Genetic Tests
MTHFR Mutation: What You Need to Know
What do MTHFR C677T and A1298C variants mean? Homocysteine, folate and thrombosis relationship with a guide to correct interpretation.
8 min · 18 June 2025
MTHFR (methylenetetrahydrofolate reductase) is the gene coding a key enzyme of the folate cycle, critical for homocysteine metabolism. The C677T and A1298C variants mildly reduce enzyme activity and are very common in the population; most carriers have no clinical problem.
What Do the Variants Do?
- C677T: Heterozygous (CT) retains about 65% of enzyme activity, homozygous (TT) about 30%. Homocysteine may rise mildly in TT carriers.
- A1298C: A milder effect alone; interpreted jointly when carried with C677T.
- It is also possible to carry the normal variants of both genes. Because of this prevalence the word "mutation" is misleading — these are polymorphisms, not disease genes.
Why Does Homocysteine Matter?
Homocysteine is an intermediate of protein metabolism. Elevated levels (hyperhomocysteinemia) can irritate the vessel lining and raise thrombotic and cardiovascular risk. But the strongest determinants are lifestyle rather than genetics: folate, B12 and B6 status, kidney function, medications (methotrexate, some antiepileptics) and thyroid function.
Relationship with Thrombosis
Scientific data show that MTHFR polymorphisms — WITHOUT elevated homocysteine — do not meaningfully raise thrombosis risk by themselves. Therefore an MTHFR result is never interpreted alone; it is always weighed with plasma homocysteine, folate/B12 status and the clinical picture. If elevated homocysteine is found, your physician guides management; folate supplementation is effective in normalizing levels.
Who Should Be Tested?
- Recurrent pregnancy loss together with elevated homocysteine.
- Unexplained thrombosis at a young age (as part of the panel).
- Dose optimization for folate-antagonist drugs like methotrexate (under oncology/rheumatology supervision).
- Generally NOT needed: routine preconception workups, general checkups, fatigue complaints.
My Result Is TT (Homozygous) — Now What?
No panic. The pathway:
- Measure plasma homocysteine, folate and B12.
- If levels are normal, no action is needed; a balanced diet suffices (leafy greens are natural folate sources).
- If homocysteine is high, your physician plans folate (usually active 5-MTHF) supplementation and follow-up.
- If planning pregnancy, start folate support after consulting your physician — standard for every pregnancy.
Conclusion
MTHFR variants are common genetic differences usually without clinical significance. The result is meaningful when interpreted with homocysteine and clinical context. A standalone "MTHFR positive" finding requires no treatment.