Is it Possible to avoid rh-TSH test in Patientswith Differentiated Thyroid Carcinoma byUsing the Association between Ablation andSuppressive Thyroglobulin?

Background: The follow-up of differentiated thyroid cancers is based on neck ultrasonography and serum thyroglobulin assay (Tg), during l-T4 therapy and after recombinant human TSH administration; this test appears quite expensive, considering that only a small percentage of patients with undetectable Tg on TSH suppression therapy shows a response after TSH stimulation. Objectives: The aim of our study was to verify whether low levels of serum thyroglobulin at the time of remnant ablation (A-Tg) associated with undetectable thyroglobulin levels on TSH suppression (S-Tg), have sufficient negative predictive value for recurrence of disease, thus avoiding rh-TSH test in Differentiated Thyroid Cancer patients. Methods: we retrospectively enrolled 975 DTC patients treated with thyroidectomy+131-I remnant ablation showing undetectable S-Tg measured after 12 months follow-up. The availability of A-Tg and rh-TSH stimulated Tg (R-Tg) obtained 1 year later were considered as inclusion criteria. Patients with positive circulating Ab-Tg and/or histological dedifferentiation were excluded. Patients were subdivided in high and low risk of recurrence according to the criteria proposed by the European Thyroid Cancer Taskforce. Results: Using rh-TSH test as gold standard, the NPV for A-Tg<10 μg/L was 98.5% in group A (low risk patients) and 95.5% in group B (high risk patients); it significantly raised to 99.2% in group A (p-value 0.03) and 99.3% in group B (p-value 0.02) when the association between A-Tg<10 μg/L and S-Tg<0.6 μg/L was considered. When we evaluated the whole population the negative predictive value was 97% for A-Tg<10 μg/L alone, raising to 99.3% when associated with S-Tg<0.6 μg/L (p-value<0.008). Conclusion: our data confirmed the very high negative predictive value of the association between low levels of A-Tg and undetectable S-Tg in the early risk stratification of differentiated thyroid cancer patients, leading to avoid rh-TSH test with an important economic impact.