Free Normetanephrine

Plasma-free normetanephrine and metanephrine (metanephrines) are the recommended biomarkers for testing of pheochromocytoma and paraganglioma (PPGL). Circulating cell-free ribonucleic acids (ccfRNAs), including messenger RNA (mRNA) and miRNA, are potential cancer biomarkers, with ccfmiRNA being relatively more stable than ccfmRNA. Label-free molecular and cellular imaging with MRI can be achieved using endogenous probes to report the chemical and biological properties of specific metabolites and biomolecules.

Sources:

 https://pubmed.ncbi.nlm.nih.gov/33993248/

 https://pubmed.ncbi.nlm.nih.gov/31215265/

 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10207347/

What is Free Normetanephrine?  [5.] 

Normetanephrine is a catabolite of norepinephrine (noradrenaline) that is found in urine and blood. 

It is one of the metanephrines, along with metanephrine, which are O-methylated metabolites of catecholamines.

Free normetanephrine refers to the unconjugated form of normetanephrine in plasma. Total normetanephrine, on the other hand, includes both free and sulfate-conjugated forms of normetanephrine.

Normetanephrine sulfation occurs in the liver. 

Free metanephrines are cleared rapidly from circulation, while sulfate-conjugated (total) metanephrines are cleared more slowly, primarily by the kidneys.

How are Metanephrine and Normetanephrine Related? [5., 6., 7.] 

Metanephrine and normetanephrine are both O-methylated catecholamine metabolites produced by the enzyme catechol-O-methyl transferase (COMT), but they have distinct differences and similarities. 

Metanephrine is primarily derived from the metabolism of epinephrine (adrenaline), while normetanephrine is derived from norepinephrine (noradrenaline). 

Chemically, metanephrine has a methyl group on the amine group, whereas normetanephrine lacks this methyl group. 

Both metabolites are produced by the enzyme COMT, with metanephrine being produced mainly in the adrenal medulla and normetanephrine in both the adrenal medulla and sympathetic nerve endings.

Clinically, elevated metanephrine levels are more specific for pheochromocytomas originating in the adrenal glands, while elevated normetanephrine levels can indicate pheochromocytomas, neuroblastomas or paragangliomas from various locations. [1., 8.] 

Metanephrine typically has a slightly longer half-life in the body compared to normetanephrine. 

In laboratory testing, both metanephrine and normetanephrine are often measured together in plasma or urine tests for diagnosing catecholamine-producing tumors. 

It is important to know that some labs will offer fractionated metanephrine tests, which include the levels of both metanephrine and normetanephrine.  

Understanding these differences is crucial for accurately interpreting test results and diagnosing conditions related to catecholamine excess, such as pheochromocytomas and paragangliomas.

Free Normetanephrine Biosynthesis and Metabolism [10.] 

Catecholamine Production

The catecholamines epinephrine and norepinephrine are synthesized in chromaffin cells of the adrenal medulla and in postganglionic sympathetic neurons. 

The biosynthesis involves a series of enzymatic reactions, starting with the conversion of tyrosine to L-DOPA by tyrosine hydroxylase, the rate-limiting step. 

Subsequent steps include the decarboxylation of L-DOPA to dopamine and the conversion of dopamine to norepinephrine by dopamine β-hydroxylase, with norepinephrine then methylated to epinephrine by phenylethanolamine N-methyltransferase (PNMT) in the adrenal medulla.

Catecholamine Metabolism and Metabolite Formation

Epinephrine and norepinephrine are normally broken down into their metabolites metanephrine and normetanephrine.  

Metanephrine is formed from the methylation of epinephrine by the enzyme catechol-O-methyltransferase (COMT).

Normetanephrine is similarly produced by the methylation of norepinephrine by COMT.

Both reactions involve the transfer of a methyl group to the catecholamine molecule, converting them into their respective O-methylated metabolites, metanephrine and normetanephrine. 

These metabolites are used as important biomarkers in the diagnosis of pheochromocytoma and other catecholamine-secreting tumors.

Free Normetanephrine Clinical Significance

Measuring Free vs. Total Normetanephrine  [5.] 

Normetanephrine levels are generally tested as part of a test panel for fractionated metanephrines, which includes levels of both metanephrine and normetanephrine. [1., 2.]  

