Something like 15% of newly diagnosed MG patients have an abnormal thymus meaning a tumor or enlargement of some sort. The tumor can be benign or malignant or benign and turning malignant. If you have a thymus tumor, they will probably want to remove it.

When young, we need the thymus to help build an immune system. As we age, the thymus shrinks, turns to fat and becomes less important for good health. The shrunken thymus is often called residual thymus tissue. It is not tumor tissue but the remnants of an unused organ.

There is debate as to whether it is worthwhile to remove a 'normal' thymus to combat MG. Some newer studies say removing a normal thymus is helpful in reducing MG symptoms and some studies say it is not effective.

Removing a tumorous thymus seems to provide a benefit to a majority of MG patients at least for some period of time.