Endocrine diseases are disorders of the endocrine system. The branch of medicine associated with endocrine disorders is known as endocrinology.

Broadly speaking, endocrine disorders may be subdivided into three groups:

1. Endocrine gland hyposecretion (leading to hormone deficiency)

2. Endocrine gland hypersecretion (leading to hormone excess)

3. Tumours (benign or malignant) of endocrine glands

Endocrine disorders are often quite complex, involving a mixed picture of hyposecretion and hypersecretion because of the feedback mechanisms involved in the endocrine system. For example, most forms of hyperthyroidism are associated with an excess of thyroid hormone and a low level of thyroid stimulating hormone.

The various signs and symptoms in Sheehan's syndrome are caused by damage to the pituitary, which causes a decrease in one or more hormones it normally secretes (see Pathophysiology section). Since the pituitary controls many glands in the endocrine system, partial or complete loss of a variety of functions may result.

Most common initial symptoms of Sheehan's syndrome are agalactorrhea (absence of lactation) and/or difficulties with lactation. Many women also report amenorrhea or oligomenorrhea after delivery. In some cases, a woman with Sheehan syndrome might be relatively asymptomatic, and the diagnosis is not made until years later, with features of hypopituitarism. Such features include secondary hypothyroidism with tiredness, intolerance to cold, constipation, weight gain, hair loss and slowed thinking, as well as a slowed heart rate and low blood pressure. Another such feature is secondary adrenal insufficiency, which, in the rather chronic case is similar to Addison's disease with symptoms including fatigue, weight loss, hypoglycemia (low blood sugar levels), anemia and hyponatremia (low sodium levels). Such a woman may, however, become acutely exacerbated when her body is stressed by, for example, a severe infection or surgery years after her delivery, a condition equivalent with an Addisonian crisis. The symptoms of adrenal crisis should be treated immediately and can be life-threatening. Gonadotropin deficiency will often cause amenorrhea, oligomenorrhea, hot flashes, or decreased libido. Growth hormone deficiency causes many vague symptoms including fatigue and decreased muscle mass.

Uncommonly, Sheehan syndrome may also appear acutely after delivery, mainly by hyponatremia. There are several possible mechanisms by which hypopituitarism can result in hyponatremia, including decreased free-water clearance by hypothyroidism, direct syndrome of inappropriate antidiuretic hormone (ADH) hypersecretion, decreased free-water clearance by glucocorticoid deficiency (independent of ADH). The potassium level in these situations is normal, because adrenal production of aldosterone is not dependent on the pituitary.

Hypothalamic disease is a disorder presenting primarily in the hypothalamus, which may be caused by damage resulting from malnutrition, including anorexia and bulimia eating disorders, genetic disorders, radiation, surgery, head trauma, lesion, tumour or other physical injury to the hypothalamus. The hypothalamus is the control center for several endocrine functions. Endocrine systems controlled by the hypothalamus are regulated by anti-diuretic hormone (ADH), corticotropin-releasing hormone, gonadotropin-releasing hormone, growth hormone-releasing hormone, oxytocin, all of which are secreted by the hypothalamus. Damage to the hypothalamus may impact any of these hormones and the related endocrine systems. Many of these hypothalamic hormones act on the pituitary gland. Hypothalamic disease therefore affects the functioning of the pituitary and the target organs controlled by the pituitary, including the adrenal glands, ovaries and testes, and the thyroid gland.

Numerous dysfunctions manifest as a result of hypothalamic disease. Damage to the hypothalamus may cause disruptions in body temperature regulation, growth, weight, sodium and water balance, milk production, emotions, and sleep cycles. Hypopituitarism, neurogenic diabetes insipidus, tertiary hypothyroidism, and developmental disorders are examples of precipitating conditions caused by hypothalamic disease.

Sheehan's syndrome, also known as postpartum pituitary gland necrosis, is hypopituitarism (decreased functioning of the pituitary gland), caused by ischemic necrosis due to blood loss and hypovolemic shock during and after childbirth.

