The thymus gland is situated just below the neck, behind the breastbone and in front of the heart. It is a flattish structure divided into two lobes, and it plays a part in the normal functioning of the immune system. An enlarged thymus gland can occur for a number of reasons, including simple overgrowth of thymus tissue, which is found in some autoimmune disorders where the body's immune system attacks itself. Other causes of enlargement are: cysts, benign tumors and cancers.
Normally, the thymus gland grows until puberty, when it begins to reduce in size, with the tissue being replaced gradually by fat as one gets older. It is relatively large in babies, so any conditions causing thymus enlargement at this age risk interfering with breathing. The thymus is the place where specialized white blood cells known as T-lymphocytes are able to mature. Mature T-lymphocytes only leave the thymus when they are able to recognize cells and substances in the body that are not part of the self and attack them as part of the immune response. This is why thymus problems can be associated with autoimmune disease, as defective cells may enter the circulation and mistakenly attack the self.
When an enlarged thymus gland is due to overgrowth of thymus tissue, it is usually a sign of autoimmune disease, although there is also a condition in children where the thymus enlarges hugely with no apparent cause, known as idiopathic hyperplasia. The autoimmune condition known as myasthenia gravis is generally associated with an enlarged thymus in adults. People with myasthenia gravis experience muscle weakness as a result of antibodies made by the body which interfere with the way nerves function. This disease can be serious if the muscles of breathing are severely affected, requiring artificial ventilation, but it is generally treatable with medication. The thymus can also be removed surgically, which relieves symptoms in most people, although the specific part the thymus plays in the disease is not fully understood.
A benign, or non-cancerous, growth or tumor may be the cause of an enlarged thymus. Examples of benign growths include cysts, fatty lumps called thymolipomas and tumors known as thymomas. Up to half of all people with thymomas are found to have myasthenia gravis as well. Benign tumors of the thymus often cause no symptoms, unless they are pressing on the windpipe or another important structure, and the usual treatment is to remove them using surgery.
Rarely, malignant, or cancerous, tumors can give rise to an enlarged thymus gland. As malignant tumors tend to spread into surrounding structures, they are more likely to cause symptoms than benign growths. Symptoms such as pain, a hoarse voice and a swollen face may occur. Treatments include surgical removal, chemotherapy and radiotherapy. Sometimes thymomas may spread, in which case they are treated in the same way as cancers of the thymus.