The thymus gland is located in the chest behind the breastbone. Its functions include producing white blood cells known as T cells, which aid in immunity. It also contributes to the production of hormones such as insulin.

The organ’s primary function is maturing T cells, or T lymphocytes. These are white blood cells responsible for fighting infections.

Additionally, the thymus produces an array of hormones. Some of these, like thymulin and thymosin, regulate immune cell production. The thymus also synthesizes hormones such as insulin and melatonin.

It is relatively large in infants and children. After puberty, it decreases in size, and is very small in older adults.

Read on to learn about the thymus’s function, location, and more.

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The thymus is an essential part of the immune system. Without it, the immune system cannot function properly.

Early studies in the 1960s tested its importance by removing it from mice. The experiments showed mice without a thymus had immunodeficiency, which is a failure of the immune system to fight infection.

The thymus is responsible for producing and maturing lymphocytes, or immune cells. These include T cells, a type of white blood cell that defends the body from infections.

Additionally, the thymus suppresses the effects of aging, according to a 2016 study.

Hormones released by the thymus inhibit the aging processes. They also help retain learning and memory capabilities.

The thymus is located in the upper chest, behind the breastbone.

It is in front of the ascending aorta, a large blood vessel coming from the heart.

Click on the BodyMap above to interact with a 3D model of the thymus.

The thymus is a soft gland with two lobes. Each lobe has a central par, known as the medulla, and an outer part, known as the cortex.

It is large in infants and young children, but it decreases in size after puberty. By the time a person reaches early adulthood, fat has largely replaced the tissues in the thymus.

The size of the thymus is very small in older adults.

The thymus produces several hormones. Most notably, it produces thymosin, which stimulates T cell production.

Researchers think thymosin acts upon the T cells that have matured in the thymus and prepares them for use throughout the body.

When T cells mature within the thymus, they are not capable of fighting viruses, bacteria, and foreign antigens. After the T cells leave the thymus, thymosin ensures they undergo complete maturation. This makes them capable of performing their duties as part of the immune system and protecting the body from harm.

Additionally, research indicates that thymosin and other thymic hormones inhibit aging.

Some of these effects involve helping preserve learning and memory capacity as people age.

The thymus also makes hormones that are similar to the hormones other glands in the body produce. They include:

  • Melatonin: a sleep-regulating hormone that comes from the pineal gland
  • Insulin: a blood sugar regulating hormone made by the pancreas
  • Growth hormone: a growth-regulating hormone from the pituitary gland
  • Prolactin: a breast-development hormone from the pituitary gland

The thymus contains various types of cells including epithelial and lymphatic cells. They include:

T cells

The thymus matures T cells which critically support the immune system.

Once fully mature, they help fight:

  • Tumors: Cancerous masses
  • Allergens: Substances that cause allergic reactions
  • Pathogens: Microbes, such as viruses and bacteria, that can cause an infection

B cells

The thymus also has a small population of B lymphocytes, or B cells.

Evidence suggests these B cells develop in the thymus and are different from B cells elsewhere in the body. They are referred to as thymic B cells.

Research shows that thymic B cells aid in negative selection of lymphocytes. This means they detect potentially harmful cells that interact too strongly with antigens, which are toxins.

Although thymic B cells play an important role in maintaining healthy cells, their function is not fully understood. Researchers are further investigating how they work.

Conditions causing dysfunction of the thymus gland can drastically reduce a person’s quality of life.

Here are some of the most common conditions:


Various disorders can enlarge the thymus and cause it to become hyperactive, or overly active.

These may include:

Symptoms of a hyperactive thymus include:

Small size or atrophy

Usually, the thymus is largest in babies and young children. However, some preterm newborns may have a small thymus. This can increase their risk of infections.

When people age, their thymus atrophies, or decreases in size. This raises their likelihood of developing:

  • cancer
  • viral and bacterial infections
  • autoimmune diseases

Thymus cancer

The thymus may develop two types of cancer: thymoma and thymic carcinoma. They are both rare forms of cancer, and they grow in the cells covering the thymus.

Thymoma is a slow-growing cancer that rarely spreads beyond the thymus.

Thymic carcinoma is a faster-growing cancer that is harder to treat.

Symptoms of these cancers may include:

  • persistent cough
  • shortness of breath
  • hoarse voice
  • chest pain
  • swelling in the neck, face, arms, or upper body

The thymus is an organ in the upper chest. It is largest in infants and young children, and it decreases in size after puberty.

It plays a critical role in immunity. Its primary function involves T cell maturation. These cells help the body fight diseases, cancers, allergens, and more.

The thymus also contains B cells. They help the thymus select the best T cells while disposing of potentially harmful ones.

Conditions affecting the thymus include hyperactivity from enlargement, cancer, and atrophy.