The hypothalamus and pituitary gland are tightly integrated. Damage to the hypothalamus will impact the responsiveness and normal functioning of the pituitary. Hypothalamic disease may cause insufficient or inhibited signalling to the pituitary leading to deficiencies of one or more of the following hormones: thyroid-stimulating hormone, adrenocorticotropic hormone, beta-endorphin, luteinizing hormone, follicle-stimulating hormone, and melanocyte–stimulating hormones. Treatment for hypopituitarism involves hormone replacement therapy.

The most common clinical manifestations are related to mental status and gastrointestinal function; they include lethargy, anorexia, vomiting, weight loss, and weakness. Additional findings may include dehydration, bradycardia, weak femoral pulses, and abdominal pain. Polyuria and polydipsia, diarrhea, and shivering are occasionally reported.

Symptoms of hypoadrenocorticism can include vomiting, diarrhea, lethargy, lack of appetite, tremors or shaking, muscle weakness, low body temperature, collapse, low heart rate, and pain in the hind quarters. Hypoglycemia can also be present, and initially may be confused with seizure disorders, insulin-secreting pancreatic tumor (insulinoma), food poisoning, parvovirus enteritis, gastric volvulus, spinal or joint problems, earning hypoadrenocorticism the nicknames of "the Great Mimic" and "the Great Imitator". It is possible not to see any signs of the disease until 90% of the adrenal cortex is no longer functioning.

If hyponatremia (low sodium) and hyperkalemia (high potassium) are severe, the resulting hypovolemia, prerenal azotemia, and cardiac arrhythmias may result in an Addisonian crisis. In severe cases, the patient may be presented in shock and moribund. Addisonian crisis must be differentiated from other life-threatening disorders such as diabetic ketoacidosis, necrotizing pancreatitis, and septic peritonitis.

Various psychiatric manifestations have been associated with pituitary disorders including pituitary adenomas. Psychiatric symptoms such as depression, anxiety apathy, emotional instability, easy irritability and hostility have been noted.

Hormone secreting pituitary adenomas cause one of several forms of hyperpituitarism. The specifics depend on the type of hormone. Some tumors secrete more than one hormone, the most common combination being GH and prolactin, which present as unexpected bone growth and unexpected lactation (in both men and women).

A patient with pituitary adenoma may present with visual field defects, classically bitemporal hemianopsia. It arises from the compression of the optic nerve by the tumor. The specific area of the visual pathway at which compression by these tumours occurs is at the optic chiasma.

The anatomy of this structure causes pressure on it to produce a defect in the temporal visual field on both sides, a condition called bitemporal hemianopsia. If originating superior to the optic chiasm, more commonly in a craniopharyngioma of the pituitary stalk, the visual field defect will first appear as bitemporal inferior quadrantanopia, if originating inferior to the optic chiasm the visual field defect will first appear as bitemporal superior quadrantanopia. Lateral expansion of a pituitary adenoma can also compress the abducens nerve, causing a lateral rectus palsy.

Also, a pituitary adenoma can cause symptoms of increased intracranial pressure.

Prolactinomas often start to give symptoms especially during pregnancy, when the hormone progesterone increases the tumor's growth rate.

Various types of headaches are common in patients with pituitary adenomas. The adenoma may be the prime causative factor behind the headache or may serve to exacerbate a headache caused by other factors. Amongst the types of headaches experienced are both chronic and episodic migraine, and more uncommonly various unilateral headaches; primary stabbing headache, short-lasting unilateral neuralgiform headache attacks with conjunctival injection and tearing (SUNCT) - another type of stabbing headache characterized by short stabs of pain -, cluster headache, and hemicrania continua (HS).

Non-secreting adenomas can go undetected for an extended time because no obvious abnormalities are seen; the gradual reduction in normal activities due to decreased production of hormones is rather less evident. For example, insufficient adrenocorticotropic hormone means that the adrenal glands will not produce sufficient cortisol, resulting in slow recovery from illness, inflammation and chronic fatigue; insufficient growth hormone in children and adolescents leads to diminished stature but which can have many other explanations.

The signs and symptoms of primary hyperparathyroidism are those of hypercalcemia. They are classically summarized by "stones, bones, abdominal groans, thrones and psychiatric overtones".

