[2][3] A neuroendocrine paraneoplastic syndrome involves neoplastic secretion of functional peptides, hormones, cytokines, growth factors, and/or immune cross-reactivity between tumor tissues and normal host tissues, resulting in a syndrome of clinical signs and symptoms.

They are known to grow in the liver, but this finding is usually a manifestation of metastatic disease from a primary carcinoid occurring elsewhere in the body.

This is due to the over-production of many substances, including serotonin, which are released into the systemic circulation, and which can lead to symptoms of cutaneous flushing, diarrhea, bronchoconstriction, and right-sided cardiac valve disease.

[8] This is considered to be a hybrid between an exocrine and endocrine tumor derived from crypt cells of the appendix.

The growth pattern is distinctive: typically producing a concentric band of tumor nests interspersed among the muscle and stroma of the appendiceal wall extending up the shaft of the appendix.

The term 'crypt cell carcinoma' has been used for them, and though perhaps more accurate than considering them carcinoids, has not been a successful competitor.

The incidence of gastric carcinoids is increased in achlorhydria, Hashimoto's thyroiditis, and pernicious anemia.

If the tumor has metastasized (most commonly, to the liver) and is considered incurable, there are some promising treatment modalities, such as the radiopharmaceuticals Lutetium (177Lu) DOTA-octreotate) and 131I-mIBG (meta iodo benzyl guanidine[9]) for arresting the growth of the tumors and prolonging survival in patients with liver metastases, though these are currently experimental.

Octreotide or lanreotide (somatostatin analogues) may decrease the secretory activity of the carcinoid, and may also have an anti-proliferative effect.

They were first characterized in 1907 by Siegfried Oberndorfer, a German pathologist at the University of Munich, who coined the term karzinoide, or "carcinoma-like", to describe the unique feature of behaving like a benign tumor despite having a malignant appearance microscopically.

The recognition of their endocrine-related properties was later described by Gosset and Masson in 1914, and these tumors are now known to arise from the enterochromaffin (EC) and enterochromaffin-like (ECL) cells of the gut.

Specific to rodents, chronically reduced gastric acid secretion encourages carcinoid development.