Leprechaunism is an autosomal recessive disorder of severe insulin-resistance, characterized by intrauterine and neonatal growth retardation, distended abdomen, decreased subcutaneous fat, acanthosis nigricans, genitomegaly, muscular hypertrophy, and lipoatrophy. Extreme resistance to insulin leads to severe hyperinsulinemia, postprandial hyperglycemia, and a characteristic fasting hypoglycemia. 

The severe insulin resistance seen in leprechaunism is due to mutations in the insulin receptor (INSR) gene located on 19p13.2. Recent studies have shown that along with the insulin receptor, functional abnormalities in the EGF (epidermal growth factor) receptor are also present in leprechaunism, which may contribute to the severity of the disease.

These mutations lead to production of inactive receptors, which are unable to carry out the uptake of insulin into the cells.