The thymus gland is a small organ with a remarkably important job. Located behind the breastbone, it serves as the body’s central training center for T-cells—specialized immune cells that help protect us from infections, cancer cells, and other threats. Although the thymus naturally becomes smaller with age, the foundation it builds for long-term immunity is …
Understanding the Thymus Gland: Function, Disorders, and Treatment Options
The thymus gland is a small organ with a remarkably important job. Located behind the breastbone, it serves as the body’s central training center for T-cells—specialized immune cells that help protect us from infections, cancer cells, and other threats. Although the thymus naturally becomes smaller with age, the foundation it builds for long-term immunity is essential for lifelong health.
This guide explains how the thymus works, what happens when it becomes enlarged or develops disease, and how modern diagnostics and treatments can help patients.
What Is the Thymus Gland?
The thymus is a primary lymphoid organ located in the anterior mediastinum, just behind the sternum. It plays a central role in the development of T-lymphocytes (T-cells)—the white blood cells responsible for defending the body against viruses, cancer cells, and abnormal tissues.
The thymus also produces small signaling molecules (thymic peptides) that help regulate immune cell development.
How the Thymus Supports Immunity
1. T-Cell Maturation
Immature T-cells from the bone marrow enter the thymus, where they learn to:
- Recognize harmful threats
- Cooperate with other immune cells
- Avoid attacking the body’s own tissues
2. Positive and Negative Selection
These two steps are essential for preventing autoimmune disease:
- Positive selection: Cells must recognize the body’s Major Histocompatibility Complex (MHC) proteins, which are essential for presenting antigens to survive. This step ensures that the mature T-cells will be able to interact with other immune cells and respond to infections.
- Negative selection: This stage is critical for preventing autoimmunity. Cells that react too strongly to self-antigens are removed. This rigorous screening process ensures that the cells released into circulation will not mistakenly attack healthy body tissues, thus safeguarding the integrity of the immune system.
3. Lifelong Immune Balance
The activity of the thymus changes significantly throughout an individual’s life. It is most active and largest during fetal development and early childhood, when the body is building its initial repertoire of immune defenders. After puberty, the organ gradually begins to involute, shrinking in size as much of its functional tissue is progressively replaced by fat.
Anatomy of the Thymus
The thymus is a primary lymphoid organ with a unique structure that supports its key role in T-cell development. It is made up of two lobes that sit closely together in the center of the chest. In infants and young children—when the thymus is most active—it appears as a soft, slightly lobulated gland with a pinkish-gray color.
Anatomically, the thymus is located in the anterior superior mediastinum, the front upper part of the chest cavity. It lies directly behind the breastbone (sternum) and in front of the heart, resting on the pericardium. The gland is positioned just anterior to major structures such as the aorta and trachea, placing it at the core of the mediastinum. This central location allows the thymus to create the specialized environment needed for immune system development, especially the maturation of T-lymphocytes.
Conditions That Affect the Thymus
1. Genetic Disorders
DiGeorge syndrome, a congenital condition results from a deletion on chromosome 22, leads to thymic hypoplasia or aplasia (the thymus may be underdeveloped or, in rare cases, completely absent), causing severe T-cell deficiency and recurrent infections.
2. Autoimmune Associations
The thymus may become enlarged in myasthenia gravis (MG) and other autoimmune diseases.
- MG causes muscle weakness, drooping eyelids, difficulty speaking, and fatigue.
3. Thymic Tumors
The two main types are:
- Thymoma (more common, usually slow-growing)
- Thymic carcinoma (rarer, more aggressive)
The WHO classification further refines tumor type and guides prognosis.