Free metanephrines (unconjugated) and total metanephrines (free plus sulfate-conjugated), including free and total normetanephrine, differ significantly in their diagnostic utility. 

Free metanephrines are produced continuously within pheochromocytoma cells, independent of catecholamine release, and are rapidly cleared from circulation. 

In contrast, sulfate-conjugated metanephrines, including normetanephrine, are metabolized and cleared more slowly by the kidneys.  [5.] 

Free metanephrine levels are significantly elevated in malignant pheochromocytoma compared to benign cases, suggesting its importance in distinguishing between malignant and benign tumors.  [13.] 

Because normetanephrine is also made in the sympathetic nerve endings, it can also indicate paraganglioma as well as pheochromocytoma.  [9.] 

Although total metanephrines are easier to measure due to their higher plasma concentrations, free metanephrines provide a more specific and sensitive marker for pheochromocytoma and paraganglioma, particularly as they are less affected by renal function. [5., 6., 7.] 

Clinical Utility of Measuring Metanephrine and Normetanephrine [7.] 

Both plasma metanephrine and normetanephrine measurements are highly sensitive and specific for detecting pheochromocytomas, with sensitivity rates approaching 97% and specificity around 96%. [7.] 

Elevated levels of metanephrine are more specific for pheochromocytomas originating in the adrenal glands.

Elevated levels of normetanephrine can indicate pheochromocytomas, paragangliomas or neuroblastomas from various locations, including extra-adrenal sites.

Pheochromocytomas and paragangliomas are rare neuroendocrine tumors derived from neural crest progenitor cells, with approximately 10% of pheochromocytomas and 35% of paragangliomas being malignant. [2.] 

These tumors are considered familial, or sporadic.

Familial cases are often associated with genetic syndromes like von Hippel-Lindau syndrome, multiple endocrine neoplasia type 2 (MEN 2), and neurofibromatosis type 1, with various germline mutations identified as causes. [2.] 

Neuroblastomas, originating from immature embryonic neuroblast cells, typically present in childhood and can form tumors in both adrenal and extra-adrenal locations.

Sporadic cases are not associated with classical genetic patterns that are seen in familial cases, although as of 2019, up to 40% of patients with sporadic PPGLs have been found to possess somatic mutations in the same genes associated with hereditary PPGLs or other genes known to initiate tumorigenesis.  [4.] 

Signs and Symptoms of Elevated Metanephrines (Including Normetanephrine) [4.] 

Elevated metanephrines, often indicative of pheochromocytomas or paragangliomas, can lead to a variety of signs and symptoms due to excessive catecholamine production. 

Symptoms of elevated catecholamines include: 

• High blood pressure (hypertension)

• Headaches

• Excessive sweating

• Heart palpitations (rapid or irregular heartbeat)

• Paleness (pallor)

• Tremors

• Nausea

• Anxiety and panic attacks

• Tingling in the hands and feet

It may be helpful to use the 6 P’s of pheochromocytoma pneumonic:

1. Pressure (Hypertension): Pheochromocytomas often cause high blood pressure, which can be persistent or episodic.

2. Pain (Headache): Severe headaches are a common symptom, often described as one of the classic triad of symptoms.

3. Perspiration: Excessive sweating, especially during symptomatic episodes, is frequently reported.

4. Palpitations: Rapid or irregular heartbeat is common due to the excess catecholamine production.

5. Pallor: Patients may experience paleness of the skin, particularly during symptomatic episodes.

6. Panic: Anxiety or a sense of doom is often reported, likely due to the surge of catecholamines.

These symptoms are often episodic and can be triggered by physical or emotional stress, certain foods, or medications. 

Factors Affecting Free Normetanephrine Levels [1.]

Borderline elevations of plasma metanephrine and normetanephrine can result from inappropriate sampling conditions and various medications and foods, such as:

• Selective serotonin and norepinephrine reuptake inhibitors (SSNRIs)

• Tricyclic antidepressants

• Alpha blockers

• Monoamine oxidase inhibitors

• Caffeine

• Cigarette smoking

• Strenuous exercise

Laboratory Testing for Free Normetanephrine

Test Information, Sample Collection and Preparation

Free metanephrines may be assessed in blood or urine.  Typically, plasma free metanephrine measurements are preferred for initial screening due to higher diagnostic accuracy and decreased susceptibility to external factors such as diet and medications.  [1.] 

