Reference Range

Somatostatin is a polypeptide that is released in the gastrointestinal tract by delta cells and the hypothalamus. It functions as a key regulatory peptide that has many physiologic effects as an inhibitor for many other hormones, including gastrin, cholecystokinin, glucagon, growth hormone, insulin, secretin, pancreatic polypeptide, vasoactive intestinal peptide, 5-hydroxytryptamine (5-HT), and some anterior pituitary hormones.

The reference range for plasma somatostatin in adults is 10-22 pg/mL, the conversion factor is 0.426, and the SI units are 4.26-9.37 pmol/L. Draw in prechilled tube, separate plasma, and freeze immediately.^[1]

Background/Interpretation

Serum and cerebrospinal fluid (CSF) somatostatin values less than 100 pg/mL are within normal limits for healthy individuals. Decreased levels have been found to be seen in CSF in autopsy samples of people with Alzheimer disease.^[2]No change was found in normal, younger subjects to suggest that this could be a predisposing laboratory screen for dementia patients. CSF values were also found to be elevated in young patients with febrile seizures.^[3]Elevations in serum somatostatin are seen in somatostatinomas and neuroendocrine tumors.^{[4, 5]}The levels are found to be on the scale of nanograms per milliliter, which is nearly 1000-fold greater than the standard unit. These tumors are very rare and slow-growing, but most patients are symptomatic. The following is the classic pentad of symptoms seen in a somatostatinoma syndrome:^[6]

Diabetes mellitus
Cholelithiasis
Weight loss
Steatorrhea and diarrhea
Hypochlorhydria and achlorhydria

Serum somatostatin

See the list below:

Specimen: Plasma
Condition: Fasting
Container: Lavender (EDTA; see image below) or pink (K₂ EDTA) pre-chilled tube

Lavender-top tube.
Collection method: Routine venipuncture
Processing: Separate plasma from cells within 2 hours of collection in lab; an adequate sample requires a minimum of 0.6-1.8 mL. If multiple samples need to be analyzed, the specimen should be kept frozen. Ambient temperatures and exposure to room air will make sample unacceptable for analysis.^[7]

Cerebrospinal fluid somatostatin

See the list below:

Specimen: CSF
Collection method: Lumbar puncture
Typically part of same panel as CSF cell count, gram stain, albumin, glucose

Background

Because intravenously injected somatostatin has a short half-life in the circulation (3 minutes), it has limited clinical usefulness.^[8]Therefore, synthetic preparations, including octreotide and lanreotide, that mimic the properties of somatostatin are used. Octreotide is an octapeptide that can be given intravenously (30 min), subcutaneously (6-12 hours), or intramuscularly (monthly) for various applications. The most common indications for use include imaging for somatostatinomas, other neuroendocrine tumors, variceal bleeding, refractory diarrhea, and hypoglycemia.^[8]

Lanreotide injection is used as long-term treatment for patients with acromegaly in whom surgery and/or radiotherapy have led to an inadequate response or are not an option. It is also employed to improve progression-free survival in patients with unresectable, well- or moderately differentiated, locally advanced or metastatic gastroenteropancreatic neuroendocrine tumors.^[9]

A study by Benderradji et al found that in patients with newly diagnosed acromegaly caused by pituitary macroadenomas, who had optic chiasm compression (grade 2 or below) and/or cavernous sinus invasion, primary therapy with monthly injections of lanreotide 120 mg led to a significant reduction in somatotroph macroadenoma size at 1 month. More specifically, volume reduction was 25% or greater in 61.9% of individuals at 1 month, while half of those patients with optic chiasm compression and visual field defects experienced visual field normalization or improvement at 1 month.^[10]