Carcinoids most commonly affect the small bowel, particularly the ileum, and are the most common malignancy of the appendix. Many carcinoids are asymptomatic and are discovered only upon surgery for unrelated causes. These coincidental carcinoids are common; one study found that one person in ten has them. Many tumors do not cause symptoms even when they have metastasized. Other tumors even if very small can produce adverse effects by secreting hormones.

Ten per cent (10%) or less of carcinoids, primarily some midgut carcinoids, secrete excessive levels of a range of hormones, most notably serotonin (5-HT) or substance P, causing a constellation of symptoms called carcinoid syndrome:

- flushing

- diarrhea

- asthma or wheezing

- congestive heart failure (CHF)

- abdominal cramping

- peripheral edema

- heart palpitations

A carcinoid crisis with profound flushing, bronchospasm, tachycardia, and widely and rapidly fluctuating blood pressure can occur if large amounts of hormone are acutely secreted, which is occasionally triggered by factors such as diet, alcohol, surgery chemotherapy, embolization therapy or radiofrequency ablation.

Chronic exposure to high levels of serotonin causes thickening of the heart valves, particularly the tricuspid and the pulmonic valves, and over a long period can lead to congestive heart failure. However, valve replacement is rarely needed. The excessive outflow of serotonin can cause a depletion of tryptophan leading to niacin deficiency, and thus pellagra, which is associated with dermatitis, dementia, and diarrhea. Many other hormones can be secreted by some of these tumors, most commonly growth hormone that can cause acromegaly, or cortisol, that can cause Cushing's syndrome.

Occasionally, haemorrhage or the effects of tumor bulk are the presenting symptoms. Bowel obstruction can occur, sometimes due to fibrosing effects of NET secretory products with an intense desmoplastic reaction at the tumor site, or of the mesentery.

Conceptually, there are two main types of NET within this category: those which arise from the gastrointestinal (GI) system and those that arise from the pancreas. In usage, the term "carcinoid" has often been applied to both, although sometimes it is restrictively applied to NETs of GI origin (as herein), or alternatively to those tumors which secrete functional hormones or polypeptides associated with clinical symptoms, as discussed.

Patients with insulinomas usually develop neuroglycopenic symptoms. These include recurrent headache, lethargy, diplopia, and blurred vision, particularly with exercise or fasting. Severe hypoglycemia may result in seizures, coma, and permanent neurological damage. Symptoms resulting from the catecholaminergic response to hypoglycemia (i.e. tremulousness, palpitations, tachycardia, sweating, hunger, anxiety, nausea) are not as common. Sudden weight gain is sometimes seen.

Adrenocortical carcinoma may present differently in children and adults. Most tumors in children are functional, and virilization is by far the most common presenting symptom, followed by Cushing's syndrome and precocious puberty. Among adults presenting with hormonal syndromes, Cushing's syndrome alone is most common, followed by mixed Cushing's and virilization (glucocorticoid and androgen overproduction). Feminization and Conn syndrome (mineralocorticoid excess) occur in less than 10% of cases. Rarely, pheochromocytoma-like hypersecretion of catecholamines has been reported in adrenocortical cancers. Non-functional tumors (about 40%, authorities vary) usually present with abdominal or flank pain, varicocele and renal vein thrombosis or they may be asymptomatic and detected incidentally.

All patients with suspected adrenocortical carcinoma should be carefully evaluated for signs and symptoms of hormonal syndromes. For Cushing's syndrome (glucocorticoid excess) these include weight gain, muscle wasting, purple lines on the abdomen, a fatty "buffalo hump" on the neck, a "moonlike" face, and thinning, fragile skin. Virilism (androgen excess) is most obvious in women, and may produce excess facial and body hair, acne, enlargement of the clitoris, deepening of the voice, coarsening of facial features, cessation of menstruation. Conn syndrome (mineralcorticoid excess) is marked by high blood pressure which can result in headache and hypokalemia (low serum potassium, which can in turn produce muscle weakness, confusion, and palpitations) low plasma renin activity, and high serum aldosterone. Feminization (estrogen excess) is most readily noted in men, and includes breast enlargement, decreased libido and impotence.

An insulinoma is a tumor of the pancreas that is derived from beta cells and secretes insulin. It is a rare form of a neuroendocrine tumor. Most insulinomas are benign in that they grow exclusively at their origin within the pancreas, but a minority metastasize. Insulinomas are one of the functional PanNET group ("functional" because it increases production of insulin; "PanNET" as an abbreviation of pancreatic neuroendocrine tumor). In the Medical Subject Headings classification, insulinoma is the only subtype of "islet cell adenoma".

