The thymus gland plays an important role in your immune system.
Removing it is controversial, but does help with certain medical problems.
Myasthenia Gravis is one cause.
http://www.medicinenet.com/myasthenia_gravis/article.htm
Searching doesn't turn up many methods of shrinking the gland.
What is your doctor telling you?
I am 49 and also have an enlarged Thymus gland with NO symptoms. Doctors seem to have no advice other than to remove it but I have concerns about how it will effect my imune system. Are there any other otpions?