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Glycogen storage disease type I (GSD-I), also called von Gierke disease, is an inherited disease caused by a defect in the body’s ability to break down glycogen (the form in which the body stores sugar) to glucose (a free form of sugar and the body’s main source of energy). Symptoms associated with GSD-I are attributed to low blood glucose levels and excessive storage of glycogen in the liver and kidneys. GSD I occurs in two forms: GSD-Ia and GSD-Ib. GSD-Ib is caused by a deficiency of the enzyme glucose-6-phosphate transporter (G6PT) whose function is to help maintain normal blood glucose levels. View testing options