Beta cells secrete insulin in response to increases in blood glucose. The resulting increase in insulin acts to lower blood glucose back to normal levels, at which point further secretion of insulin is stopped. In contrast, the secretion of insulin by insulinomas is not properly regulated by glucose, and the tumors continue to secrete insulin causing glucose levels to fall further than normal.

As a result, patients present symptoms of low blood glucose (hypoglycemia), which are improved by eating. The diagnosis of an insulinoma is usually made biochemically with low blood glucose, elevated insulin, proinsulin, and C-peptide levels, and confirmed by localizing the tumor with medical imaging or angiography. The definitive treatment is surgery.

Thyroid cancers are mainly papillary, follicular, medullary or anaplastic thyroid cancer. Most patients are 25 to 65 years of age when first diagnosed; women are more affected than men. Nearly 80 percent of thyroid cancer is papillary and about 15 percent is follicular; both types grow slowly and can be cured if caught early. Medullary thyroid cancer makes up about 3 percent of this cancer. It grows slowly and can be controlled if caught early. Anaplastic is the most deadly and makes up around 2 percent. This type grows quickly and is hard to control. The classification is determined by looking at the sample of cells under a microscope and by which type of thyroid cell is present.

Other thyroid malignancies include thyroid lymphoma, various types of thyroid sarcoma, smooth muscle tumors, teratoma, squamous cell thyroid carcinoma and other rare types of tumors.

Thyroid adenoma is a benign neoplasm of the thyroid. Thyroid nodules are very common and around 80 percent of adults will have at least one by the time they reach 70 years of age. Approximately 90 to 95 percent of all nodules are found to be benign.

Some PanNETs do not cause any symptoms, in which case they may be discovered incidentally on a CT scan performed for a different purpose. Symptoms such as abdominal or back pain or pressure, diarrhea, indigestion, or yellowing of the skin and whites of the eyes can arise from the effects of a larger PanNET tumor, either locally or at a metastasis. About 40% of PanNETS have symptoms related to excessive secretion of hormones or active polypeptides and are accordingly labeled as "functional"; the symptoms reflect the type of hormone secreted, as discussed below. Up to 60% of PanNETs are nonsecretory or nonfunctional, in which there is no secretion, or the quantity or type of products, such as pancreatic polypeptide (PPoma), chromogranin A, and neurotensin, do not cause a clinical syndrome although blood levels may be elevated. In total, 85% of PanNETs have an elevated blood marker.

Functional tumors are often classified by the hormone most strongly secreted, for example:

- gastrinoma: the excessive gastrin causes Zollinger–Ellison syndrome (ZES) with peptic ulcers and diarrhea

- insulinoma: hypoglycemia occurs with concurrent elevations of insulin, proinsulin and C peptide

- glucagonoma: the symptoms are not all due to glucagon elevations, and include a rash, sore mouth, altered bowel habits, venous thrombosis, and high blood glucose levels

- VIPoma, producing excessive vasoactive intestinal peptide, which may cause profound chronic watery diarrhea and resultant dehydration, hypokalemia, and achlorhydria (WDHA or pancreatic cholera syndrome)

- somatostatinoma: these rare tumors are associated with elevated blood glucose levels, achlorhydria, cholelithiasis, and diarrhea

- less common types include ACTHoma, CRHoma, calcitoninoma, GHRHoma, GRFoma, and parathyroid hormone–related peptide tumor

In these various types of functional tumors, the frequency of malignancy and the survival prognosis have been estimated dissimilarly, but a pertinent accessible summary is available.

Pancreatic islet cell tumors occur in 60 to 70% of patients. Tumors are usually multicentric. Multiple adenomas or diffuse islet cell hyperplasia commonly occurs; such tumors may arise from the small bowel rather than the pancreas. About 30% of tumors are malignant and have local or distant metastases. Malignant islet cell tumors due to MEN 1 syndrome often have a more benign course than do sporadically occurring malignant islet cell tumors.About 40% of islet cell tumors originate from a β-cell, secrete insulin (insulinoma), and can cause fasting hypoglycemia. β-cell tumors are more common in patients 40 years of age. Non-β-cell tumors are somewhat more likely to be malignant.

