Why is tyrosine supplementation used in treatment for phenylketonuria?

In individuals with phenylketonuria (PKU), the enzyme phenylalanine hydroxylase is inactive or deficient, preventing the normal conversion of the amino acid phenylalanine into tyrosine.12

Because tyrosine is a vital precursor for the production of important neurotransmitters—such as dopamine—as well as the pigment melanin, the metabolic block in PKU leads to a deficiency of this amino acid in the body. Tyrosine supplementation is therefore used to ensure that the body has adequate levels of this essential precursor for normal physiological function, particularly in the central nervous system and for pigment formation, while patients simultaneously maintain a strict, low-phenylalanine diet to prevent the toxic accumulation of phenylalanine.12