Hypogonadism or testosterone deficiency is a condition in which the body does not produce sufficient levels of testosterone as a result of an underlying medical condition or other causes. It is likely that testosterone deficiency is underdiagnosed and is often mistaken for other medical conditions due to the fact that its signs and symptoms resemble other diseases, especially psychiatric disorders.
Testosterone deficiency is classified according to the location of its cause: 
- Primary: This type of testosterone deficiency is also known as primary testicular failure. The causes of the deficiency originate from a testicular problem.
- Secondary: In this type of testosterone deficiency, the testicles are normal but its functions are altered due to a problem with the pituitary gland or hypothalamus (brain region that regulates temperature).
Hypogonadism may be present at birth (congenital) or may develop later in life (acquired). Congenital causes of hypogonadism include the following:
- Klinefelter’s Syndrome: This condition results from a congenital abnormality of the sex chromosomes, X and Y. Normally, a male only has one X and one Y chromosome.  In this condition, two or more X chromosomes are present in addition to one Y chromosome. This causes the testicles to develop abnormally, which in turn affects the production of testosterone.
- Undescended testicles (Cryptorchidism): Normally, the testicles move down the scrotum after birth.  Sometimes, one or both of the testicles may not descend and remain inside the abdomen. If not corrected in early childhood, it may affect the function of the testicles and reduce the rate at which testosterone is produced.
- Kallmann syndrome: This condition is referred to as an abnormal development of the hypothalamus.  This abnormality can also affect the ability to smell (anosmia) and can lead to red-green color blindness.
- Hemochromatosis: This condition occurs when there is too much iron in the blood.  As a result, the testicles or pituitary gland malfunctions, affecting the production of testosterone.
Acquired causes of testosterone deficiency include the following:
- Injury or trauma to the testicles: This can impair blood flow going to the testicles and affect the production of testosterone.
- Cancer treatment: Chemotherapy or radiation therapy can interfere with the production of sperm and testosterone.  Although the effects are temporary, there is still a chance that permanent infertility may occur.
- Pituitary disorders: Any abnormality in the pituitary gland such as tumors can hinder the release of hormones from the pituitary gland to the testicles, thus affecting the production of testosterone.
- Inflammatory disease: Tuberculosis, sarcoidosis and histiocytosis can affect the hypothalamus and pituitary gland, which in turn impair the production of testosterone. 
- Autoimmune disorders: HIV/AIDS (Acquired immunodeficiency syndrome) can impair testosterone production by affecting the hypothalamus, the pituitary gland and the testes. 
- Medications: Long-term use of medications for high cholesterol such as statins can lower testosterone levels as a side effect. 
- Stress: Chronic stress raises the levels of the stress hormone called cortisol in response to the situation. This in turn suppresses the central hormone pathways which includes testosterone.