Most islet cell tumors secrete pancreatic polypeptide, the clinical significance of which is unknown. Gastrin is secreted by many non–β-cell tumors (increased gastrin secretion in MEN 1 also often originates from the duodenum). Increased gastrin secretion increases gastric acid, which may inactivate pancreatic lipase, leading to diarrhea and steatorrhea. Increased gastrin secretion also leads to peptic ulcers in > 50% of MEN 1 patients. Usually the ulcers are multiple or atypical in location, and often bleed, perforate, or become obstructed. Peptic ulcer disease may be intractable and complicated. Among patients presenting with Zollinger-Ellison syndrome, 20 to 60% have MEN 1.

A severe secretory diarrhea can develop and cause fluid and electrolyte depletion with non–β-cell tumors. This complex, referred to as the watery diarrhea, hypokalemia and achlorhydria syndrome (VIPoma) has been ascribed to vasoactive intestinal polypeptide, although other intestinal hormones or secretagogues (including prostaglandins) may contribute. Hypersecretion of glucagon, somatostatin, chromogranin, or calcitonin, ectopic secretion of ACTH resulting in Cushing's syndrome, and hypersecretion of somatotropin–releasing hormone (causing acromegaly) sometimes occur in non–β-cell tumors. All of these are rare in MEN 1.Nonfunctioning pancreatic tumors also occur in patients with MEN 1 and may be the most common type of pancreatoduodenal tumor in MEN 1. The size of the nonfunctioning tumor correlates with risk of metastasis and death.

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.

Almost all thyroid adenomas are follicular adenomas. Follicular adenomas can be described as "cold", "warm" or "hot" depending on their level of function. Histopathologically, follicular adenomas can be classified according to their cellular architecture and relative amounts of cellularity and colloid into the following types:

- Fetal (microfollicular) - these have the potential for microinvasion. These consist of small, closely packed follicles lined with epithelium.

- colloid (macrofollicular) - these do "not" have any potential for microinvasion

- embryonal (atypical) - have the potential for microinvasion.

- Hürthle cell adenoma (oxyphil or oncocytic tumor) - have the potential for microinvasion.

- Hyalinizing trabecular adenoma

Papillary adenomas are very rare.

Hyperparathyroidism is present in ≥ 90% of patients. Asymptomatic hypercalcemia is the most common manifestation: about 25% of patients have evidence of nephrolithiasis or nephrocalcinosis. In contrast to sporadic cases of hyperparathyroidism, diffuse hyperplasia or multiple adenomas are more common than solitary adenomas.

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.

The first signs of a parathyroid adenoma and the resulting primary hyperparathyroidism can include bone fractures and urinary calculi such as kidney stones.

Oftentimes parathyroid adenoma is not diagnosed until found on standard blood-tests that reveal high calcium content in the blood, it can appear in urine tests as well. Patients may not be experiencing any noticeable symptoms but could be producing excessive amounts of calcium and eventually experience problems later in life if untreated. However, patients can experience common symptoms that can range from joint, muscle, and abdominal pain to slight discomfort. Additionally patients might be experiencing feelings of depression due to the hormonal imbalance. Constipation and exhaustion can also be experienced as a result of the irregularity in the bloodstream. There is also a potential that the kidneys could be damaged with the excess of calcium in the blood.

Adrenocortical carcinoma (ACC, adrenal cortical carcinoma, adrenal cortical cancer, adrenal cortex cancer, etc.) is an aggressive cancer originating in the cortex (steroid hormone-producing tissue) of the adrenal gland. Adrenocortical carcinoma is a rare tumor, with incidence of 1–2 per million population annually. Adrenocortical carcinoma has a bimodal distribution by age, with cases clustering in children under 5, and in adults 30–40 years old. Adrenocortical carcinoma is remarkable for the many hormonal syndromes which can occur in patients with steroid hormone-producing ("functional") tumors, including Cushing's syndrome, Conn syndrome, virilization, and feminization. Adrenocortical carcinoma has often invaded nearby tissues or metastasized to distant organs at the time of diagnosis, and the overall 5-year survival rate is only 20–35%. The widely used angiotensin-II-responsive steroid-producing cell line H295R was originally isolated from a tumor diagnosed as adrenocortical carcinoma.

A thyroid adenoma may be clinically silent ("cold" or "warm" adenoma), or it may be a functional tumor, producing excessive thyroid hormone ("hot" adenoma). In this case, it may result in symptomatic hyperthyroidism, and may be referred to as a toxic thyroid adenoma.

