The incidence of SIADH rises with increasing age. Residents of nursing homes are at highest risk.

Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is characterized by excessive unsuppressible release of antidiuretic hormone (ADH) either from the posterior pituitary gland, or an abnormal non-pituitary source. Unsuppressed ADH causes an unrelenting increase in solute-free water being returned by the tubules of the kidney to the venous circulation.

ADH is derived from a preprohormone precursor that is synthesized in cells in the hypothalamus and stored in vesicles in the posterior pituitary. "Appropriate" ADH secretion is regulated by osmoreceptors on the hypothalamic cells that synthesize and store ADH: plasma hypertonicity activates these receptors, ADH is released into the blood stream, the kidney increases solute-free water return to the circulation, and the hypertonicity is alleviated. "Inappropriate" ADH secretion causes a "unrelenting increase" in solute-free water ("free water") absorption by the kidneys, with two consequences. First, in the extracellular fluid (ECF) space, there is a dilution of blood solutes, causing hypoosmolality, including a low sodium concentration - hyponatremia. Then virtually simultaneously, in the intracellular space, cells swell, i.e. intracellular volume increases. Swelling of brain cells causes various neurological abnormalities which in severe or acute cases can result in convulsions, coma, and death.

The causes of SIADH are grouped into six categories: 1) central nervous system diseases that directly stimulate the hypothalamus, the site of control of ADH secretion; 2) various cancers that synthesize and secrete ectopic ADH; 3) various pulmonary diseases; 4) numerous (at least seventeen) drugs that chemically stimulate the hypothalamus; 5) inherited mutations that cause aquaporins always to be "turned on"; and 6) miscellaneous largely transient conditions.

Potential treatments of SIADH include restriction of fluid intake, correction of an identifiable reversible underlying cause, and/or medication which promotes solute-free water excretion by the kidney. The presence of cerebral edema may necessitate intravenous isotonic or hypertonic saline administration.

SIADH was originally described in 1957 in two people with small-cell carcinoma of the lung.

The condition is due to:

- Bilateral idiopathic (micronodular) adrenal hyperplasia (66%)

- Adrenal adenoma (Conn's syndrome) (33%)

- Primary (unilateral) adrenal hyperplasia—2% of cases

- Aldosterone-producing adrenocortical carcinoma—<1% of cases

- Familial Hyperaldosteronism (FH)

- Glucocorticoid-remediable aldosteronism (FH type I)—<1% of cases

- FH type II (APA or IHA)—<2% of cases

- Ectopic aldosterone-producing adenoma or carcinoma—< 0.1% of cases

There is volume expansion in the body, no edema, but hyponatremia occurs

- SIADH (and its many causes)

- Hypothyroidism

- Not enough ACTH

The hypovolemia (extracellular volume loss) is due to total body sodium loss. The hyponatremia is caused by a relatively smaller loss in total body water.

- any cause of hypovolemia such as prolonged vomiting, decreased oral intake, severe diarrhea

- diuretic use (due to the diuretic causing a volume depleted state and thence ADH release, and not a direct result of diuretic-induced urine sodium loss)

- Addison's disease and congenital adrenal hyperplasia in which the adrenal glands do not produce enough steroid hormones (combined glucocorticoid and mineralocorticoid deficiency)

- pancreatitis

Prolonged periods of exercise may be a cause, known as exercise-associated hyponatremia (EAH). It is common in marathon runners and participants of other endurance events. The use of MDMA can result in hyponatremia. This likely occurs as a result of fluid loss via sweating and replacement with water without electrolytes.

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.

Radiation exposure increases the risk of primary hyperparathyroidism. A number of genetic conditions including multiple endocrine neoplasia syndromes also increase the risk.

Autoimmune adrenalitis is the most common cause of Addison's disease in the industrialised world. Autoimmune destruction of the adrenal cortex is caused by an immune reaction against the enzyme 21-hydroxylase (a phenomenon first described in 1992). This may be isolated or in the context of autoimmune polyendocrine syndrome (APS type 1 or 2), in which other hormone-producing organs, such as the thyroid and pancreas, may also be affected.

Adrenal destruction is also a feature of adrenoleukodystrophy (ALD), and when the adrenal glands are involved in metastasis (seeding of cancer cells from elsewhere in the body, especially lung), hemorrhage (e.g. in Waterhouse-Friderichsen syndrome or antiphospholipid syndrome), particular infections (tuberculosis, histoplasmosis, coccidioidomycosis), or the deposition of abnormal protein in amyloidosis.

A calcimimetic (such as "cinacalcet") is a potential therapy for some people with severe hypercalcemia and primary hyperparathyroidism who are unable to undergo parathyroidectomy and for secondary hyperparathyroidism on dialysis.

In the treatment of secondary hyperparathyroidism due to chronic kidney disease on dialysis calcimimetics do not appear to affect the risk of early death. They do decrease the need for a parathyroidectomy but cause more issues with low blood calcium levels and vomiting.

