SAT-359 Thymic Hyperplasia During Pregnancy with Graves’ Disease

Abstract Disclosure: S. Douglas: None. J. Martone: None. M.M. Kristan: None. Graves’ disease (GD) is an autoimmune condition characterized by antibodies stimulating TSH receptors, affecting 0.2% of pregnancies. Identification and management of GD is essential as the disease can result in adverse outcomes in both mother and fetus, including thyrotoxicosis. Pregnant women with thyrotoxicosis are at a higher risk of preterm birth, low birth weight, preeclampsia and cesarean section compared to low risk pregnant women. A finding of thymic hyperplasia during imaging has been associated with GD, though pathogenesis is less clear [3]. Thymic hyperplasia can lead to symptoms such as dyspnea and pain attributable to compression of structures including the superior vena cava, which can be dangerous in pregnancy. Case A 38 year old woman at the 16th week of her first pregnancy presented to the emergency department (ED) with two weeks of headache and shortness of breath. She was found to have abnormal thyroid function tests suggestive of hyperthyroidism; free thyroxine (FT4) of 5.72 pmol/L, free triiodothyronine (FT3) 23.8 pmol/L, total triiodothyronine (TT3) of 651 pmol/L, TSH of <0.01. Thyroid stimulating immunoglobulin was 2.3 pmol/L, suggestive of Graves’ disease (GD). Propranolol (10 mg) and methimazole (MMI, 20 mg) were initiated. A CT scan showed a 7.7 cm lobulated solid anterior mediastinal mass, favored to represent thymic hyperplasia in the setting of pregnancy. At follow-up, biopsy was deferred until 6 months after treatment for GD. Two weeks later, thyroid function tests worsened and MMI dose increased to 30 mg. She developed pretibial myxedema related to GD, MMI increased to 40 mg. She returned to the ED two months after initial presentation with shortness of breath; labs consistent with GD and CT showed an 8.6 cm anterior mediastinal mass. Two weeks later, ultrasound showed macrosomia of the fetus at 25 weeks, with growth >99th percentile. Labs showed continued hyperthyroidism and MMI dose increased to 50 mg. Two weeks later, MMI dose increased to 60 mg daily. Over the next month she remained hyperthyroid, on MMI, throughout pregnancy. She was admitted for cesarean section and the baby was successfully delivered at >5000 grams. Five days after delivery she returned to the ED with chest pain. Cardiac testing was unremarkable, thyroid panel consistent with hyperthyroidism, and CT showed an anterior mediastinal mass with decreased size to 7.1 cm compared to previous. Discussion Thymic hyperplasia has been associated with GD, though it generally regresses with treatment. In the setting of thyrotoxicosis, tissue biopsy or thymectomy may be contraindicated due to potential for triggering thyroid storm. In this case, thymic hyperplasia was treated medically. In the absence of suspicious mass findings, medical treatment of GD should take priority over biopsy or thymectomy due to the invasive nature of the procedure and anticipated regression with subsequent treatment of GD. Presentation: Saturday, July 12, 2025