BH4D is an autosomal recessive hereditary disease that can be divided into five types (van Spronsen et al., 2021), 6-pyruvyltetrahydrotrexate synthase (PTPS) deficiency, dihydroterine reductase (DHPR) deficiency, guanylate triphosphate cyclization hydrolase (GTPCH) deficiency, trexin-4A-dimethoxylamine dehydrase deficiency (PCD), and sepiapterin reductase (SR) deficiency. Here, SPR is linked to hyperinsulinemic hypoglycemia, familial, 4.