The thymus is a two-lobed lymphoid organ behind the breastbone in the upper chest where T cells mature and learn to tell the body's own tissues from foreign invaders. It is large in childhood, reaching its greatest weight around puberty, then is gradually replaced by fat. It also produces hormones such as thymosin and thymulin, so it is sometimes classed as an endocrine gland too.
| Location | Anterior (superior and anterior) mediastinum, behind the manubrium and upper sternum, in front of the pericardium, great vessels and trachea; may extend up into the neck |
|---|---|
| Secretes |
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| Nerve Supply | Vagus nerve branches; Sympathetic fibres from the cervical and thoracic ganglia |
| Blood Supply | Internal thoracic artery branches; Inferior thyroid artery branches |
| Venous drainage |
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| Lymphatic drainage |
Provides the environment in which immature T cells from the bone marrow develop, with self-reactive cells deleted and useful ones released to populate lymph nodes and spleen. It is essential for establishing T cell immunity early in life.
Thymomas, tumours of thymic epithelium, are the commonest anterior mediastinal masses in adults and are strongly linked with myasthenia gravis; thymectomy can improve myasthenia. Congenital absence or underdevelopment of the thymus (DiGeorge syndrome, 22q11 deletion) causes T cell deficiency and recurrent infections. In infants a large normal thymus can look like a mass on chest X-ray (the sail sign).
A tumour of thymic epithelial cells in the anterior mediastinum, often associated with myasthenia gravis and other autoimmune conditions.
A 22q11.2 deletion causing an absent or small thymus with T cell immunodeficiency, along with heart defects and low calcium.
Enlargement of the thymus, either true hyperplasia after stress or chemotherapy or lymphoid follicular hyperplasia in myasthenia gravis.
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