Blood samples typically require a venipuncture.  Blood samples may be collected after the patient has been resting in a supine position for at least 20-30 minutes to minimize the effects of posture and physical activity on catecholamine levels.  In settings where this is not possible, a 24 hour urine collection may be preferred. 

Urine samples are usually collected over a 24-hour period to account for diurnal variations in metanephrine and normetanephrine excretion.

In addition to recumbency for at least 20-30 minutes prior to a blood sample, patients are often asked to avoid caffeine for at least 12 hours prior to sample collection.  

Interpreting Free Normetanephrine Levels

Optimal Results of Free Normetanephrine Testing

Free normetanephrine is a metabolite of norepinephrine, a catechol molecule with important functions in human physiology.  Too much free normetanephrine may indicate a pathological process.  

One lab reports expected plasma normetanephrine levels as: [1.]

Males:

0-4 years: not established

5-11 years: 0.0-165.9 pg/mL

12-19 years: 0.0-150.8 pg/mL

20-40 years: 0.0-210.1 pg/mL

41-50 years: 0.0-218.9 pg/mL

51-60 years: 0.0-244.0 pg/mL

61-80 years: 0.0-285.2 pg/mL

>80 years: 0.0-297.2 pg/mL

Females:

0-4 years: not established

5-11 years: 0.0-148.0 pg/mL

12-19 years: 0.0-150.8 pg/mL

20-40 years: 0.0-210.1 pg/mL

41-50 years: 0.0-218.9 pg/mL

51-60 years: 0.0-244.0 pg/mL

61-80 years: 0.0-285.2 pg/mL

>80 years: 0.0-297.2 pg/mL

Expected 24-hour urine normetanephrine levels have been reported as: [2.] 

Males: 

0-8 years: 48-229 pg/mL

9-12 years: 80-384 pg/mL

13-20 years: 105-405 pg/mL

21-30 years: 110-553 pg/mL

>30 years: 156-729 pg/mL

Females:

0-8 years: 48-200 pg/mL

9-12 years: 71-296 pg/mL

13-20 years: 90-315 pg/mL

21-30 years: 95-449 pg/mL

>30 years: 131-612 pg/mL

Clinical Significance of Elevated Levels of Free Normetanephrine

Due to the low prevalence of pheochromocytoma and paragangliomas, as well as limited diagnostic specificity, false-positive results are common and outnumber true positives. [1.] 

According to the Endocrine Society Guidelines, all patients with positive results should undergo appropriate follow-up based on the level of increase and clinical presentation. [12.] The follow-up should be guided by clinical judgment, considering the pretest probability of a tumor and the pattern of test results. [12.] 

Typically, free metanephrine levels are markedly elevated in pheochromocytoma, up to and over 4 times normal levels.  [4.] 

Additional workup may include a clonidine suppression test and abdominal imaging to localize the tumor, typically as an MRI or CT. [4.] 

A diagnosis of pheochromocytoma or paraganglioma may be followed by genetic testing.  

Clinical Significance of Decreased Levels of Free Normetanephrine

Decreased levels of free metanephrine are not considered clinically relevant.  

Free Normetanephrine Related Biomarkers

In addition to free metanephrines, several other biomarkers are relevant in the evaluation of pheochromocytoma and paraganglioma, as well as other conditions associated with catecholamine excess.

Catecholamines

Catecholamines, including epinephrine and norepinephrine, can also be measured in plasma or urine as part of the evaluation of pheochromocytoma and paraganglioma. 

However, these biomarkers are less specific and can be influenced by various physiological and pathological conditions, making them less reliable as initial screening tests.  For assessment of symptoms of pheochromocytoma or paraganglioma, metanephrines are recommended.  [11.] 

Chromogranin A

Chromogranin A is a protein co-released with catecholamines from neuroendocrine cells. While not as sensitive as metanephrines for the diagnosis of pheochromocytoma and paraganglioma, chromogranin A can be useful in the follow-up and monitoring of these tumors, particularly in cases where metanephrine levels have normalized after treatment.

Chromogranin A testing also helps distinguish between true-positive and false-positive results in cases where plasma metanephrines are mildly elevated, thereby reducing unnecessary imaging and invasive diagnostic procedures. [3.] 

Vanillylmandelic Acid (VMA) and Homovanillic Acid (HVA) [12.] 

