SAT551 Significantly Elevated Thyroglobulin Levels with Negative Whole-Body Scan and PET/CT After Treatment of Thyroid Cancer: A Case Report

Disclosure: A. Bassatne: None. J.T. Chaiban: None. Background: Thyroglobulin (Tg) levels are used to monitor recurrence or persistence of disease after treatment of differentiated thyroid cancer. Some cases may have persistently elevated Tg and negative whole-body scans (WBS). We present a case of substantially elevated Tg levels despite negative WBS and PET/CT after thyroidectomy and radioactive iodine (RAI) treatment in a patient with papillary thyroid carcinoma (PTC). Clinical Case: A 66 years old lady was referred to the endocrinology clinic almost 5 years ago after incidental findings of calcified thyroid nodules on CT. Thyroid US confirmed 2cm aggregate calcified nodules. FNA revealed PTC for which she underwent thyroidectomy. Pathology showed a unifocal 3.5 cm left lobe conventional PTC without angioinvasion nor extrathyroidal extension, 3/13 positive lymph nodes (LN) on the lateral dissection of the left neck as well as multiple small LN on the central dissection. 6 days postop, her Tg was still significantly high at 40.5ng/mL (Ref 1.2-35 ng/mL) and then increased to 397ng/mL after thyrogen preparation for RAI ablation, antithyroglobulin antibodies were negative. WBS post RAI was negative except for the physiologic neck uptake. Almost a year later, a diagnostic WBS after thyrogen stimulation was still negative but the stimulated Tg increased again to 235 ng/mL with negative thyroglobulin antibodies. Tg result were confirmed by LC-MS method. Heterophile Antibodies were negative. Patient confirmed that every single time she followed at least 2 weeks of low iodine diet in preparation for the WBS. Unfortunately, we were not able to get a 24 hour urine collection for iodine. A PET/CT was reassuring. CT neck/chest did not show any suspicious metastases. TSH was maintained at an adequately suppressed level all through and Tg levels were ranging around 6-8 ng/mL. After another year, a stimulated Tg was 302.4 ng/mL with a TSH of 198 mcU/mL (Ref 0.35-5.0 mcU/mL). At that time, a repeat WBS was negative and a repeat PET/CT demonstrated an interval development of a small thyroid focus thought to be consistent with recurrence of malignancy. However, a CT of neck/chest did not reveal any abnormal thyroid findings or metastases. Patient received 100 mCi of RAI after thyroid hormone withdrawal. Subsequent WBS was negative. Currently her Tg levels are stable at around 8-9ng/mL for the past 6 months with a suppressed TSH. Patient had another PET/CT through her breast surgeon to follow on a previous breast lesion and no thyroid abnormality was noted. Conclusion: Elevated Tg levels despite negative imaging may be seen in patients with PTC as illustrated in this case. We are reporting it due to the extent of elevation of Tg levels seen on more than one occasion in the absence of structural recurrence. Evaluation and treatment of such patients remain controversial. Presentation Date: Saturday, June 17, 2023