Ablative Free Thyroxine to Thyroglobulin Ratio as a Predictor of Differentiated Thyroid Cancer Recurrence

Background: Serum thyroglobulin (Tg), a widely used thyroid cancer marker, is limited at the time of ablation, unable to differentiate between diseased and normal residual tissue. Objective: We evaluated the use of the ablation free thyroxine to thyroglobulin ratio (fT(4):Tg) as a tumour-specific ratio for predicting persistence or recurrence in differentiated thyroid cancer. Design: Retrospective chart review. Setting: McGill University Health Centre. Methods: Of 234 patients, 84 were analyzed after exclusion of those with anti-Tg antibodies, ablation Tg <= 2, and follow-up < 3 months. Ablation thyroxine and Tg levels were recorded and patients were followed to detect recurrence. The relationship between the ablation fT(4):thyroglobulin ratio and recurrence was evaluated. Main Outcome Measures: Hazards ratio (HR) for predictive fT(4):Tg ratio cutoff value and disease-free survival based on the fT(4):Tg ratio. Results: Thirty-eight percent of patients developed recurrence: 8 pathologically proven and 24 suspected. Eighty-one percent of patients with recurrence had an fT(4):Tg < 27%, in contrast to 23% of those without recurrence (HR 6.2; p < .001). Of all patients with fT(4):Tg < 27%, 68% developed evidence of recurrence compared with 13% with fT(4):Tg >= 27% (p < .001). Recurrences in the fT(4):Tg < 27% group occurred twice as early. Conclusion: Ablation fT(4):Tg < 27% is predictive of recurrence and should be used to identify high-risk patients.