SAT-356 - Marked Thyroglobulin Elevation after Thyroid Lobectomy Due to Graves Disease Mimicking Recurrent Papillary Thyroid Carcinoma

Background: Thyroglobulin (Tg) interpretation after thyroid lobectomy is challenging, as residual thyroid tissue and functional changes may influence levels. Progressive Tg elevation may raise concern for recurrent papillary thyroid carcinoma (PTC), but alternative etiologies should be considered.

Clinical

Case: A 48-year-old woman underwent right hemithyroidectomy with isthmusectomy in 2016 for a suspicious thyroid nodule. Pathology demonstrated two foci of papillary microcarcinoma (0.3 cm in the right lobe and 0.1 cm in the isthmus), without lymphovascular invasion, extrathyroidal extension, or nodal metastasis. A concurrent 4.6 cm noninvasive follicular thyroid neoplasm with papillary-like nuclear features was identified in the isthmus. She subsequently developed hypothyroidism and was maintained on stable levothyroxine therapy. Postoperative surveillance ultrasounds initially showed no recurrence with fluctuating left-lobe nodules, including spontaneous regression and later resolution. A fine needle aspiration from the left thyroid nodule was benign consistent with Bethesda 2. Serum Tg remained detectable but stable (59-79 ng/mL) over several years, consistent with residual thyroid tissue following lobectomy. In December 2025, the patient presented with significant unintentional weight loss. Evaluation revealed marked biochemical thyrotoxicosis (free T4 54.1 pmol/L, TSH 0.005 mIU/L) with a sharp rise in Tg to 377 ng/mL, later increasing to 517 ng/mL despite discontinuation of levothyroxine for several weeks. Initial impression was over-treated hypothyroidism with possible recurrence in the left lobe so completion thyroidectomy was considered. Despite discontinuation of levothyroxine patient remained hyperthyroid so radioactive iodine uptake scan was done in January 2026 and demonstrated diffuse uptake consistent with Graves disease, therefore, radioactive iodine therapy was arranged.

Conclusion: This case highlights that marked Tg elevation after lobectomy may reflect hyper-functioning residual thyroid tissue rather than recurrent PTC. Many causes should be taken into consideration when facing a patient with biochemical evidence of hyperthyroidism and elevated TG. Up to our knowledge, Grave’s disease reported after lobectomy is rare but should be considered especially if there is lack of improvement with levothyroxine cessation.

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