Iatrogenic Cushing's syndrome (caused by treatment with corticosteroids) is the most common form of Cushing's syndrome. Cushing's disease is rare; a Danish study found an incidence of less than one case per million people per year. However, asymptomatic microadenomas (less than 10 mm in size) of the pituitary are found in about one in six individuals.

People with Cushing's syndrome have increased morbidity and mortality as compared to the general population. The most common cause of mortality in Cushing's syndrome is cardiovascular events. People with Cushing's syndrome have nearly 4 times increased cardiovascular mortality as compared to the general population.

Outcomes are typically good when treated. Most can expect to live relatively normal lives. Someone with the disease should be observant of symptoms of an "Addison's crisis" while the body is strained, as in rigorous exercise or being sick, the latter often needing emergency treatment with intravenous injections to treat the crisis.

Individuals with Addison's disease have more than a doubled mortality rate. Furthermore, individuals with Addison's disease and diabetes mellitus have an almost 4 time increase in mortality compared to individuals with only diabetes.

The frequency rate of Addison's disease in the human population is sometimes estimated at roughly one in 100,000. Some put the number closer to 40–144 cases per million population (1/25,000–1/7,000). Addison's can affect persons of any age, sex, or ethnicity, but it typically presents in adults between 30 and 50 years of age. Research has shown no significant predispositions based on ethnicity.

The most common cause of Cushing's syndrome is the taking of glucocorticoids prescribed by a health care practitioner to treat other diseases (called iatrogenic Cushing's syndrome). This can be an effect of corticosteroid treatment of a variety of disorders such as asthma and rheumatoid arthritis, or in immunosuppression after an organ transplant. Administration of synthetic ACTH is also possible, but ACTH is less often prescribed due to cost and lesser utility. Although rare, Cushing's syndrome can also be due to the use of medroxyprogesterone acetate. In this form of Cushing's, the adrenal glands atrophy due to lack of stimulation by ACTH, since glucocorticoids downregulate production of ACTH. Cushing's syndrome in childhood usually results from use of glucocorticoid medication.

Endogenous Cushing's syndrome results from some derangement of the body's own system of secreting cortisol. Normally, ACTH is released from the pituitary gland when necessary to stimulate the release of cortisol from the adrenal glands.

- In pituitary Cushing's, a benign pituitary adenoma secretes ACTH. This is also known as Cushing's disease and is responsible for 70% of endogenous Cushing's syndrome.

- In adrenal Cushing's, excess cortisol is produced by adrenal gland tumors, hyperplastic adrenal glands, or adrenal glands with nodular adrenal hyperplasia.

- Tumors outside the normal pituitary-adrenal system can produce ACTH (occasionally with CRH) that affects the adrenal glands. This etiology is called ectopic or paraneoplastic Cushing's disease and is seen in diseases such as small cell lung cancer.

- Finally, rare cases of CRH-secreting tumors (without ACTH secretion) have been reported, which stimulates pituitary ACTH production.

Several studies have shown that hypopituitarism is associated with an increased risk of cardiovascular disease and some also an increased risk of death of about 50% to 150% the normal population. It has been difficult to establish which hormone deficiency is responsible for this risk, as almost all patients studied had growth hormone deficiency. The studies also do not answer the question as to whether the hypopituitarism itself causes the increased mortality, or whether some of the risk is to be attributed to the treatments, some of which (such as sex hormone supplementation) have a recognized adverse effect on cardiovascular risk.

The largest study to date followed over a thousand people for eight years; it showed an 87% increased risk of death compared to the normal population. Predictors of higher risk were: female sex, absence of treatment for sex hormone deficiency, younger age at the time of diagnosis, and a diagnosis of craniopharyngioma. Apart from cardiovascular disease, this study also showed an increased risk of death from lung disease.

Quality of life may be significantly reduced, even in those people on optimum medical therapy. Many report both physical and psychological problems. It is likely that the commonly used replacement therapies do not completely mimic the natural hormone levels in the body. Health costs remain about double those of the normal population.

Hypopituitarism is usually permanent. It requires lifelong treatment with one or more medicines.

Cases of Cushing's disease are rare, and little epidemiological data is available on the disease. An 18-year study conducted on the population of Vizcaya, Spain reported a 0.004% prevalence of Cushing's disease. The average incidence of newly diagnosed cases was 2.4 cases per million inhabitants per year. The disease is often diagnosed 3–6 years after the onset of illness.

Several studies have shown that Cushing's disease is more prevalent in women than men at a ratio of 3-6:1, respectively. Moreover, most women affected were between the ages of 50 and 60 years.