Primary aldosteronism, also known as primary hyperaldosteronism or Conn's syndrome, is excess production of the hormone aldosterone by the adrenal glands resulting in low renin levels. Often it produces few symptoms. Most people have high blood pressure which may cause poor vision or headaches. Occasionally there may be muscular weakness, muscle spasms, tingling sensations, or excessive urination. Complications include cardiovascular disease such as stroke, myocardial infarction, kidney failure, and abnormal heart rhythms.

Primary hyperaldosteronism has a number of causes. About 66% of cases are due to enlargement of both adrenal glands and 33% of cases are due to an adrenal adenoma that produces aldosterone. Other uncommon causes include adrenal cancer and an inherited disorder called familial hyperaldosteronism. Some recommend screening people with high blood pressure who are at increased risk while others recommend screening all people with high blood pressure for the disease. Screening is usually done by measuring the aldosterone-to-renin ratio in the blood with further testing used to confirm positive results. While low blood potassium is classically described this is only present in about a quarter of people. To determine the underlying cause medical imaging is carried out.

Some cases may be cured by removing the adenoma by surgery. A single adrenal gland may also be removed in cases where only one is enlarged. In cases due to enlargement of both glands treatment is typically with medications known as aldosterone antagonists such as spironolactone or eplerenone. Other medications for high blood pressure and a low salt diet may also be needed. Some people with familial hyperaldosteronism may be treated with the steroid dexamethasone.

Primary aldosteronism is present in about 10% of people with high blood pressure. It occurs more often in women than men. Often it begins in those between 30 and 50 years of age. Conn's syndrome is named after Jerome W. Conn (1907–1994), the American endocrinologist who first described adenomas as a cause of the condition in 1955.

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.

Dipsogenic DI or primary polydipsia results from excessive intake of fluids as opposed to deficiency of arginine vasopressin. It may be due to a defect or damage to the thirst mechanism, located in the hypothalamus; or due to mental illness. Treatment with DDAVP may lead to water intoxication.

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).

Gestational DI occurs only during pregnancy and the postpartum period. During pregnancy, women produce vasopressinase in the placenta, which breaks down ADH. Gestational DI is thought to occur with excessive production and/or impaired clearance of vasopressinase.

Most cases of gestational DI can be treated with desmopressin (ddAVP), but not vasopressin. In rare cases, however, an abnormality in the thirst mechanism causes gestational DI, and desmopressin should not be used.

Diabetes insipidus is also associated with some serious diseases of pregnancy, including pre-eclampsia, HELLP syndrome and acute fatty liver of pregnancy. These cause DI by impairing hepatic clearance of circulating vasopressinase. It is important to consider these diseases if a woman presents with diabetes insipidus in pregnancy, because their treatments require delivery of the baby before the disease will improve. Failure to treat these diseases promptly can lead to maternal or perinatal mortality.

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.

Type B adipsia occurs when vasopressin responses are at decreased levels in the presence of osmotic stimuli. Although minimal, there is still some secretion of AVP. This type may be due to some elimination of osmoreceptors.

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.

Adipsia, also known as hypodipsia, is a symptom of inappropriately decreased or absent feelings of thirst. It involves an increased osmolality or concentration of solute in the urine, which stimulates secretion of antidiuretic hormone (ADH) from the hypothalamus to the kidneys. This causes the person to retain water and ultimately become unable to feel thirst. Due to its rarity, the disorder has not been the subject of many research studies.

Adipsia may be seen in conditions such as diabetes insipidus and may result in hypernatremia. It can occur as the result of abnormalities in the hypothalamus, pituitary and corpus callosum, as well as following pituitary/hypothalamic surgery.

It is possible for hypothalamic dysfunction, which may result in adipsia, to be present without physical lesions in the hypothalamus, although there are only four reported cases of this. There are also some cases of patients experiencing adipsia due to a psychiatric disease. In these rare psychogenic cases, the patients have normal levels of urine osmolality as well as typical ADH activity.

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.

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.

Other causes of acquired NDI include: low blood potassium, post-obstructive polyuria, sickle cell disease/trait, amyloidosis, Sjogren syndrome, renal cystic disease, Bartter syndrome, and various medications (Amphotericin B, Orlistat, Ifosfamide, Ofloxacin, Cidofovir, Vaptanes).

In addition to kidney and systemic disorders, nephrogenic DI can present itself as a side-effect to some medications. The most common and well known of these medications is lithium, although there are many other medications that cause this effect with lesser frequency.

Growth hormone-releasing hormone (GHRH) is another releasing factor secreted by the hypothalamus. GHRH stimulates the pituitary gland to secrete growth hormone (GH), which has various effects on body growth and sexual development. Insufficient GH production may cause poor somatic growth, precocious puberty or gonadotropin deficiency, failure to initiate or complete puberty, and is often associated with rapid weight gain, low T, and low levels of sex hormones.

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.

Neurogenic diabetes insipidus may occur due to low levels of ADH production from the hypothalamus. Insufficient levels of ADH result in increased thirst and urine output, and prolonged excessive urine excretion increases the risk of dehydration.