Biotinidase deficiency is an inherited metabolic disorder in which the body cannot process the vitamin biotin due to a deficiency in the enzyme biotinidase. Children affected with biotinidase deficiency can experience seizures, poor muscle tone, difficulty with movement and balance, developmental delay, vision problems, hearing loss, and skin abnormalities. There are two forms of biotinidase deficiency; a profound form with severe symptoms and a partial form with milder symptoms. Both forms of biotinidase deficiency can be successfully treated with lifelong treatment of supplemental biotin pills. Daily biotin medication prevents symptoms from appearing and often can reverse some damage caused by the disease. However, developmental delay, vision and hearing loss, if present, are irreversible. Biotinidase deficiency is caused by pathogenic variants in the BTD gene.