Thymus hyperplasia (or thymic hyperplasia) refers to an enlargement ("hyperplasia") of the thymus.
It is not always a disease state. The size of the thymus usually peaks during adolescence, and atrophies in the following decades. Before the immune function of the thymus was well understood, the enlargement was sometimes seen as a cause for alarm, and justification for surgical reduction. This approach is much less common today.
It can be associated with myasthenia gravis.
MRI can be used to distinguish it from thymoma.