Hi @dinahk, welcome.
The thymus is a small, irregular-shaped gland in the top part of the chest, just under the breastbone and between the lungs. It is located in an area of the body called the mediastinum. The thymus is part of both the lymphatic system and the endocrine system. For this reason, I moved your question to the Endocrine System group here: https://connect.mayoclinic.org/group/diabetes-and-endocrine-problems/

The thymus appears in a variety of shapes and sizes, even in the same person. It gradually involutes (rolls in) with age and may acutely shrink during periods of bodily stress. There isn't a standard "normal" size.

It sounds like there may have been unexpected changes in your thymus that your doctor would like to investigate. Do you know what they suspect?