- "Stones" refers to kidney stones, nephrocalcinosis, and diabetes insipidus (polyuria and polydipsia). These can ultimately lead to renal failure.

- "Bones" refers to bone-related complications. The classic bone disease in hyperparathyroidism is osteitis fibrosa cystica, which results in pain and sometimes pathological fractures. Other bone diseases associated with hyperparathyroidism are osteoporosis, osteomalacia, and arthritis.

- "Abdominal groans" refers to gastrointestinal symptoms of constipation, indigestion, nausea and vomiting. Hypercalcemia can lead to peptic ulcers and acute pancreatitis. The peptic ulcers can be an effect of increased gastric acid secretion by hypercalcemia.

- "Thrones" refers to polyuria and constipation

- "Psychiatric overtones" refers to effects on the central nervous system. Symptoms include lethargy, fatigue, depression, memory loss, psychosis, ataxia, delirium, and coma.

Left ventricular hypertrophy may also be seen.

Other signs include proximal muscle weakness, itching, and band keratopathy of the eyes.

When subjected to formal research, symptoms of depression, pain, and gastric dysfunction seem to correlate with mild cases of hypercalcemia.

In contrast with primary hyperparathyroidism in adults, primary hyperparathyroidism in pediatric patients is considered a rare endocrinopathy. Pediatric primary hyperparathyroidism can be distinguished by its more severe manifestations, in contrast to the less intense manifestations in adult primary hyperparathyroidism. Multiple endocrine neoplasia is more likely to be associated with childhood and adolescent primary hyperparathyroidism. The fundamental skeletal radiologic manifestation include diffuse osteopenia, pathologic fractures and the coexistence of resorption and sclerosis at numerous sites. Skeletal lesions can be specifically bilateral, symmetric and multifocal, exhibiting different types of bone resorption. Pathologic fractures of the femoral neck and spine can potentially initiate serious complications. Because pediatric primary hyperparathyroidism is frequently associated with pathologic fractures it can be misdiagnosed as osteogenesis imperfecta. Pediatric patients with primary hyperparathyroidism are best remedied by parathyroidectomy. Early diagnosis of pediatric primary hyperparathyroidism is all-important to minimize disease complications and start off timely and relevant treatment.

Euthyroid sick syndrome (ESS), sick euthyroid syndrome (SES), thyroid allostasis in critical illness, tumours, uremia and starvation (TACITUS), non-thyroidal illness syndrome (NTIS) or low T low T syndrome is a state of adaptation or dysregulation of thyrotropic feedback control where the levels of T3 and/or T4 are at unusual levels, but the thyroid gland does not appear to be dysfunctional.

This condition is often seen in starvation, critical illness or patients in intensive care unit. Similar endocrine phenotypes are observed in fetal life and in hibernating mammals The most common hormone pattern in sick euthyroid syndrome is a low total and unbound T3 levels with normal T4 and TSH levels.

Endocrine syndromes associated with acanthosis nigricans can develop in many conditions, particularly:

- starts with insulin resistance, such as diabetes mellitus and metabolic syndrome

- excess circulating androgens, particularly Cushing's disease, acromegaly, polycystic ovarian disease

- Addison's disease and hypothyroidism

- Rare diseases, including pinealoma, leprechaunism, lipoatrophic diabetes, pineal hyperplasia syndrome, pituitary basophilism, ovarian hyperthecosis, stromal luteoma, ovarian dermoid cysts, Prader-Willi syndrome, and Alstrom syndrome.

Acanthosis nigricans associated with endocrine dysfunction is more insidious in its onset, is less widespread, and the patients are often concurrently obese.

Pickardt syndrome (also Pickardt's syndrome or Pickardt–Fahlbusch syndrome) denotes a rare form of tertiary hypothyroidism that is caused by interruption of the portal veins connecting hypothalamus and pituitary.

It was characterized in 1972 and 1973.

It typically occurs in individuals younger than age 40, may be genetically inherited, and is associated with obesity or endocrinopathies, such as hypothyroidism, acromegaly, polycystic ovary disease, insulin-resistant diabetes, or Cushing's disease

Pickardt's syndrome may cause difficulties in differential diagnosis of pituitary adenomas, as both suprasellar hormone-inactive adenomas and prolactinomas may be associated with increased prolactin levels, central hypgogonadism and central hypothyroidism. Usually, the prolactin levels are higher in case of a true prolactinoma, but the concentration ranges overlap.

