Ablative free thyroxine to thyroglobulin ratio as a predictor of differentiated thyroid cancer recurrence

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.

We evaluated the use of the ablation free thyroxine to thyroglobulin ratio (fT4:Tg) as a tumour-specific ratio for predicting persistence or recurrence in differentiated thyroid cancer.

Retrospective chart review.

McGill University Health Centre.

Of 234 patients, 84 were analyzed after exclusion of those with anti-Tg antibodies, ablation Tg < or = 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 fT4:thyroglobulin ratio and recurrence was evaluated.

Hazards ratio (HR) for predictive fT4:Tg ratio cutoff value and disease-free survival based on the fT4:Tg ratio.

Thirty-eight percent of patients developed recurrence: 8 pathologically proven and 24 suspected. Eighty-one percent of patients with recurrence had an fT4:Tg < 27%, in contrast to 23% of those without recurrence (HR 6.2; p < .001). Of all patients with fT4:Tg < 27%, 68% developed evidence of recurrence compared with 13% with fT4:Tg > or = 27% (p < .001). Recurrences in the fT4:Tg < 27% group occurred twice as early.

Ablation fT4:Tg < 27% is predictive of recurrence and should be used to identify high-risk patients.