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.