What causes Adrenal Insufficiency?

Primary Adrenal Insufficiency (Addison's Disease)

Surgical removal of both adrenal glands
Autoimmune Diseases (when the body's own immune system attacks itself)
Granulomatous Diseases (e.g. tuberculosis and histoplasmosis)
Metastatic malignancies (Spread of cancers e.g. lung or breast cancer to the adrenal gland)
Pharmacological steroid therapy
Haemorrhage (bleeding in the adrenal gland, often as a result of anticoagulant therapy)
Meningococcal septicaemia (Waterhouse-Friderichsen syndrome) - septicaemia (infection in the bloodstream) followed by adrenal
Rare hereditary diseases
Pneumocystis infection (as a complication of AIDS)

Hypothalamic and Pituitary lesion
Trauma
Surgery
Radiation
Infection
Tumours
Suppression of the adrenals after long-term steroid use (this is covered in more detail in the section on the pituitary)