Ectopic Cervical Thymic Tissue Mimicking Parathyroid Hyperplasia: A Rare Occurrence Linked to Graves Disease

Thymic hyperplasia has occasionally been reported in patients with Graves disease (GD). However, ectopic cervical thymic hyperplasia in the setting of hyperthyroid GD is exceptionally rare. We describe a case of a 54-year-old Thai woman who presented with hyperthyroidism, diplopia, and pretibial myxedema. She underwent a total thyroidectomy because of diplopia caused by Graves ophthalmopathy. During the surgery, 3 macroscopically abnormal enlargements of parathyroid gland-like tissue were identified and removed. Histopathology revealed hyperplastic thymic tissue mixed with 1 normal-sized parathyroid gland at the location of the left upper parathyroid gland, and thymic tissue was found in the sample labeled as the right upper parathyroid gland. Notably, the sample labeled as the right lower parathyroid gland was actually determined to be a lymph node. Preoperative blood samples showed normal serum calcium and parathyroid hormone levels. Postoperatively, computed tomography of the chest showed thymic hyperplasia in the anterior mediastinum, which slightly regressed at the 9-month follow-up. The patient had transient hypoparathyroidism requiring oral calcium and active vitamin D supplements for 6 months postoperatively. Ectopic cervical thymic hyperplasia can be found in GD and might be indistinguishable from parathyroid hyperplasia. Biochemical evaluations are required to exclude concomitant hyperparathyroidism, and a conservative approach should be considered.