Hyperhomocysteinemia (High Homocysteine)

What homocysteine is

Homocysteine is produced when the body breaks down methionine, an amino acid found in protein foods. It is normally recycled quickly into other substances using enzymes that depend on three B vitamins:

• Folate and vitamin B12 convert homocysteine back to methionine
• Vitamin B6 converts it to cysteine

When any of these vitamins is lacking, or an enzyme does not work properly, homocysteine builds up in the blood.

Homocysteine levels

Labs may use slightly different reference ranges. Levels tend to be higher in men and rise with age.

Causes

Nutritional

• Vitamin B12 deficiency: common in older adults, vegans, people with pernicious anemia, and those taking metformin or acid-reducing medicines long term
• Folate deficiency: from poor diet, alcohol misuse or malabsorption
• Vitamin B6 deficiency

Medical conditions

• Chronic kidney disease, the most common cause of moderate elevation
• Hypothyroidism
• Psoriasis and some cancers

Medicines

• Methotrexate
• Certain anticonvulsants (phenytoin, carbamazepine)
• Metformin (through B12 deficiency)
• Nitrous oxide exposure
• Levodopa

Lifestyle

• Smoking
• Heavy alcohol use
• High coffee intake
• Physical inactivity

Genetic

• Homocystinuria: a rare inherited enzyme deficiency (most often cystathionine beta-synthase) causing very high levels. It causes lens dislocation, a tall thin build, intellectual disability, osteoporosis and early blood clots.
• MTHFR gene variants: common variants such as C677T can mildly raise homocysteine, mainly when folate intake is low. The American College of Medical Genetics advises against routine MTHFR testing, because variants have little clinical significance on their own.

Symptoms

Mild to moderate hyperhomocysteinemia usually causes no symptoms. Symptoms come from the underlying cause or complications:

• B12 deficiency: tiredness, numbness, tingling, unsteadiness, memory problems, anemia
• Blood clots: leg swelling and pain, breathlessness
• Homocystinuria: vision problems, skeletal features, developmental delay, clots

Health risks

High homocysteine is associated with:

• Venous blood clots: deep vein thrombosis and pulmonary embolism
• Stroke
• Coronary heart disease
• Cognitive decline and dementia
• Pregnancy complications such as neural tube defects and placental problems
• Osteoporotic fractures

Homocysteine may damage the lining of blood vessels and promote clotting. However, large clinical trials found that lowering homocysteine with B vitamins did not reduce heart attacks in people with heart disease, although a modest reduction in stroke was seen in some studies, particularly where foods are not fortified with folic acid. High homocysteine may be partly a marker of other risk factors rather than a cause.

Diagnosis

A fasting blood test measures total homocysteine. Doctors may also check:

• vitamin B12, folate and sometimes methylmalonic acid
• kidney function
• thyroid function
• a full blood count for anemia
• genetic and amino acid tests if homocystinuria is suspected

Homocysteine testing is not recommended as a routine screening test for heart disease. It is mainly used to investigate B12 or folate deficiency, unexplained blood clots, or suspected homocystinuria.

Treatment

• Correct deficiencies: vitamin B12 (tablets or injections), folic acid and vitamin B6 as needed. Check and treat B12 deficiency before giving folic acid alone, because folic acid can mask B12 deficiency while nerve damage progresses.
• Treat underlying conditions: manage kidney disease and hypothyroidism.
• Review medicines with your doctor.
• Lifestyle: stop smoking, limit alcohol, eat folate-rich foods (leafy greens, legumes, citrus, fortified grains) and B12 sources (meat, fish, eggs, dairy or fortified foods).
• Homocystinuria: specialist treatment with high-dose B6 in responsive forms, a low-methionine diet, betaine, and B12 and folate.
• Blood clot prevention: people with clots are managed with anticoagulants according to standard guidelines.

Frequently asked questions

What causes high homocysteine?

The most common causes are low vitamin B12, folate or B6, kidney disease, an underactive thyroid, smoking, heavy alcohol use and certain medicines. Rare inherited conditions such as homocystinuria cause very high levels. MTHFR variants cause only mild increases.

What are the symptoms of high homocysteine?

High homocysteine usually has no symptoms itself. Symptoms come from the cause, such as tiredness and numbness from B12 deficiency, or from complications such as blood clots or stroke. Inherited homocystinuria causes vision, skeletal and developmental problems.

Is high homocysteine dangerous?

It is linked to a higher risk of blood clots, stroke, heart disease and cognitive decline. However, lowering it with vitamins has not been shown to prevent heart attacks in most studies, so it is managed mainly by finding and treating its cause.

How can I lower my homocysteine?

Correct any B12, folate or B6 deficiency with diet or supplements, stop smoking, limit alcohol and treat conditions such as kidney disease or hypothyroidism. Check B12 levels before taking folic acid supplements alone.

Should I be tested for MTHFR?

Routine MTHFR testing is not recommended. Common MTHFR variants have little effect on health when folate intake is adequate. Testing homocysteine and B vitamin levels is more useful if there is a clinical concern.