High Homocysteine: What Does It Mean and What Causes It?

High homocysteine means there is more of this amino acid in the blood than expected. The body normally breaks homocysteine down using vitamins B12, B6 and folate, so a high level can mean that this process is not working well or that one of these vitamins is low. Kidney disease, thyroid problems and some inherited conditions can also raise it.

On its own, a high homocysteine result is not a diagnosis. It is a clue that is read alongside symptoms, other blood tests and your medical history. This guide is for adults looking at a result that has already been reported.

A homocysteine number does not decide how urgent things are. Symptoms do. The NHS advises calling emergency services straight away for signs of a stroke or heart attack:

• Face drooping on one side, or difficulty smiling.
• Weakness or numbness in one arm, or being unable to lift both arms and keep them up.
• Slurred speech or sudden confusion, even if it settles within 24 hours.
• Tight, squeezing or crushing chest pain, especially if it spreads to the arm, neck or jaw.
• Severe difficulty breathing, or pale, blue or grey lips or skin.
• Get a prompt, non-emergency appointment for numbness or pins and needles, balance problems, memory changes or new tiredness, which the NHS lists as possible signs of B12 or folate deficiency.

Vitamins B12, B6 and folate (vitamin B9) help convert homocysteine into other substances the body needs. When one of them runs low, homocysteine can build up. That is why MedlinePlus and Cleveland Clinic both list B-vitamin deficiency as a main cause of a high result.

A raised homocysteine does not, by itself, say which vitamin is low. Clinicians usually look at B12 and folate levels directly. MedlinePlus explains that a methylmalonic acid (MMA) test, which rises when B12 is low, may be compared with homocysteine and vitamin tests to help make a diagnosis.

B12 deficiency can be present even when the blood count looks normal, and low folate has its own range of causes. Our separate guides cover both in more detail.

Tell the person testing you about any B-vitamin supplements or other products you take, because they can affect results. MedlinePlus advises not stopping any medicine unless your provider says so.

Yes. MedlinePlus lists chronic kidney disease and hypothyroidism (an underactive thyroid) among conditions linked with higher levels. Cleveland Clinic adds that older age, being male, menopause and tobacco use are associated with higher homocysteine.

Because these conditions are on the list of causes, it is reasonable to ask whether your kidney function and thyroid have been checked, and how they fit with the homocysteine result.

• Low vitamin B12, B6 or folate.
• Chronic kidney disease.
• Underactive thyroid.
• Homocystinuria, a rare inherited condition.
• Older age, male sex and smoking.

Homocystinuria is a rare inherited disorder in which the body cannot process certain amino acids properly, so homocysteine builds up to high levels. MedlinePlus Genetics says the classic form can cause eye lens problems, weak bones, a higher risk of blood clots and sometimes learning difficulties. Symptoms usually start in childhood, though milder forms may not be noticed until adulthood.

Most people with a mildly raised homocysteine do not have homocystinuria. If a level is very high, or there is a family history, a clinician may consider it. MedlinePlus notes that homocysteine-lowering treatment is recommended for people with homocystinuria, which is one situation where lowering the level has a clearer purpose.

High homocysteine is associated with damage to artery linings and blood clots, which may raise the risk of heart attack, stroke and other blood vessel disease. However, MedlinePlus states that in most cases, lowering homocysteine has not been shown to reduce heart attack or stroke risk.

Cleveland Clinic makes a similar point: vitamins alone do not lower heart disease risk. It highlights not smoking, limiting salt, staying active and controlling blood pressure.

MedlinePlus explains that routine screening is not recommended for everyone because the effect of homocysteine on heart disease is uncertain. Cleveland Clinic describes the test as appropriate when someone has symptoms of B-vitamin deficiency.

So a result is most useful when there is a clear question behind it, such as suspected low B12 or folate, or a possible inherited condition. If you are unsure why the test was ordered, ask.

Ranges differ between laboratories, so use the range printed on your report. Cleveland Clinic gives a typical range of about 5 to 15 micromoles per litre, and describes 15 to 30 as mildly raised, 30 to 100 as moderately raised and above 100 as severely raised.

MedlinePlus notes you may be asked to fast for 8 to 12 hours before the test, so check whether you fasted, as preparation can matter when comparing results. Treatment, if needed, depends on the cause and is decided with a clinician. Do not start, stop or change a prescribed medicine or supplement without talking to the prescriber.

Bring the report with its units, range and date, plus a list of all medicines and supplements.

• Why was homocysteine tested, and what question should it answer?
• Have my B12, folate and kidney function been checked, and what did they show?
• Would an MMA test or a thyroid test help?
• Is this level mildly raised, or high enough to consider an inherited cause?
• Did fasting or any supplement affect the result? Should it be repeated?
• What should I do about heart and stroke risk factors regardless of this result?

On SameCase, patients and verified doctors share de-identified cases. Seeing how others with high homocysteine were followed up, such as which vitamin or kidney tests were checked, can help you prepare focused questions. Similar cases cannot explain your own result, and your clinician remains the person to interpret it.