Causes of euthyroid sick syndrome include a number of acute and chronic conditions, including pneumonia, fasting, starvation, anorexia nervosa, sepsis, trauma, cardiopulmonary bypass, malignancy, stress, heart failure, hypothermia, myocardial infarction, chronic renal failure, cirrhosis, and diabetic ketoacidosis.

Euthyroid sick syndrome (non-thyroidal illness syndrome) has been assumed closely related with a series of diseases, (such as inflammatory bowel disease).

Multiple endocrine neoplasia or MEN is part of a group of disorders that affect the body's network of hormone-producing glands (the endocrine system). Hormones are chemical messengers that travel through the bloodstream and regulate the function of cells and tissues throughout the body. Multiple endocrine neoplasia involves tumors in at least two endocrine glands; tumors can also develop in other organs and tissues. These growths can be noncancerous (benign) or cancerous (malignant). If the tumors become cancerous, some cases can be life-threatening.

The two major forms of multiple endocrine neoplasia are called type 1 and type 2. These two types are often confused because of their similar names. However, type 1 and type 2 are distinguished by the genes involved, the types of hormones made, and the characteristic signs and symptoms.

These disorders greatly increase the risk of developing multiple cancerous and noncancerous tumors in glands such as the parathyroid, pituitary, and pancreas. Multiple endocrine neoplasia occurs when tumors are found in at least two of the three main endocrine glands (parathyroid, pituitary, and pancreatico-duodenum). Tumors can also develop in organs and tissues other than endocrine glands. If the tumors become cancerous, some cases can be life-threatening. The disorder affects 1 in 30,000 people.

Although many different types of hormone-producing tumors are associated with multiple endocrine neoplasia, tumors of the parathyroid gland, pituitary gland, and pancreas are most frequent in multiple endocrine neoplasia type 1. MEN1-associated overactivity of these three endocrine organs are briefly described here:

- Overactivity of the parathyroid gland (hyperparathyroidism) is the most common sign of this disorder. Hyperparathyroidism disrupts the normal balance of calcium in the blood, which can lead to kidney stones, thinning of the bones (osteoporosis), high blood pressure (hypertension), loss of appetite, nausea, weakness, fatigue, and depression.

- Neoplasia in the pituitary gland can manifest as prolactinomas whereby too much prolactin is secreted, suppressing the release of gonadotropins, causing a decrease in sex hormones such as testosterone. Pituitary tumor in MEN1 can be large and cause signs by compressing adjacent tissues.

- Pancreatic tumors associated with MEN-1 usually form in the beta cells of the islets of Langerhans, causing over-secretion of insulin, resulting in low blood glucose levels (hypoglycemia). However, many other tumors of the pancreatic Islets of Langerhans can occur in MEN-1. One of these, involving the alpha cells, causes over-secretion of glucagon, resulting in a classic triad of high blood glucose levels (hyperglycemia), a rash called necrolytic migratory erythema, and weight loss. Another is a tumor of the non-beta islet cells, known as a gastrinoma, which causes the over-secretion of the hormone gastrin, resulting in the over-production of acid by the acid-producing cells of the stomach (parietal cells) and a constellation of sequelae known as Zollinger-Ellison syndrome. Zollinger-Ellison syndrome may include severe gastric ulcers, abdominal pain, loss of appetite, chronic diarrhea, malnutrition, and subsequent weight loss. Other non-beta islet cell tumors associated with MEN1 are discussed below.

Pituitary tumors occur in 15 to 42% of MEN 1 patients. From 25 to 90% are prolactinomas. About 25% of pituitary tumors secrete growth hormone or growth hormone and prolactin. Excess prolactin may cause galactorrhea (see Pituitary Disorders: Galactorrhea), and excess growth hormone causes acromegaly clinically indistinguishable from sporadically occurring acromegaly. About 3% of tumors secrete ACTH, producing Cushing's disease. Most of the remainder are nonfunctional. Local tumor expansion may cause visual disturbance, headache, and hypopituitarism. Pituitary tumors in MEN 1 patients appear to be larger and behave more aggressively than sporadic pituitary tumors.

