MODY (Maturity-Onset Diabetes of the Young) is a monogenic (single-gene) form of diabetes, inherited in an autosomal dominant pattern. Unlike classic type 1 (autoimmune) or typical type 2 (insulin resistance-driven), MODY results from a gene defect that impairs beta-cell function. Many patients are misclassified for years, which matters because treatment and prognosis differ by gene.

When should you suspect MODY?

  1. Think MODY in a person with:
  2. Diabetes diagnosed
  3. Non-obese or only mildly overweight (BMI often normal/near-normal)
  4. No ketosis or DKA despite years of disease
  5. Good control on low-dose oral therapy (sometimes diet alone)
  6. Negative autoantibodies (GAD, IA-2, ZnT8) and preserved C-peptide
  7. Family history of diabetes across ≥2 generations (autosomal dominant pattern)

Exam/clinic pearl: If a lean 20- or 30-something is still making insulin (C-peptide preserved), has never had DKA, and doesn’t look insulin-resistant—pause and consider MODY.

Why genetic testing changes everything

Confirms the diagnosis (prevents years of misclassification).

Tailors therapy:

HNF1A/HNF4A: switch from insulin/metformin to low-dose sulfonylurea (often dramatic HbA1c improvement).

GCK: usually no pharmacotherapy needed; avoid overtreatment.

Guides family screening: autosomal dominant → test first-degree relatives.

Impacts pregnancy management: particularly for GCK, where treatment depends on whether the fetus inherits the variant.

Which test to order? Request a MODY gene panel, or if phenotype strongly fits, start with HNF1A. In primary care, start with antibodies and C-peptide to justify referral/testing:

Autoantibodies: negative

C-peptide: detectable/preserved (outside the honeymoon period)