Many authors confuse incidence with prevalence. There are many different classification systems used for carcinoid tumours, mostly put forward by histopathologists. The prognosis and activity of disease only correlates moderately with the classification, and it is particularly difficult to determine an individual prognosis for patients with endocrine tumours such as those given above. Ovarian carcinoids have also been described. Many patients have undiagnosed carcinoid tumours in the midgut that may secrete various hormones, that are only found incidentally at post mortem.
Appendiceal carcinoids are an example of such.
These tumours do not cause the carcinoid syndrome unless they metastasise to the liver. This is because the mediators of the syndrome that may be secreted are fully metabolised by the liver on first pass, so that systemic concentrations of these peptides or hormones is not increased. Once such a tumour metastasises to the liver, these metastases continue to secrete vasoactive mediators into the systemic circulation. Patients who present with the features of systemic exposure to the secreted mediators are said to have the carcinoid syndrome. The mediators of these effects are in fact unknown.