Besides the clinical picture, fasting VIP plasma level may confirm the diagnosis, and CT scan and somatostatin receptor scintigraphy are used to localise the tumor, which is usually metastatic at presentation.

Tests include:

- Blood chemistry tests (basic or comprehensive metabolic panel)

- CT scan of the abdomen

- MRI of the abdomen

- Stool examination for cause of diarrhea and electrolyte levels

- Vasoactive intestinal peptide (VIP) level in the blood

Normally, endogenous insulin production is suppressed in the setting of hypoglycemia. A 72-hour fast, usually supervised in a hospital setting, can be done to see if insulin levels fail to suppress, which is a strong indicator of the presence of an insulin-secreting tumor.

The insulinoma might be localized by noninvasive means, using ultrasound, CT scan, or MRI techniques. An indium-111 pentetreotide scan is more sensitive than ultrasound, CT, or MRI for detection of somatostatin receptor positive tumors, but not a good diagnostic tool for insulinomas. An endoscopic ultrasound has a sensitivity of 40-93% (depending on the location of the tumor) for detecting insulinomas.

Sometimes, angiography with percutaneous transhepatic pancreatic vein catheterization to sample the blood for insulin levels is required. Calcium can be injected into selected arteries to stimulate insulin release from various parts of the pancreas, which can be measured by sampling blood from their respective veins. The use of calcium stimulation improves the specificity of this test.

During surgery to remove an insulinoma, an intraoperative ultrasound can sometimes localize the tumor, which helps guide the surgeon in the operation and has a higher sensitivity than noninvasive imaging tests.

Surgery can usually cure VIPomas. However, in one-third to one-half of patients, the tumor has spread by the time of diagnosis and cannot be cured.

Zollinger–Ellison syndrome may be suspected when the above symptoms prove resistant to treatment, when the symptoms are especially suggestive of the syndrome, or when endoscopy is suggestive. The diagnosis is made through several laboratory tests and imaging studies:

- Secretin stimulation test, which measures evoked gastrin levels

- Fasting gastrin levels on at least three separate occasions

- Gastric acid secretion and pH (normal basal gastric acid secretion is less than 10 mEq/hour; in Zollinger–Ellison patients, it is usually more than 15 mEq/hour)

- An increased level of chromogranin A is a common marker of neuroendocrine tumors.

In addition, the source of the increased gastrin production must be determined using MRI or somatostatin receptor scintigraphy.

For localization of both primary lesions and metastasis, the initial imaging method is Octreoscan, where indium-111 labelled somatostatin analogues (octreotide) are used in scintigraphy for detecting tumors expressing somatostatin receptors. Median detection rates with octreoscan are about 89%, in contrast to other imaging techniques such as CT scan and MRI with detection rates of about 80%. Gallium-68 labelled somatostatin analogues such as Ga-DOTA-Octreotate (DOTATATE), performed on a PET/CT scanner is superior to conventional Octreoscan.

Usually, on a CT scan, a spider-like/crab-like change is visible in the mesentery due to the fibrosis from the release of serotonin. F-FDG PET/CT, which evaluate for increased metabolism of glucose, may also aid in localizing the carcinoid lesion or evaluating for metastases. Chromogranin A and platelets serotonin are increased.

With a certain degree of clinical suspicion, the most useful initial test is the 24-hour urine levels of 5-HIAA (5-hydroxyindoleacetic acid), the end product of serotonin metabolism. Patients with carcinoid syndrome usually excrete more than 25 mg of 5-HIAA per day.

An adrenal "incidentaloma" is an adrenal tumor found by coincidence without clinical symptoms or suspicion. It is one of the more common unexpected findings revealed by computed tomography (CT), magnetic resonance imaging (MRI), or ultrasonography.

In these cases, a dexamethasone suppression test is often used to detect cortisol excess, and metanephrines or catecholamines for excess of these hormones. Tumors under 3 cm are generally considered benign and are only treated if there are grounds for a diagnosis of Cushing's syndrome or pheochromocytoma. Radiodensity gives a clue in estimating malignancy risk, wherein a tumor with 10 Hounsfield units or less on an unenhanced CT is probably a lipid-rich adenoma.