The prevalence of hypertension, and abnormalities in glucose metabolism are major predictors of mortality and morbidity in untreated cases of the disease. The mortality rate of Cushing's disease was reported to be 10-11%, with the majority of deaths due to vascular disease Women aged 45–70 years have a significantly higher mortality rate than men.

Moreover, the disease shows a progressive increase with time. Reasons for the trend are unknown, but better diagnostic tools, and a higher incidence rate are two possible explanations.

All causes in this category are genetic, and generally very rare. These include mutations to the "SF1" transcription factor, congenital adrenal hypoplasia due to "DAX-1" gene mutations and mutations to the ACTH receptor gene (or related genes, such as in the Triple A or Allgrove syndrome). "DAX-1" mutations may cluster in a syndrome with glycerol kinase deficiency with a number of other symptoms when "DAX-1" is deleted together with a number of other genes.

There is only one study that has measured the prevalence (total number of cases in a population) and incidence (annual number of new cases) of hypopituitarism. This study was conducted in Northern Spain and used hospital records in a well-defined population. The study showed that 45.5 people out of 100,000 had been diagnosed with hypopituitarism, with 4.2 new cases per year. 61% were due to tumors of the pituitary gland, 9% due to other types of lesions, and 19% due to other causes; in 11% no cause could be identified.

Recent studies have shown that people with a previous traumatic brain injury, spontaneous subarachnoid hemorrhage (a type of stroke) or radiation therapy involving the head have a higher risk of hypopituitarism. After traumatic brain injury, as much as a quarter have persistent pituitary hormone deficiencies. Many of these people may have subtle or non-specific symptoms that are not linked to pituitary problems but attributed to their previous condition. It is therefore possible that many cases of hypopituitarism remain undiagnosed, and that the annual incidence would rise to 31 per 100,000 annually if people from these risk groups were to be tested.

Causes of acute adrenal insufficiency are mainly sudden withdrawal of long-term corticosteroid therapy, Waterhouse-Friderichsen syndrome, and stress in people with underlying chronic adrenal insufficiency. The latter is termed critical illness–related corticosteroid insufficiency.

For chronic adrenal insufficiency, the major contributors are autoimmune adrenalitis (Addison's Disease), tuberculosis, AIDS, and metastatic disease. Minor causes of chronic adrenal insufficiency are systemic amyloidosis, fungal infections, hemochromatosis, and sarcoidosis.

Autoimmune adrenalitis may be part of Type 2 autoimmune polyglandular syndrome, which can include type 1 diabetes, hyperthyroidism, and autoimmune thyroid disease (also known as autoimmune thyroiditis, Hashimoto's thyroiditis, and Hashimoto's disease). Hypogonadism may also present with this syndrome. Other diseases that are more common in people with autoimmune adrenalitis include premature ovarian failure, celiac disease, and autoimmune gastritis with pernicious anemia.

Adrenoleukodystrophy can also cause adrenal insufficiency.

Adrenal insufficiency can also result when a patient has a craniopharyngioma, which is a histologically benign tumor that can damage the pituitary gland and so cause the adrenal glands not to function. This would be an example of secondary adrenal insufficiency syndrome.

Causes of adrenal insufficiency can be categorized by the mechanism through which they cause the adrenal glands to produce insufficient cortisol. These are adrenal dysgenesis (the gland has not formed adequately during development), impaired steroidogenesis (the gland is present but is biochemically unable to produce cortisol) or adrenal destruction (disease processes leading to glandular damage).

In a study of 1,034 symptomatic adults, Sheehan syndrome was found to be the sixth most frequent etiology of growth hormone deficiency, being responsible for 3.1% of cases (versus 53.9% due to a pituitary tumor).

Drug induced (iatrogenic) hypoadrenocorticism is caused during abrupt cessation of a steroid medication. During steroid treatment, the adrenal glands do not function fully. The body senses the levels of the exogenous steroids in the system and therefore does not signal for additional production. The usual protocol for stopping steroid medications is not to eliminate them suddenly, but to withdraw from them gradually in a "tapering off" process, which allows the production to adjust to normal. If steroids are abruptly withdrawn, the dormant adrenal glands may not able to reactivate, and the body will need to have its adrenal glucocorticoid hormones replaced by medication.

The cause of hyperpituitarism in most cases is due to pituitary adenomas. They usually come from the anterior lobe, are functional and secrete the hormone, GH and prolactin.

Pituitary ACTH hypersecretion (or Cushing disease) is a form of hyperpituitarism characterized by an abnormally high level of ACTH produced by the anterior pituitary. It is one of the causes of Cushing's syndrome. (However, Cushing's syndrome can be caused by many other causes, including exogenous administration.)

