What is phenylketonuria?

Phenylketonuria, or PKU, is a rare genetic disorder. People with this disorder lack a substance in the body called phenylalanine hydroxylase (PAH). This substance is an enzyme needed to break down an amino acid called phenylalanine. Phenylalanine is a ‘building block’ that we get from our food that is required for proper growth and development. Because people with PKU lack the enzyme PAH, the amino acid, phenylalanine builds up in the blood, brain and other tissues of the body. When phenylalanine builds up, it can damage the body and harm the brain. If untreated, PKU can eventually cause serious problems including intellectual disabilities, seizures, and behavioral problems. The main form of the disorder, classic PKU, is usually identified through newborn screening so treatment is often started before symptoms begin. People get phenylalanine mostly through what they eat. Phenylalanine is found in all foods containing protein and in some artificial sweeteners. . PKU is treated by a low-protein diet and the use of Medical Foods (including specialized formulas) that contain very little or no phenylalanine. PKU is caused by changes (alterations) in the PAH gene and is passed on in a family in an autosomal recessive manner. There is a mild form called non-PKU hyperphenylalaninemia. These people have a mild buildup of phenylalanine in the body, but often do not require treatment.

SOURCE: Emory University - Department of Human Genetics in collaboration with ThinkGenetic • • DATE UPDATED: 2016-07-07


Mitchell JJ. Phenylalanine Hydroxylase Deficiency. GeneReviews website. Accessed June 5, 2016.

Phenylketonuria. Genetics Home Reference website. Accessed June 5, 2016.

Contact a ThinkGenetic Counselor

Do you have additional questions that haven't been answered? Ask a ThinkGenetic Counselor — a real expert in the field of genetics. Just fill out this form to send an email. We will be in touch within 48 hours.

Please sign me up for your mailing list.

This content comes from a hidden element on this page.

The inline option preserves bound JavaScript events and changes, and it puts the content back where it came from when it is closed.

Remember Me

Welcome to the ThinkGenetic beta. More content is added weekly so come back and check often. Please provide your feedback by filling out a contact us form.