Endocrine Abstracts

Introduction: There is a rare but well documented association between thymic hyperplasia and Graves’ disease particularly in young patients. It is important to recognise this as patients presenting with an anterior mediastinal mass will often undergo extensive investigations to exclude an underlying lymphoproliferative disorder.

Case study: A 19-year-old female with symptomatic Graves’ thyrotoxicosis (TSH <0.01 mU/l; FT₄ 46.8 pmol/l; TBII 5 U/l) presented with pleuritic chest pain. A CT pulmonary angiogram excluded pulmonary embolism but an incidental large anterior mediastinal soft tissue mass measuring 7.6×2.1×7.1 cm was noted. Hodgkin’s lymphoma was suspected but histology from CT guided biopsy of the mediastinal mass showed a T-cell lymphoid population and features consistent with true thymic hyperplasia. PET CT showed diffuse moderate grade FDG activity within the mediastinal mass. An association between Graves’ disease and thymic hyperplasia was recognised. The patient underwent an uneventful total thyroidectomy after euthyroidism was achieved with carbimazole therapy. CT scan 6 months post-surgery showed near complete resolution of the anterior mediastinal mass.

Discussion: The association between Graves’ disease and thymic hyperplasia is rare but well documented.In the majority of cases, thymic enlargement is minimal, but occasionally it may be massive. The exact pathophysiology in unknown, but a postulated mechanism is the possible presence of TSH receptors in thymic epithelial cells that may be involved in the autoimmune response. Given that the differential diagnosis of an anterior mediastinal mass includes pathologies that may be managed with surgical resection for both diagnosis and treatment, recognition of the association between Graves’ and thymic hyperplasia can potentially avoid unnecessary major surgery.

Treatment of Graves’ hyperthyroidism usually results in complete resolution of the thymic hyperplasia.