In secondary hypoadrenocorticism (also known as atypical hypoadrenocorticism) the problem is not in the adrenal gland but in the pituitary gland. Usually, the anterior portion of the pituitary gland produces a hormone, adrenocorticotropic hormone (ACTH), that signals the zona fasciculata and zona reticularis to produce their steroids. When the pituitary is unable to produce ACTH, these zones stop production of their hormones. The zona glomerulosa is not controlled by ACTH, and remains able to produce a normal amount of mineralocorticoids. A dog with secondary hypoadrenocorticism is not at risk of an Addisonian crisis, and only needs to have medication to replace the glucocorticoid steroid cortisol. One dog in every 42 diagnosed with hypoadrenocorticism has the secondary form of the disease where mineralocorticoid production remains intact.

Secondary adrenocortical insufficiency involves only a deficiency of glucocorticoid secretion. Destructive lesions (e.g. neoplasia, inflammation) in the pituitary gland or hypothalamus and chronic administration of exogenous glucocorticoids or megestrol acetate (cats) are the most common causes.

In some dogs with secondary hypoadrenocorticism, the disease progresses to primary hypoadrenocorticism, and mineralocorticoid replacement becomes necessary, while others retain their ability to continue production of mineralocorticoids for years, requiring glucocorticoid replacement only.

Evidence indicates that the mechanism of hyperpituitarism can originate from genetic disruption causing pituitary tumorigenesis, most pituitary adenomas are monoclonal, which in turn indicates their origin from an event in a single cell. There are three hormones that are oversecreted resulting in the pituitary adenoma: prolactin, adrenocorticotropic hormone (ACTH), and growth hormone (GH). Excess prolactin may result in a prolactinoma Excess GH results in gigantism, the severity of gigantism depends on whether the epiphyseal plate is open. The four most common types of hyperpituitarism are prolactinoma, corticotropinoma (Cushing's disease), somatotropinoma (gigantism), and thyrotropinoma .

Cushing's disease is a cause of Cushing's syndrome characterised by increased secretion of adrenocorticotropic hormone (ACTH) from the anterior pituitary (secondary hypercortisolism). This is most often as a result of a pituitary adenoma (specifically pituitary basophilism) or due to excess production of hypothalamus CRH (corticotropin releasing hormone) (tertiary hypercortisolism/hypercorticism) that stimulates the synthesis of cortisol by the adrenal glands. Pituitary adenomas are responsible for 80% of endogenous Cushing's syndrome, when excluding Cushing's syndrome from exogenously administered corticosteroids.

This should not be confused with ectopic Cushing syndrome or exogenous steroid use.

Common causes include bilateral adrenalectomy for the treatment of Cushing's disease, and hypopituitarism. The onset of the disease can occur up to 24 years after a bilateral adrenalectomy has been performed, with an average of up to 15 years after. A preventative measure that can be utilized is prophylactic radiotherapy when a bilateral adrenalectomy is being performed in order to prevent Nelson's syndrome from manifesting. Screening can also be done with the help or an MRI in order to visualize the pituitary for tumors. If tumors are not present then an MRI should be performed at intervals. Hyper-pigmentation and fasting ACTH levels within plasma above 154 pmol/l are predictive of Nelson's syndrome after an adrenalectomy. Risk factors include being younger in age and pregnancy.

PPNAD is a rare cause of high cortisol levels in the blood and often manifests as ACTH-independent Cushing's syndrome. The effects of PPNAD can often be cyclical so the symptoms of Cushing's syndrome will not always be as severe, which may complicate diagnosis. The classic symptoms of Cushing's syndrome include rapid central weight gain, a puffy red face and a buffalo hump at the back of the neck due to fat deposits. Skin changes in Cushing's syndrome include thinning and bruising easily, developing striae and hyperpigmentation at skin folds. The hormonal changes can lead to hirsuitism, males developing breast tissue, females no longer having periods and both sexes may become infertile. High cortisol levels can lead to psychological disturbances such as anxiety or depression and insomnia. Bone health can deteriorate, leading to an increased fracture risk in people with Cushing's syndrome. PPNAD is unique as it often causes Cushing's at a young age, in children and adolescents. In addition to the other symptoms of Cushing's syndrome, the patient may have a short stature due to interrupted growth because of ACTH suppression.