VMA is a breakdown product of epinephrine and norepinephrine. HVA is a metabolite of dopamine.  VMA was historically used for diagnosing pheochromocytoma but is now less favored due to lower sensitivity and specificity compared to metanephrine levels.

As complementary markers, testing both HVA and VMA can provide a more comprehensive picture of catecholamine metabolism.

VMA and HVA testing are still used to help detect and rule out pheochromocytoma, although they are more commonly used for diagnosing neuroblastomas in children.

The VMA and HVA tests are sensitive to many external factors such as diet, medications, and stress, leading to a high false-positive rate. They are not the preferred test for pheochromocytoma due to these limitations.  Instead, the Endocrine Society Clinical Practice Guideline recommends testing metanephrines.  [12.]

Genetic Testing

Genetic testing for mutations in genes associated with hereditary forms of pheochromocytoma and paraganglioma, such as succinate dehydrogenase (SDH) subunit genes, can be performed in selected cases, particularly in patients with a positive family history or specific clinical features suggestive of a hereditary syndrome. [4.] 

FAQ: Free Normetanephrine

What is Free Normetanephrine?

Free normetanephrine is a metabolite of norepinephrine, a hormone and neurotransmitter involved in the body's "fight or flight" response. It is produced when norepinephrine is broken down by the enzyme catechol-O-methyltransferase (COMT).

What is the Function of Free Normetanephrine in the Body?

Free normetanephrine does not have a direct function but serves as a marker for the breakdown of norepinephrine. It is useful in medical diagnostics to assess the activity of the adrenal glands and the sympathetic nervous system.

How is Free Normetanephrine Measured?

Free normetanephrine is measured through blood or urine tests. These tests help in diagnosing and monitoring conditions related to excess production of catecholamines, such as pheochromocytoma and paraganglioma.

What is a Normal Free Normetanephrine Level?

Normal levels of free normetanephrine can vary slightly depending on the laboratory and the methods used. 

Typically, for adults, the normal range in blood plasma is about 18 to 111 pg/mL (picograms per milliliter). I

n a 24-hour urine test, normal values are usually between 105 and 354 mcg/24 hours (micrograms per 24 hours). 

Consult with a healthcare provider for accurate interpretation of test results.

What Do Elevated Levels of Free Normetanephrine indicate?

Elevated levels of free normetanephrine can indicate:

• Pheochromocytoma: A rare tumor of the adrenal glands that produces excess catecholamines.

• Paraganglioma: A rare tumor arising from the nerve tissue outside the adrenal glands.

• Stress or illness: Severe stress, acute illness, or certain medications can also cause elevated levels.  However, elevations due to pheochromocytoma or paraganglioma will be excessively elevated.  

What Do Low Levels of Free Normetanephrine Indicate?

Low levels of free normetanephrine are generally not clinically significant. However, they may occur in situations where there is decreased production of norepinephrine, which could be due to various physiological or pathological conditions.

How are Abnormal Free Normetanephrine Levels Managed?

Management of abnormal free normetanephrine levels depends on the underlying cause:

• Elevated levels:

• Pheochromocytoma or paraganglioma: surgical removal of the tumor, medications to control symptoms, and regular monitoring.

• Medication-related: Adjusting or changing medications under the guidance of a healthcare provider.

Can Lifestyle Changes Affect Free Normetanephrine Levels?

Yes, lifestyle changes can affect free normetanephrine levels, especially if elevated levels are due to stress or medication use. However, elevated levels due to pheochromocytoma or paraganglioma will not respond to lifestyle interventions.

Consider the following:

• Stress management: Techniques such as mindfulness, meditation, and yoga can help reduce stress.

• Healthy diet: A balanced diet can support overall health and hormone balance.

• Regular exercise: Physical activity can help regulate stress hormones.

• Avoiding stimulants: Reducing intake of caffeine and other stimulants can help manage norepinephrine levels.

Where can I find more information about Free Normetanephrine and related conditions?

For more information about free normetanephrine and related conditions, consider consulting:

• Healthcare providers: Medical professionals can provide personalized advice and diagnosis.

• Scientific literature: Research articles and reviews on normetanephrine and its clinical significance.
• Reputable health organizations: Websites of organizations such as the National Institutes of Health (NIH) and the American Association of Clinical Endocrinologists (AACE).

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