The classic feature of gynecomastia is male breast enlargement with soft, compressible, and mobile subcutaneous chest tissue palpated under the areola of the nipple in contrast to softer fatty tissue. This enlargement may occur on one side or both. Dimpling of the skin and nipple retraction are not typical features of gynecomastia. Milky discharge from the nipple is also not a typical finding, but may be seen in a gynecomastic individual with a prolactin secreting tumor. An increase in the diameter of the areola and asymmetry of chest tissue are other possible signs of gynecomastia.

Males with gynecomastia may appear anxious or stressed due to concerns about the possibility of having breast cancer.

Endocrine insufficiency of the pancreas occurs with JBS, though it is sometimes less common and less pronounced than the more prominent effects on exocrine function. The islets of Langerhans are ducts in the pancreas where endocrine activity such as the release of hormones glucagon, somatostatin and insulin takes place. Pancreatic endocrine insufficiency in JBS can be associated with either a buildup of connective tissue in the islet regions, congenital replacement of the islets with fatty tissue, or improper nerve signalling to the islets. Endocrine dysfunction of the pancreas often results in diabetes mellitus. Both insulin resistance and diabetes have been observed with JBS, and it is suggested that diabetes should be considered as a complication of JBS and its course.

Ductular output of fluids and electrolytes is preserved in the pancreas of many with JBS, as well as moderate to normal levels of functioning bicarbonate.

Endocrine abnormalities in other areas have also been present with the disorder. These include hypothyroidism, growth hormone deficiency and hypopituitarism. Findings affecting pituitary function in some JBS patients have included such anomalies as the formation of a glial hamartoma (a neoplasm, or tumor composed of glial cells) on a lobe of the pituitary gland, as well congenital underdevelopment of the anterior pituitary. Growth failure and associated short stature (dwarfism) in JBS can be attributed to growth hormone deficiency caused by diminished anterior pituitary function, with malabsorption of fats playing a subsequent role.

The primary malformation apparent with JBS is hypoplasia (underdevelopment) of the nasal alae, or "wing of the nose". Both hypoplasia and aplasia (partial or complete absence) of structural cartilage and tissue in this area of the nose, along with the underlying alae nasi muscle, are prevailing features of the disorder. Together, these malformations give the nose and nostrils an odd shape and appearance.

MEN2 can present with a sign or symptom related to a tumor or, in the case of multiple endocrine neoplasia type 2b, with characteristic musculoskeletal and/or lip and/or gastrointestinal findings.Medullary thyroid carcinoma (MTC) represent the most frequent initial diagnosis. Occasionally pheochromocytoma or primary hyperparathyroidism may be the initial diagnosis.

Pheochromocytoma occurs in 33-50% of MEN2 cases. In MEN2A, primary hyperparathyroidism occurs in 10–50% of cases and is usually diagnosed after the third decade of life. Rarely, it may present in childhood or be the sole clinical manifestation of this syndrome.

MEN2A associates medullary thyroid carcinoma with pheochromocytoma in about 20–50% of cases and with primary hyperparathyroidism in 5–20% of cases.MEN2B associates medullary thyroid carcinoma with pheochromocytoma in 50% of cases, with marfanoid habitus and with mucosal and digestive neurofibromatosis.

In familial isolated medullary thyroid carcinoma the other components of the disease are absent.

In a review of 85 patients 70 had Men2A and 15 had Men2B. The initial manifestation of MEN2 was medullary thyroid carcinoma in 60% of patients, medullary thyroid carcinoma synchronous with pheochromocytoma in 34% and pheochromocytoma alone in 6%. 72% had bilateral pheochromocytomas.

The adrenal cortex is composed of three distinct layers of endocrine cells which produce critical steroid hormones. These include the glucocorticoids which are critical for regulation of blood sugar and the immune system, as well as response to physiological stress, the mineralcorticoid aldosterone, which regulates blood pressure and kidney function, and certain sex hormones. Both benign and malignant tumors of the adrenal cortex may produce steroid hormones, with important clinical consequences.