Thymic hyperplasia is frequent in patients with Graves disease (GD) but it rarely is large enough to be detected radiologically as an anterior mediastinal mass. In the few cases operated, lymphoid hyperplasia (i.e. lymphoid follicle proliferation with expansion of both the cortical and the medullary component) has been documented histologically in 38% of cases, while true thymic hyperplasia, i.e. thymic enlargement with normal tissue architecture, was found more rarely. In only 4 out of 107 patients with GD a malignant thymic tumor was reported.
The mechanisms of this association remain unclear, both autoimmunity and hyperthyroidism being a potential cause. Hyperthyroidism persists after thymectomy but the treatment of hyperthyroidism with antithyroid drugs usually results in a decrease of the thymus.
We describe the evolution of 3 patients with GD and thymic mass on the computed tomography. In 2 patients, a 49 year-old female and a 28 year-old male, the thymic mass (3.3/1.6 cm and 5.5/2.5 cm, respectively) shrinked to normal after 4 and 6 months of treatment with methymazole. In the 3rd patient, a 37 year-old female, the thymic mass 4.8/3.7 cm persisted after 5 months of treatment with methymazole (TSH normalization was obtained only in the last 2 months). She underwent thymic surgery; the pathological exam showed thymic lymphoid hyperplasia. Two months later, the patient had overt myasthenia gravis with antibodies to acetilcholine receptor and required treatment with glucocorticoids and pyridostigmine. She was diagnosed 2 years later with seronegative rheumatoid arthritis and systemic lupus erythematosus. Her GD was cured after 2.5 years of medical treatment, with persistence of TPOAb. It is debatable if the thymus removal or a severe autoimmune background was the cause for this unfavorable evolution.
Conclusion: When thymic hyperplasia is diagnosed in association with GD, in the absence of myasthenia gravis and/or suspect CT findings, only antithyroid treatment and radiological follow-up are indicated. Thymic surgery may carry unnecessary risks for these patients.