What causes thymus swelling?

Also consider... Good's syndrome, which is a rare immunodeficiency that has been described in adults and is manifested by thymoma in conjunction with a combined cellular/humoral immunodeficiency.
Many causes. Thymomas are benign tumors associated with autoimmune neuromuscular disease myastenia gravis.Thymolipoma is another benign tumor mainly consisting of fatty tissue and enlargement of thymus.Thymic cysts can occur and be mutiloculated.Cancer or thymus very rare. Hyperthyroidism forms hormone that causes enlargement of medullary portion of thymus. This overgrowth is seen usually in teen age patient.
Thymoma. Thymus problems are not that common. Thymomas are usually benign tumors of the thymus. Often they are manifested with the symptoms of myasthenia graves - muscle weakness etc. There are some malignant tumors as well. Other thymus problems except for tumors are not common.