Symptoms of Thymus Disorders
Some patients have no symptoms. When present, symptoms may include:
Mass Effect
- Persistent cough
- Chest pain
- Shortness of breath
- Hoarseness
- Facial or neck swelling [suggesting superior vena cava (SVC) syndrome]
Autoimmune or Paraneoplastic Symptoms
- Muscle weakness
- Double vision
- Difficulty swallowing
- Drooping eyelids
Diagnosis: How Thymus Conditions Are Identified
Evaluation typically includes:
- Chest X-ray to detect masses
- CT scan for detailed anatomy
- MRI to evaluate complex mediastinal involvement
- Biopsy → the only way to confirm tumor type
Blood tests may be used when autoimmune diseases such as MG are suspected.
If you have persistent chest pain, shortness of breath, facial or neck swelling, difficulty swallowing or unexplained muscle weakness, seek medical evaluation promptly.
Even if you do not have any symptoms, do not forget to schedule regular check-up examinations.
Treatment Options for Thymus Disorders
1. Surgery (Thymectomy)
- First-line treatment for most thymomas
- Often improves symptoms in patients with myasthenia gravis
- Can be performed through open, VATS, or robotic-assisted techniques depending on tumor size and anatomy
2. Radiation Therapy
Used after surgery or when the tumor involves nearby structures.
3. Chemotherapy
Used for advanced thymic carcinoma or recurrent disease.
4. Long-Term Management for Autoimmune Disease
MG patients may benefit from:
- Immunosuppressive medications
- Symptomatic therapies
- Specialized follow-up
Living Without a Thymus
- Adults can function well because their immune repertoire is already established.
- Children who lack a thymus require specialized care because early T-cell development is essential for immunity.
The Thymus and the Immune System
The thymus plays a central role in building a strong and balanced immune system. Often described as the body’s “training center” for T-cells, it provides the highly controlled environment where these cells learn to recognize harmful invaders while avoiding attacks on the body’s own healthy tissues. This careful maturation and selection process is essential for preventing autoimmune diseases and ensuring effective immune defense.
After puberty, the thymus naturally becomes smaller through a process called thymic involution, where some of its active tissue is gradually replaced by fat. This change is normal and expected, as most of the body’s T-cell diversity has already been established during childhood. Even so, the thymus continues to produce a smaller number of new T-cells throughout adult life, supporting ongoing immune surveillance.
When the thymus does not function properly, the consequences can be significant. Reduced or faulty T-cell development may lead to immunodeficiency, increasing susceptibility to infections. Conversely, disrupted selection processes may contribute to autoimmune disorders, in which the immune system mistakenly targets the body’s own cells.
FAQs About the Thymus Gland
What does the thymus do?
It trains T-cells to protect the body without attacking healthy tissues.
Where is it located?
Behind the breastbone (sternum), in the upper front part of the chest (anterior mediastinum).
Can thymus diseases be serious?
Yes. Aggressive thymic cancer and severe congenital immunodeficiency can be life-threatening.
How does the thymus change with age?
It is largest in childhood and gradually shrinks and its activity diminishes significantly after puberty, though it continues producing some T-cells.
What are signs of thymus cancer?
Many individuals with early-stage thymus cancer are asymptomatic, making detection difficult. For this reason, scheduling regular health evaluations and comprehensive check-up programs is essential for early detection and timely treatment.
When symptoms do appear, they often result from the tumor pressing on nearby chest structures, leading to a persistent cough, chest pain, shortness of breath, or hoarseness. In some cases, symptoms may arise from associated autoimmune conditions like myasthenia gravis, which can cause muscle weakness, drooping eyelids, and double vision.
When to Seek Specialist Care
If you experience:
- Persistent chest symptoms
- Unexplained muscle weakness
- Facial or neck swelling
- Difficulty swallowing or breathing
…a consultation with a thoracic surgeon or neurologist is recommended.
Need Expert Evaluation or Treatment?
Florence Nightingale Hospitals offer:
- Comprehensive check-up programs
- Advanced imaging
- Robotic thoracic surgery
- Multidisciplinary tumor boards
- Comprehensive MG and autoimmune disease care
Contact us to schedule an evaluation or request a medical second opinion.
