The Cut-off Value of Serum Thyroglobulin (Tg) for Well Differentiated Thyroid Carcinoma treated by thyroidectomy and 131I Used as an indicator of disease free period

The follow-up of well differentiated thyroid carcinoma (DTC) after thyroidectomy and 131 I ablation is mainly based on serum thyroglobulin (Tg) level determination since serum Tg as a tumor marker reveals high sensitivity and specificity. Determination of cut-off value of serum Tg level in each thyroid cancer clinic is essential for the prediction of disease persistence, recurrence or metastasis. This retrospective study included 68 patients (9 males, 59 females) diagnosed consecutively between 1991 and 1999 with DTC and treated by total / near total thyroidectomy and remnant ablation with 100-150 mCi of iodine 131 at Rajavithi Hospital. All patients underwent 131 I Whole Body Scan ( 131 I WBS) between 6-12 months after ablative doses and had two negative scans consecutively at 6-12 month intervals coinciding with low serum Tg level after 4-6 weeks of thyroid hormone withdrawal. These patients remained 4-7 years disease free (average 5.87 ฑ 1.16 yrs). No interference in the serum Tg assay from antithyroglobulin antibodies (TgAb) was found in these 68 patients. Serum Tg levels were below 0.5 ng/ml (undetectable) in 50 patients (73.5%). The 9.0 ng/ ml cut-off value was then derived as 95 percentile value. In conclusion, the cut-off value of serum Tg level 9.0 ng/ml after 4-6 weeks thyroid hormone withdrawal was obtained from this group of patients who had disease free period of 5.87 ฑ 1.16 years after adequate treatment with surgery and 131 I ablation. It is therefore likely that this cut-off value might be useful as a critical level for discerning the disease in the follow-up of DTC patients in Rajavithi Hospital. Key words: thyroglobulin, well differentiated thyroid carcinoma, cut-off value