SUN-623 Markedly Elevated Thyroglobulin Levels in a Patient with Papillary Thyroid Carcinoma in the Setting of Hashimoto’s Thyroiditis and Negative Imaging: A Case Report

Background: Despite the high survival rate of differentiated thyroid cancer (DTC), recurrence may occur. After tumor resection, radioactive iodine (RAI) can be considered if necessary, and serum thyroglobulin (Tg) and thyroglobulin antibody (TgAb) levels as well as neck ultrasound (US) are the main tools used for surveillance. Discrepancies between laboratory testing and imaging findings may present. This can occur due to assay interference or less commonly in the setting of Hashimoto’s thyroiditis (HT). We present the case of a patient with low risk DTC and chronic markedly elevated Tg levels in the setting of no evidence of structural disease and a history of HT. Clinical Case: A 72-year-old woman with a history of primary hypothyroidism and non-toxic multinodular goiter underwent left hemithyroidectomy in 2012. Pathology revealed incidentally discovered papillary thyroid microcarcinoma, 0.2 cm with negative margins and negative lymphovascular invasion in a background of chronic lymphocytic thyroiditis. Due to the low risk of recurrence, she did not undergo completion thyroidectomy nor RAI. Post operatively, she was noted to have a serum Tg level of 752 ng/mL (no reference range), TgAb <20 IU/mL (reference range < or =1 IU/mL), with a TSH of 4.3 mIU/L (reference range 0.4-4.5 mIU/L). Since then she has remained with elevated Tg levels ranging from 236 to 786 ng/mL, including Tg levels by liquid chromatography mass spectrometry (LC/MS) and negative TgAb. She did have a thyroid peroxidase antibody measured at > 1000 IU/mL (reference range < 35 IU/mL). Imaging included multiple serial neck US negative for right hemi-thyroid nodules and no suspicious lymph nodes. CT scan of the chest was negative. PET/CT showed FDG uptake in the right thyroid lobe (SUV max 8.6) without any evidence of FDG avid tumor in the rest of the body. She is otherwise clinically stable on levothyroxine therapy to maintain her TSH between 0.5 to 2 mIU/L. Discussion: This patient presents persistently elevated serum Tg levels significantly above what is expected for a patient status post hemithyroidectomy. We suspect the most likely explanation for this finding is her underlying HT causing autoimmune thyroid tissue destruction. Moving forward, we suggest measuring heterophilic antibodies and remeasuring Tg level with serial dilutions to rule out the possibility of assay interference. This case is especially challenging as she has an intact hemi-thyroid, which limits some of the imaging diagnostic modalities available. HT has been shown to result in a diffuse distribution of FDG uptake in the thyroid gland on PET/CT as seen in this case. Conclusion: Even though in most cases with DTC Tg levels serve as a sensitive and reliable tumor marker, in the setting of elevated thyroglobulin levels and no clear structural evidence of disease, it is important to consider other possible etiologies and not only disease recurrence.