1 and 2 years after treatment. Receiver operating curve analysis was used for definition of optimal fTg cut off for detection of incomplete response. 218 female and 55 male with DTC had negative anti-Tg antibody (mean age: 37.5±14.5 years) and analyzed separately. fTg≥33.5 ng/mL and fTg/TSH ratio of ≥0.36 had the optimal sensitivity and specificity for detection of incomplete response 1 and 2 years after treatment. fTg<33.5 ng/mL had NPV of 98.5% for exclusion of distant metastases. Patients with fTg≥33.5 ng/mL had longer "time to excellent response" (3.6±2.3 vs. 2.0±1.8 yrs) and needed more additional treatments compared to patients with fTg<33.5 ng/mL. Multivariate analysis showed that fTg was the most potent risk factor for prediction of treatment failure 1 and 2 years after RIT. fTg of ≥33.5 ng/mL was the most important risk factor for prediction of treatment failure after RIT and could be included in decision algorithms regarding intensity of treatments in low/intermediate risk patients with DTC.