Hormonal evaluation includes:

- 1-mg overnight dexamethasone suppression test

- 24-hour urinary specimen for measurement of fractionated metanephrines and catecholamines

- Blood plasma aldosterone concentration and plasma renin activity, "if hypertension is present"

On CT scan, benign adenomas typically are of low radiographic density (due to fat content) and show rapid washout of contrast medium (50% or more of the contrast medium washes out at 10 minutes). If the hormonal evaluation is negative and imaging suggests benign, followup should be considered with imaging at 6, 12, and 24 months and repeat hormonal evaluation yearly for 4 years

Proton pump inhibitors (such as omeprazole and lansoprazole) and histamine H2-receptor antagonists (such as famotidine and ranitidine) are used to slow acid secretion. Once gastric acid is suppressed, symptoms normally improve.

CT-scans, MRIs, sonography (ultrasound), and endoscopy (including endoscopic ultrasound) are common diagnostic tools. CT-scans using contrast medium can detect 95 percent of tumors over 3 cm in size, but generally not tumors under 1 cm.

Advances in nuclear medicine imaging, also known as molecular imaging, has improved diagnostic and treatment paradigms in patients with neuroendocrine tumors. This is because of its ability to not only identify sites of disease but also characterize them. Neuronedocrine tumours express somatostatin receptors providing a unique target for imaging. Octreotide is a synthetic modifications of somatostatin with a longer half-life. OctreoScan, also called somatostatin receptor scintigraphy (SRS or SSRS), utilizes intravenously administered octreotide that is chemically bound to a radioactive substance, often indium-111, to detect larger lesions with tumor cells that are avid for octreotide.

Somatostatin receptor imaging can now be performed with positron emission tomography (PET) which offers higher resolution, three-dimensional and more rapid imaging. Gallium-68 receptor PET-CT is much more accurate than an OctreoScan.

Imaging with fluorine-18 fluorodeoxyglucose (FDG) PET may be valuable to image some neuroendocrine tumors. This scan is performed by injected radioactive sugar intravenously. Tumors that grow more quickly use more sugar. Using this scan, the aggressiveness of the tumor can be assessed.

The combination of somatostatin receptor and FDG PET imaging is able to quantify somatostatin receptor cell surface (SSTR) expression and glycolytic metabolism, respectively. The ability to perform this as a whole body study is highlighting the limitations of relying on histopathology obtained from a single site. This is enabling better selection of the most appropriate therapy for an individual patient.

Screening methods for colon cancer depend on detecting either precancerous changes such as certain kinds of polyps or on finding early and thus more treatable cancer. The extent to which screening procedures reduce the incidence of gastrointestinal cancer or mortality depends on the rate of precancerous and cancerous disease in that population. gFOBT (guaiac fecal occult blood test) and flexible sigmoidoscopy screening have each shown benefit in randomized clinical trials. Evidence for other colon cancer screening tools such as iFOBT (immunochemical fecal occult blood test) or colonoscopy is substantial and guidelines have been issued by several advisory groups but does not include randomized studies.

In 2009 the American College of Gastroenterology (ACG) suggest that colon cancer screening modalities that are also directly preventive by removing precursor lesions should be given precedence, and prefer a colonoscopy every 10 years in average-risk individuals, beginning at age 50. The ACG suggests that cancer detection tests such as any type of FOB are an alternative that is less preferred, and if a colonoscopy is declined, the FIT (fecal immunochemical test, or iFOBT) should be offered instead. Two other recent guidelines, from the US Multisociety Task Force (MSTF) and the US Preventive Services Task Force (USPSTF), while permitting immediate colonoscopy as an option, did not categorize it as preferred. The ACG and MSTF also included CT colonography every five years, and fecal DNA testing as considerations. All three recommendation panels recommended replacing any older low-sensitivity, guaiac-based fecal occult blood testing (gFOBT) with either newer high-sensitivity guaiac-based fecal occult blood testing (hs gFOBT) or fecal immunochemical testing (FIT). MSTF looked at six studies that compared high sensitivity gFOBT (Hemoccult SENSA) to FIT, and concluded that there was no clear difference in overall performance between these methods.

