Utility of stimulated serum thyroglobulin assay and I-131 whole body scan for the follow-up of patients with low risk thyroid cancer after radioablation

Objectives To determine the value of stimulated serum thyroglobulin (Tg) assay and I-131 whole body scan (WBS) in patients with undetectable Tg on thyroid hormone replacement and negative anti-Tg antibodies after total thyroidectomy and radioablation for thyroid carcinoma.

Methods 187 patients (137 women, mean age: 49±14 (17-79) with low risk differentiated thyroid carcinoma who had a normal clinical exam and undetectable serum Tg during T4 replacement were included in the study. Patients were treated with total thyroidectomy followed by radioablation with I-131. All tumors were smaller than 4cm with no evidence of extrathyroidal invasion or metastases. Tg was measured using a high sensitive solid phase chemoluminescence immunometric method with a functional sensitivity of 0.2ng/ml. After T4 withdrawal, serum Tg measurement, neck ultrasonography and I-131 WBS were performed.

Results After T4 withdrawal (TSH >30mIU/l), 29% (n=54) and 7% (n=13)of the patients serum Tg levels were measured between 0.2ng/ml and 1ng/ml (median 0.57) and >1ng/ml respectively. Out of 54 patients who had 0.2ng/ml ≤ Tg ≤ 1ng/ml, 29% did not have any uptake on WBS whereas 71% of patients had minimal I-131 uptake in the thyroid bed. Since USG was within normal limits in all, no further treatment was given. None of these patients had tumor recurrence over 7.4 (4.7-18.8) months of follow-up. Out of 13 patients with Tg >1ng/ml, 50% had minimal uptake in the thyroid bed and one patient had extrathyroidal uptake on I-131 WBS. Although WBS was negative, three of these patients were diagnosed with cervical lymph node metastases with USG. Using the institutional cut-off of 0.2ng/ml, a negative serum Tg during T4 replacement was associated a high negative predictive value (NPV:98%) which increased to 99% when combined with neck USG.

Conclusions Undetectable Tg on T4 with a high-sensitive immunoradiometric assay following thyroid ablation in the absence of antibody interference and normal USG has a high NPV in low risk thyroid cancer patients.