Primary hypogonadism: Usually results from damage directly to the testes through a congenital defect or trauma. (Age-related hormone decline may be considered primary hypogonadism if testosterone falls to pathologic levels and the patient becomes symptomatic.)

Causes include:

• Aging
• Klinefelter's syndrome (a chromosomal abnormality)
• Undescended testes (cryptorchidism)
• Trauma to the testes can be physical
• Iinfectious e.g.Postpubertal mumps orchitis
• Chemicals that can be toxic to the testes include chemotherapeutic agents (although these are more damaging to sperm production than to testosterone production), the antifungal drug ketoconazole, and long-term glucocorticoids] Use of spironolactone, cimetidine, phenytoin, and/or flutamide has been associated with hypogonadism.
• Radiation therapy can be damaging to the testes

Secondary (hypogonadotropic) hypogonadism: Caused by a disturbance in hypothalamic-pituitary function, possibly the result of a disease or defect of the anterior pituitary.Secondary hypogonadism may also be attributed to:

• chronic illness
• malnutrition
• anorexia or even more commonly to obesity- increase in body weight as we age due to inactivity negatively impacts our body fat percentage, this increase in adipocyte tissue also elevates the amount of estrogen synthase resulting also in elevated estrogen levels. Now it is these elevated levels of estrogens, which cause the liver to produce an increase in Sex hormone binding globulin (SHBG). It is this elevation in SHBG, which inactivates free or bioavailable testosterone. It is these factors that contribute to
  the downward spiral of free testosterone with aging. This is known as "Metabolic Hypogonadism", this condition can usually be reversed by weight loss.