The American College of Gastroenterology has recommended the abandoning of gFOBT testing as a colorectal cancer screening tool, in favor of the fecal immunochemical test. Though the FIT test is preferred, even the guaiac FOB testing of average risk populations may have been sufficient to reduce the mortality associated with colon cancer by about 25%. With this lower efficacy, it was not always cost effective to screen a large population with gFOBT.

If colon cancer is suspected in an individual (such as in someone with an unexplained anemia) fecal occult blood tests may not be clinically helpful. If a doctor suspects colon cancer, more rigorous investigation is necessary, whether or not the test is positive.

In 2006, the Australian Government introduced the National Bowel Cancer Program which has been updated several times since; targeted screening will be done of all Australians aged over 50 to 74 by 2017–2018. Cancer Council Australia recommended that FOBT should be done every two years. Gradually government fund disbursement meant that some people are not yet eligible for the national program and should pay for a FOBT by themselves.

The Canadian Cancer Society recommends that men and women age 50 and over have a FOBT at least every 2 years.

In colon cancer screening, using only one sample of feces collected by a doctor performing a digital rectal examination is discouraged.

The use of the M2-PK Test is encouraged over gFOBT for routine screening as it may pick up tumors that are both bleeding and non bleeding. It is able to pick up 80 percent of colorectal cancer and 44 percent for adenoma > 1 centimeter, while gFOBT picks up 13 to 50 percent of colorectal cancers.

Pituitary incidentalomas are pituitary tumors that are characterized as an incidental finding. They are often discovered by computed tomography (CT) or magnetic resonance imaging (MRI), performed in the evaluation of unrelated medical conditions such as suspected head trauma, in cancer staging or in the evaluation of nonspecific symptoms such as dizziness and headache. It is not uncommon for them to be discovered at autopsy. In a meta-analysis, adenomas were found in an average of 16.7% in postmortem studies, with most being microadenomas (<10mm); macrodenomas accounted for only 0.16% to 0.2% of the decedents. While non-secreting, noninvasive pituitary microadenomas are generally considered to be literally as well as clinically benign, there are to date scant studies of low quality to support this assertion.

It has been recommended in the current Clinical Practice Guidelines (2011) by the Endocrine Society - a professional, international medical organization in the field of endocrinology and metabolism - that all patients with pituitary incidentalomas undergo a complete medical history and physical examination, laboratory evaluations to screen for hormone hypersecretion and for hypopituitarism. If the lesion is in close proximity to the optic nerves or optic chiasm, a visual field examination should be performed. For those with incidentalomas which do not require surgical removal, follow up clinical assessments and neuroimaging should be performed as well follow-up visual field examinations for incidentalomas that abut or compress the optic nerve and chiasm and follow-up endocrine testing for macroincidentalomas.

A blood serum glucagon concentration of 1000 pg/mL or greater is indicative of glucagonoma (the normal range is 50–200 pg/mL).

However, recent studies have shown that forty percent of patients have plasma glucagon levels ranging from 500 to 1000 pg/mL. Increased levels have been reported in cases of decreased kidney function, acute pancreatitis, hypercorticism, liver diseases, severe stress, extended fasting, and familial hyperglucagonemia. Rarely do these cases result in levels over 500 pg/mL, except in the case of patients with liver diseases.

Blood tests may also reveal abnormally low concentrations of amino acids, zinc, and essential fatty acids, which are thought to play a role in the development of NME. Skin biopsies may also be taken to confirm the presence of NME.

A CBC can uncover anemia, which is an abnormally low level of hemoglobin.

The tumor itself may be localized by any number of radiographic modalities, including angiography, CT, MRI, PET, and endoscopic ultrasound. Laparotomy is useful for obtaining histologic samples for analysis and confirmation of the glucagonoma.

Heightened glucagon secretion can be treated with the administration of octreotide, a somatostatin analog, which inhibits the release of glucagon. Doxorubicin and streptozotocin have also been used successfully to selectively damage alpha cells of the pancreatic islets. These do not destroy the tumor, but help to minimize progression of symptoms.

The only curative therapy for glucagonoma is surgical resection, where the tumor is removed. Resection has been known to reverse symptoms in some patients.

Unlike tumors of the posterior Pituitary, Pituitary adenomas are classified as endocrine tumors (not brain tumors). Pituitary adenomas are classified based upon anatomical, histological and functional criteria.

