The thymus gland, located behind your sternum and between your lungs, is only active until puberty. After puberty, the thymus starts to slowly shrink and become replaced by fat. Thymosin is the hormone of the thymus, and it stimulates the development of disease-fighting T cells.
Abnormalities of the thymus can result in a decreased number of T cells and autoimmune diseases such as Autoimmune polyendocrine syndrome type 1 and myasthenia gravis. These are often associated with cancer of the tissue of the thymus, called thymoma , or tissues arising from immature lymphocytes such as T cells, called lymphoma.
Myasthenia gravis is an autoimmune disease most often due to antibodies that block acetylcholine receptors, involved in signalling between nerves and muscles. It is often associated with thymic hyperplasia or thymoma, with antibodies produced probably because of T cells that develop abnormally. Myasthenia gravis most often develops between young and middle age, causing easy fatiguing of muscle movements. Investigations include demonstrating antibodies (such as against acetylcholine receptors or muscle-specific kinase), and CT scan to detect thymoma. With regard to the thymus, removal of the thymus, called thymectomy may be considered as a treatment, particularly if a thymoma is found.