A newborn with glycogen storage disease type Ib presenting with hypoglycemia and evidence of mitochondrial dysfunction.

Glycogen storage disease type Ib (GSDIb) is an inherited disorder of carbohydrate metabolism that commonly presents in infancy. Here, we describe a case of GSDIb who presented on the first day of life with severe hypoglycemia and lactic acidosis, prompting a broad differential diagnosis. Additional testing revealed hyperlipidemia, relatively low ketones, and elevations of TCA-cycle intermediates in urine suggestive of primary or secondary mitochondrial dysfunction. The diagnosis of GSDIb was established by genome sequencing showing homozygous likely pathogenic variants in SLC37A4. This case highlights the importance of including GSDIb in the differential diagnosis of neonatal hypoglycemia and reinforces the underreported metabolic pattern of mitochondrial dysfunction that can be seen in GSDIb.