- Anatomically pituitary tumors are classified by their size based on radiological findings; either microadenomas (less than <10 mm) or macroadenomas (equal or greater than ≥10 mm).

- Histological classification utilizes an immunohistological characterization of the tumors in terms of their hormone production. Historically they were classed as either basophilic, acidophilic, or chromophobic on the basis of whether or not they took up the tinctorial stains hematoxylin and eosin. This classification has fallen into disuse, in favor of a classification based on what type of hormone is secreted by the tumor. Approximately 20-25% of adenomas do not secrete any readily identifiable active hormones ('non-functioning tumors') yet they are still sometimes referred to as 'chromophobic'.

- Functional classification is based upon the tumors endocrine activity as determined by serum hormone levels and pituitary tissue cellular hormone secretion detected via immunohistochemical staining. The "Percentage of hormone production cases" values are the fractions of adenomas producing each related hormone of each tumor type as compared to all cases of pituitary tumors, and does not directly correlate to the percentages of each tumor type because of smaller or greater incidences of absence of secretion of the expected hormone. Thus, nonsecretive adenomas may be either "null cell adenomas" or a more specific adenoma that, however, remains nonsecretive.

Surgery, if feasible, is the only curative therapy. If the tumor has metastasized (most commonly, to the liver) and is considered incurable, there are some promising treatment modalities, such as radiolabeled octreotide (e.g. Lutetium (Lu) DOTA-octreotate) or the radiopharmaceutical 131I-mIBG (meta iodo benzyl guanidine) for arresting the growth of the tumors and prolonging survival in patients with liver metastases, though these are currently experimental.

Chemotherapy is of little benefit and is generally not indicated. Octreotide or Lanreotide (somatostatin analogues) may decrease the secretory activity of the carcinoid, and may also have an anti-proliferative effect. Interferon treatment is also effective, and usually combined with somatostatin analogues.

As the metastatic potential of a coincidental carcinoid is probably low, the current recommendation is for follow up in 3 months with CT or MRI, labs for tumor markers such as serotonin, and a history and physical, with annual physicals thereafter.

Carcinoid Syndrome is multiple in 1/5 cases.

Incidence of Gastric Carcinoid is increased in Achlorhydria,Hashimoto's thyroiditis,Pernicious anemia.

There are four methods in clinical use for testing for occult blood in feces. These look at different properties, such as antibodies, heme, globin, or porphyrins in blood, or at DNA from cellular material such as from lesions of the intestinal mucosa.

- Fecal immunochemical testing (FIT), and immunochemical fecal occult blood test (iFOBT). FIT products utilize specific antibodies to detect globin. FIT screening is more effective in terms of health outcomes and cost compared with guaiac FOBT. According to the guidelines of the American College of Gastroenterology, "Annual fecal immunochemical testing is the preferred colorectal cancer detection test." A FIT test detects globin levels in feces at or above 50 nanograms per mL, the established cutoff by the World Health Organization for Colorectal Cancer Screening.

FIT testing has replaced most gFOBT tests as the colon cancer screening test of choice. This methodology can be adapted for automated test reading and to report quantitative results, which are potential factors in design of a widescale screening strategy. The number of fecal samples submitted for FIT may affect the clinical sensitivity and specificity of the methodology.[8] High sensitivity gFOBT tests such as Hemoccult SENSA remains an accepted option;[8] and may retain a role in monitoring gastrointestinal conditions such as ulcerative colitis; however the FIT test is preferred in recent guidelines.

- Stool guaiac test for fecal occult blood (gFOBT): – The stool guaiac test involves smearing some feces onto some absorbent paper that has been treated with a chemical. Hydrogen peroxide is then dropped onto the paper; if trace amounts of blood are present, the paper will change color in one or two seconds. This method works as the heme component in hemoglobin has a peroxidase-like effect, rapidly breaking down hydrogen peroxide. In some settings such as gastric or proximal upper intestinal bleeding the guaiac method may be more sensitive than tests detecting globin because globin is broken down in the upper intestine to a greater extent than is heme. There are various commercially available gFOBT tests which have been categorized as being of low or high sensitivity, and only high sensitivity tests remain an acceptable alternative to FIT testing, which is now the best-practices recommendation in colon cancer screening. Optimal clinical performance of the stool guaiac test depends on preparatory dietary adjustment.

