what symptoms would correlate with it

The short version: the retrieved evidence does not directly test whether MTHFR variants produce a specific symptom checklist, but it does show that MTHFR variation alters brain chemistry, inflammation, and methylation in ways that plausibly connect to ADHD and autism symptoms.

The 2021 review found that the MTHFR C677T variant is linked to higher autism risk in several studies, especially when combined with maternal genotype effects and higher homocysteine [1]. The same review notes that DHFR and CBS variants also associate with ASD, and that cord blood folate patterns differ by ethnicity [1]. This means MTHFR variation does not cause a single symptom but shifts the odds of a broader neurodevelopmental pattern.

The 2026 mouse study is the most direct mechanistic evidence here. Mice carrying the Mthfr 677C>T variant on a late-onset Alzheimer's background had higher plasma homocysteine, altered brain cytokines (higher IL-10 in females, higher IL-12p70, IL-2, IL-5, IL-6, and TNF-alpha in males), and brain molecular signatures related to cerebrovasculature, myelination, and synaptic biology — all without amyloid plaques or cognitive deficits [2]. That is a prodromal signature: the variant changes the brain's molecular environment before any outward symptom appears.

For ADHD specifically, this retrieval has no study that directly links MTHFR to ADHD symptoms. The 2024 metformin study found no association between MTHFR C677T and neuropathy in 103 diabetes patients [4], but that is a different population and outcome. The 2023 narrative review proposes that MTHFR-driven methylation links Long Covid, chronic fatigue, POTS, and mast cell disorders [3], but it is a review, not a primary study, and its evidence units were not extractable.

My call: MTHFR variation is a plausible contributor to the biological substrate of autism and possibly ADHD, but the evidence here does not support a specific symptom profile. The strongest finding is that MTHFR C677T changes brain inflammation and methylation without producing immediate cognitive symptoms — meaning the effects may be subtle and cumulative. Confidence: low — the retrieval lacks any study that directly measures symptom patterns in people with MTHFR variants and diagnosed ADHD or autism.