Is stimulation of thyroglobulin (Tg) useful in low-risk patients with thyroid carcinoma and undetectable Tg on thyroxin and negative neck ultrasound?

Objective: To determine the usefulness of thyroglobulin (Tg) stimulation in low-risk patients with undetectable Tg on T4 and negative neck ultrasound (US) after initial therapy of thyroid carcinoma.

Methods: We evaluated 122 consecutive patients classified as low risk 6 months to 1 year after total thyroidectomy and remnant ablation. All patients had a normal clinical exam, Tg < or = 1 ng/ml during suppressive therapy (TSH < 0.1 mIU/l), and undetectable antithyroglobulin antibodies.

Results: After T4 withdrawal and elevation of TSH to values > 30 mIU/l, 26 patients (21.3%) converted Tg to levels > 1 ng/ml. Metastases were detected in 10 patients, nine showing stimulated Tg levels > 1 ng/ml. Cervical metastases were observed in 9/10 patients and lung metastases in one patient. Neck US identified all cervical metastases. Seventeen patients with stimulated Tg levels > 1 ng/ml initially showed no apparent disease, with a reduction in Tg being observed upon subsequent measurements, and 13 patients presented undetectable Tg off T4 at the end of the study. Undetectable Tg on T4 showed a high negative predictive value (NPV; 91.8%), which increased to 99.1% when combined with neck US. Stimulated Tg levels < 1 ng/ml presented a 98.9% NPV. A total of 113 patients with undetectable Tg on T4 and negative US had to be exposed to hypothyroidism in order to diagnose one further case of metastases.

Conclusion: Undetectable Tg on T4 combined with negative neck US presented a high NPV in low-risk patients and Tg stimulation might be avoided in these patients.