In 90% of people with PPNAD it is associated with Carney Complex. Carney Complex is usually inherited, however it can also occur sporadically. A visible sign of Carney complex is abnormal skin hyperpigmentation. There may also be myxomas which can appear as lumps in the skin and breast as well as often being present in the heart, which can lead to multiple cardiovascular problems. The majority of people with PPNAD will have some of these signs/symptoms due to the strong association between PPNAD and Carney Complex.

PPNAD, the endocrine manifestation that comes from Carney Complex (CNC), can be syndromic or isolated. The main cause of isolated PPNAD is a mutation of PRKAR1α, located at 17q22-24, which is the gene encoding the regulatory R1α subunit of protein kinase A. Germline heterozygous PRKAR1α inactivation mutations are present in 80% of CNC patients affected by Cushing's syndrome. There are over 117 mutations of the PRKAR1α gene that can cause CNC, with many of these mutations producing premature stop codons, thus resulting in the complete loss of PRKAR1α protein. CNC patients have also been discovered with an unusually shortened PRKAR1α protein, detected in tumours and leukocytes, following a splice-site mutation, which causes exon-6 skipping. Therefore, both haploinsufficiency and the complete loss of PRKAR1α can lead to the increased PKA activity observed in PPNAD patients, due to the disruption of the cAMP signalling pathway.

Sahut-Barnola et al. used a mouse model to cre-lox knockout the Prkar1a gene specifically from cells of the adrenal cortex and observed that the mice subsequently developed Cushing syndrome that is independent of the pituitary. They also observed that the mutation caused increased PKA activity.

The R1α loss caused the adult adrenal gland became hyperactive and hyperplastic on both sides, as seemingly the foetal adrenal cells within it were not maintained and thus expanded. This established tumoral growths. This mouse KO model phenocopies what happens in human cases of PPNAD.

Inactivation of PDE11A4, located at 2q31-5, has also been identified in PPNAD patients without PRKAR1α mutations. PDE11A4 is the gene encoding phosphodiesterase 11A4, another participant of the cAMP signalling pathway.

Infertility observed in adult males with congenital adrenal hyperplasia (CAH) has been associated with testicular adrenal rest tumors (TART) that may originate during childhood. TART in prepubertal males with classic CAH could be found during childhood (20%). Martinez-Aguayo et al. reported differences in markers of gonadal function in a subgroup of patients, especially in those with inadequate control.

Most XY children are so undervirilized that they are raised as girls. The testes are uniformly nonfunctional and undescended; they are removed when the diagnosis is made due to the risk of cancer development in these tissues.

Adrenocorticotropic hormone deficiency (ACTH deficiency) is a result of a decreased or absent production of adrenocorticotropic hormone (ACTH) by the pituitary gland.

It can be associated with "TBX19".

80% of patients with pituitary antibodies also have antibodies to thyroid gland or its hormones. Likewise, 20% of autoimmune thyroid patients also have pituitary antibodies. It follows that a subset of thyroid patients may have a disease related to autoimmune hypophysitis. Recent research has focused on a defect at the CTLA-4 gene which, coupled with other factors, may result in autoimmunity primarily focusing on certain endocrine glands including the pituitary and thyroid.

Glucocorticoid deficiency 1 (FGD or GCCD) is an adrenocortical failure characterized by low levels of plasma cortisol produced by the adrenal gland despite high levels of plasma ACTH. This is an inherited disorder with several different causes which define the type.

FGD type 1 (FGD1 or GCCD1) is caused by mutations in the ACTH receptor (melanocortin 2 receptor; MC2R). FGD type 2 is caused by mutations in the MC2R accessory protein (MRAP). These two types account for 45% of all cases of FGD.

Some cases of FGD type 3 are caused by mutations in the steroidogenic acute regulatory protein (StAR), with similarity to the nonclassic form of lipoid congenital adrenal hyperplasia. In this case, a general impairment in not just adrenal steroid production, but gonadal steroid production can affect sexual development and fertility.

The causes of other cases of FGD type 3 not due to StAR are currently unknown.

Common treatments for Nelson's syndrome include radiation or surgical procedure. Radiation allows for the limitation of the growth of the pituitary gland and the adenomas. If the adenomas start to affect the surrounding structures of the brain, then a micro-surgical technique can be adapted in order to remove the adenomas in a transsphenoidal (bone at base of the skull) process. Death may result with development of a locally aggressive pituitary tumor. However, does not commonly occur with pituitary diseases. In the rare case, ACTH-secreting tumors can become malignant. Morbidity from the disease can occur due to pituitary tissue compression or replacement, and compression of structures that surround the pituitary fossa. The tumor can also compress the optic apparatus, disturb cerebrospinal fluid flow, meningitis, and testicular enlargement in rare cases.