An adenoma of a parathyroid gland may secrete inappropriately high amounts of parathyroid hormone and thereby cause primary hyperparathyroidism.

Hürthle cell adenoma is a rare benign tumor, typically seen in women between the ages of 70 and 80 years old. This adenoma is characterized by a mass of benign Hürthle cells (Askanazy cells). Typically such a mass is removed because it is not easy to predict whether it will transform into the malignant counterpart, a subtype of follicular thyroid cancer called a Hürthle cell carcinoma.

Hürthle cell adenoma is the benign analogue of Hürthle cell carcinoma. This adenoma is extremely rare; when it occurs, it usually occurs in women. Often the adenoma is harmless but is removed after detection because its future course cannot be trusted. This mass can be detected and removed before transformation and metastasis. The tumor is often detected by imaging such as ultrasound. The location and size of the tumor may cause pressure and pain to the patient. But often the tumor goes undetected. After detection, the mass is tested using an invasive fine-needle aspiration biopsy.

A parathyroid adenoma is a benign tumor of the parathyroid gland. It generally causes hyperparathyroidism; there are very few reports of parathyroid adenomas that were not associated with hyperparathyroidism.

A human being usually has four parathyroid glands located on the back surface of the thyroid in the neck. The parathyroids secrete parathyroid hormone (PTH), which increases the concentration of calcium in the blood by inducing the bones to release calcium into the blood and the kidneys to reabsorb it from the urine into the blood. When a parathyroid adenoma causes hyperparathyroidism, more parathyroid hormone is secreted, causing the calcium concentration of the blood to rise, resulting in hypercalcemia.

PanNETs are sometimes abbreviated as PETs or PNETs: such use should not to be confused with the primitive neuroectodermal tumor (PNET).

The majority of PanNETs are benign, while some are malignant. The World Health Organization (WHO) classification scheme places neuroendocrine tumors into three main categories, which emphasize the tumor grade rather than the anatomical origin. In practice, those tumors termed well or intermediately differentiated PanNETs in the WHO scheme are sometimes called "islet cell tumors." The high grade subtype, termed neuroendocrine cancer (NEC) in the WHO scheme, is synonymous with "islet cell carcinoma".

An endocrine gland neoplasm is a neoplasm affecting one or more glands of the endocrine system.

Examples include:

- Adrenal tumor

- Pituitary adenoma

The most common form is thyroid cancer.

Condition such as pancreatic cancer or ovarian cancer can be considered endocrine tumors, or classified under other systems.

Pinealoma is often grouped with brain tumors because of its location.

An adenoma (from Greek αδένας, "", "gland" + -ώμα, "", "tumor") (; plural adenomas or adenomata ) is a benign tumor of epithelial tissue with glandular origin, glandular characteristics, or both. Adenomas can grow from many glandular organs, including the adrenal glands, pituitary gland, thyroid, prostate, and others. Some adenomas grow from epithelial tissue in nonglandular areas but express glandular tissue structure (as can happen in familial polyposis coli). Although adenomas are benign, over time they may transform to become malignant, at which point they are called adenocarcinomas. Most adenomas do not transform. But even while benign, they have the potential to cause serious health complications by compressing other structures (mass effect) and by producing large amounts of hormones in an unregulated, non-feedback-dependent manner (causing paraneoplastic syndromes). Some adenomas are too small to be seen macroscopically but can still cause clinical symptoms.

A VIPoma (also known as Verner–Morrison syndrome, after the physicians who first described it) is a rare (1 per 10,000,000 per year) endocrine tumor, usually (about 90%) originating from non-β islet cell of the pancreas, that produce vasoactive intestinal peptide (VIP). It may be associated with multiple endocrine neoplasia type 1.

The massive amounts of VIP in turn cause profound and chronic watery diarrhea and resultant dehydration, hypokalemia, achlorhydria (hence "WDHA-syndrome", or "pancreatic cholera syndrome"), acidosis, vasodilation (flushing and hypotension), hypercalcemia and hyperglycemia.

A glucagonoma is a rare tumor of the alpha cells of the pancreas that results in the overproduction of the hormone glucagon. Alpha cell tumors are commonly associated with glucagonoma syndrome, though similar symptoms are present in cases of pseudoglucagonoma syndrome in the absence of a glucagon-secreting tumor.