Phenylketonuria (PKU) is an inherited metabolic condition where there is a defect in phenylalanine hydroxylase. This enzyme normally converts phenylalanine, an essential amino acid in dietary protein, in the body into tyrosine. Only a small quantity of phenylalanine is required to ensure normal growth. Where there is an enzyme block, due to the defect in phenylalanine hydroxylase, phenylalanine accumulates in the body tissues and affects the normal development of the brain causing learning difficulties learning disability.


Medical text written January 2004 by Dr D C Davidson, Consultant Paediatrician, Alder Hey Children’s Hospital, Liverpool, UK. Last updated July 2010 by Dr Mike Champion, Consultant in Paediatric Inherited Metabolic Disease, Evelina Children’s Hospital, London, UK.

What are the symptoms?

In the UK, infants with PKU are detected on newborn screening and commenced on treatment prior to the development of symptoms. Prior to screening being available, children would present with developmental concerns within the first year or so of life. This was associated with a mild odour, dry skin and reduced hair, skin and eye pigmentation in some patients.

How is it diagnosed?

The Guthrie test (heel prick test) is now done on all newborn babies at six to ten days of age and can identify PKU at an early age.

How is it treated?

The diet of an affected child is carefully controlled so that only the small amount of phenylalanine necessary for growth is given. With a phenylalanine-restricted diet children with PKU develop normally. It is imperative that women with PKU should be on a low phenylalanine diet before or from early pregnancy to reduce the risk of fetal abnormality.

Inheritance patterns and prenatal diagnosis

Inheritance patterns
Autosomal recessive. Genetic advice is available for families with the condition.

Prenatal diagnosis
May be made by genetic studies on chorionic villus samples in families already studied.

Is there support?

National Society for Phenylketonuria (NSPKU)

Helpline: 030 3040 1090

The Society is a Registered Charity in England and Wales No. 273670. It provides information and support to people with Phenylketonuria (PKU) and their families and carers. The Society promotes the care and treatment of PKUs and works closely with medical professionals. It organises events such as conferences and study days throughout the UK, and has a network of local support groups.

Group details last updated December 2014.

Information and support in the UK for metabolic diseases is also provided by Metabolic Support UK (see entry Inherited Metabolic diseases).

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