T
ryptophan is a precursor to serotonin, but normally only 1-3% is metabolised through the serotonin pathway - the majority being used for protein synthesis or for production of nicotinamide adenine dinucleotide (NAD/NADP).
However in carcinoid tumours, up to 60% of tryptophan can be diverted to the serotonin pathway, resulting in niacin deficiencies (frank pellagra rare, <1%). A flow diagram of the
Tryptophan metabolic pathway can be found in "Vitamin B3" entry in this test guide.
24 hour urine 5 HIAA test is primarily used to confirm the presence of midgut carcinoid tumours (sensitivity 76%, specificity around 90-100%). It is less sensitive to detect foregut carcinoid (sensitivity 31%).
Pancreatic neuroendocrine tumours occasionally increase urine 5-HIAA (~1-8%, depending on series) but are rare to induce carcinoid syndrome.
Hindgut carcinoid tumours rarely secrete serotonin or 5-HIAA and do not normally cause carcinoid syndrome. In comparison, plasma chromogranin A though non-specific if raised, has about 80-100% sensitivity for carcinoid tumours regardless of site.
Carcinoid Syndrome.
Carcinoid syndrome typically comprises episodes of diarrhoea, flushing, bronchoconstriction and cardiac manifestations (valvular/endocardial fibrosis; predominantly right-sided).
Serotonin plays a major role in mediating diarrhoea symptoms and fibrosis (cardiac and non-cardiac). On the other hand, the exact mediator of flushing is unclear although in gastric carcinoid, it can be attributed to excess histamine. Carcinoid heart disease (CHD) occurs in 20-50% of patients with carcinoid syndrome. It is a major source of morbidity and mortality. 24-h urine 5-HIAA >300 umol/day and also >3 episodes of flushing per day are independent risk factors in predicting the development or progression of CHD. NT-proBNP is a useful screening test for CHD, with level >31 pmol/L having 92% sensitivity and 91% specificity in one study.
Overall, carcinoid syndrome only occurs in about 1.7-18.7% of patients with carcinoid tumours (depending on series). Mid-gut carcinoids only manifest symptomatically as carcinoid syndrome when the tumour bulk is large and with liver metastases.
Rarely, carcinoid syndrome can be encountered in patients with primary ovarian, lung/bronchial or pancreatic carcinoid tumours without liver metastases, or in mid-gut carcinoids with retroperitoneal metastases.
Potential Interferences
Untreated coeliac disease and some small intestinal diseases e.g. Whipple's disease can increase 24 hr urine 5-HIAA by about 2-3 times the above upper reference limit.
Foods -
Tryptophan or serotonin rich foods can increase urine 5-HIAA, hence the above recommended 2 days dietary restriction recommendation before and during urine collection. Daily intake of 100mg of 5 hydroxytryptophan containing over-the-counter supplements e.g. for mood stabilisation, insomnia or even claiming to treat flushing, can increase urine 5-HIAA up to 10 times above upper reference limit.
A relevant history of supplement use is useful and a normal plasma chromogranin A level is also suggestive.
Drugs -
A number of common drugs can interfere analytically with older methods of 5HIAA measurement. Traditionally medications such as glyceryl guaiacolate containing cough mixture or Naproxen (causing falsely high results),
phenothiazines, aspirin or L-dopa (causing falsely low results) have been described to cause analytical interference. HPLC system can also be interfered by sulfasalazine, mesalazine (via 5 aminosalicylic acid).
Our LC-MS assay is not analytically interfered with by these medications.
It is unclear if methyldopa or carbidopa-levodopa exhibit
in-vivo
inhibition of L-aromatic amino acid (DOPA) decarboxylase, and thus may reduce 5-hydroxytryptophan metabolism to serotonin, or if aspirin's anti-platelet effect reduces serotonin release from platelets in-vivo.
There is no hard data to support the notion that psychiatric medications affecting serotonin metabolism (e.g. monoamine oxidase inhibitors, serotonin reuptake inhibitors or serotonin receptor antagonists) in therapeutic doses significantly alter 5-HIAA excretion in humans, causing possible false diagnosis of neuroendocrine tumours. This may be because the amount of serotonin alteration in the central nervous system is relatively low (about 10%) compared with the much larger serotonin pool in gut neuroendocrine cells.
Urine 5-HIAA has no place in the diagnosis and monitoring of such patients.
Overlap with other neuroendocrine secreting tumours
Phaeochromocytomas or paragangliomas can induce a mild increase in 5-HIAA urine excretion. However head and neck paragangliomas are unlikely to do so.
Conversely, some carcinoids can express catecholamine synthesizing enzymes, resulting in elevated metanephrine, 3 methoxy-tyramine (3MT) and catecholamine levels, and raising the possibility of phaeochromocytoma or paragangliomas.
Two series of patients with mid-gut carcinoids revealed that although the
median
urine levels of normetanephrine, metanephrine and 3MT were not noticeably raised, the normetanephrine and metanephrine could be up to 2 times and 3MT up to 5 times above their corresponding upper reference limits in some patients.
Other markers of Carcinoid Syndrome
Platelet (as whole blood) serotonin is an alternative marker for diagnosis of mid-gut carcinoids. Diagnostic performance is comparable to 24 hour urine 5-HIAA. Unlike the urine test, it is not affected by short term consumption of serotonin rich food thus prior dietary restriction is not necessary. In foregut carcinoids or in those with residual carcinoid tumours where there is a low rate of serotonin production, platelet serotonin is more sensitive than urine 5-HIAA.
However, platelet serotonin concentration is affected by platelet count and is saturable at high serotonin secretion rate, hence urine 5-HIAA and chromogranin A are preferred as markers for monitoring. Platelet serotonin level is also age dependent, about 30% lower in those older than 65 years compared with younger adults or children.
Like urine 5-HIAA, platelet serotonin has no proven value in diagnosis or monitoring of depression or treatments such as SSRI's.
Platelet serotonin measurements require pathologist approval. Special collection requirements apply - platelet serotonin is unstable unless stored frozen within 2 hours of collection.
Also see Chromogranin A
References:
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