- Stool DNA screening tests look for DNA alterations that have been associated with cancer.

Additional methods of looking for occult blood are being explored, including transferrin dipstick and stool cytology.

Symptoms from secreted hormones may prompt measurement of the corresponding hormones in the blood or their associated urinary products, for initial diagnosis or to assess the interval change in the tumor. Secretory activity of the tumor cells is sometimes dissimilar to the tissue immunoreactivity to particular hormones.

Given the diverse secretory activity of NETs there are many other potential markers, but a limited panel is usually sufficient for clinical purposes. Aside from the hormones of secretory tumors, the most important markers are:

- chromogranin A (CgA), present in 99% of metastatic carcinoid tumors

- urine 5-hydroxyindoleacetic acid (5-HIAA)

- neuron-specific enolase (NSE, gamma-gamma dimer)

- synaptophysin (P38)

Newer markers include N-terminally truncated variant of Hsp70 is present in NETs but absent in normal pancreatic islets. High levels of CDX2, a homeobox gene product essential for intestinal development and differentiation, are seen in intestinal NETs. Neuroendocrine secretory protein-55, a member of the chromogranin family, is seen in pancreatic endocrine tumors but not intestinal NETs.

Diet and lifestyle are believed to play a large role in whether colorectal polyps form. Studies show there to be a protective link between consumption of cooked green vegetables, brown rice, legumes, and dried fruit and decreased incidence of colorectal polyps.

The veterinarian will typically perform a series of tests such as blood tests and imaging studies. The most definitive way to confirm/rule out intestinal tumors is to perform a medical procedure called endoscopy to visualize the organ and do a tissue biopsy.

Because of the way familial polyposis develops, it is possible to have the genetic condition, and therefore be at risk, but have no polyps or issues so far. Therefore, an individual may be diagnosed "at risk of" FAP, and require routine monitoring, but not (yet) actually have FAP (i.e., carries a defective gene but as yet appears not to have any actual medical issue as a result of this). Clinical management can cover several areas:

- Identifying those individuals who could be at risk of FAP: usually from family medical history or genetic testing

- Diagnosis (confirming whether they have FAP)—this can be done either by genetic testing, which is definitive, or by visually checking the intestinal tract itself.

- Screening / monitoring programs involve visually examining the intestinal tract to check its healthy condition. It is undertaken as a routine matter every few years where there is cause for concern, when either (a) a genetic test has confirmed the risk or (b) a genetic test has not been undertaken for any reason so the actual risk is unknown. Screening and monitoring allows polyposis to be detected visually before it can become life-threatening.

- Treatment, typically surgery of some kind, is involved if polyposis has led to a large number of polyps, or a significant risk of cancer, or actual cancer.

Common diagnostic techniques include:

- MRIs

- CAT scans

- blood samples.

Blood samples are assessed for the absence or presence of aldosterone and cortisol. Physical examinations are also useful in patients in order to examine vision, skin pigmentation, how the body replaces steroids, and the cranial nerves. Recent advancements in high-resolution MRIs allow for adenomas to be detected during the early stages of Nelson syndrome. Physical examination including height, weight, vital signs, blood pressure, eye examination, thyroid examination, abdominal examination, neurological examination, skin examination and pubertal staging needs to be assessed. Through blood pressure and pulse readings can indicate hypothyroidism and adrenal insufficiency. Hyper-pigmentation, hyporeflexia, and loss of vision can also indicate Nelson's syndrome when assessed together. Specifically for a child who might have Nelson's syndrome, the patient should be questioned about the symptoms of the disease, and well as symptoms of other diseases to narrow down which disease the patient presents with. The patient should be questioned about how often and to what degree headaches, visual disturbances, and symptoms associated with pituitary malfunction occur. Additionally, adrenal steroid replacement should be assessed, especially in children who have prior insufficiency associated wit

Surgical treatment remains the treatment of choice for cats and dogs diagnosed with intestinal tumors who are in otherwise good health.

A recommend surveillance program for Multiple Endocrine Neoplasia Type 1 has been suggested by the International Guidelines for Diagnosis and Therapy of MEN syndromes group.