tyrosinemia type iii tyrosinemia type iii tyrosinemia type iii is a rare disorder caused by a deficiency of the enzyme 4-hydroxyphenylpyruvate dioxygenase (ec 1.13.11.27), encoded by the gene hpd. this enzyme is abundant in the liver, and smaller amounts are found in the kidneys. it is one of a series of enzymes needed to break down tyrosine. specifically, 4-hydroxyphenylpyruvate dioxygenase converts a tyrosine byproduct called 4-hydroxyphenylpyruvate to homogentisic acid. characteristic features of type iii tyrosinemia include mild mental retardation, seizures, and periodic loss of balance and coordination (intermittent ataxia). type iii tyrosinemia is very rare; only a few cases have been reported. pathophysiology of metabolic disorders of tyrosine, resulting in elevated levels of tyrosine in blood. tyrosinemia type iiiother namestyrsn3tyrosinespecialtyendocrinology