Symptoms of phenylketonuria  

Most babies with phenylketonuria (PKU) appear healthy at birth.

Symptoms usually only develop because of complications that arise if the condition isn't treated properly.

Treated PKU

If treatment is started within the first three weeks of life, people with PKU shouldn't experience the severe learning difficulties associated with the condition.

Some children and adults with PKU experience mild to moderate problems with attention, planning or problem solving. These problems can be related to an increased phenylalanine level in the blood and may improve with stricter dietary treatment.

There's some evidence that adults with PKU may be more likely to develop mental health problems at some point compared with the population at large.

The most widely reported conditions in people with PKU are:

  • depression  feelings of extreme sadness that last for weeks or months
  • anxiety disorders  feelings of unease, such as worry or fear, that can be mild or severe
  • phobias – particularly agoraphobia (intense fear of crowded places or enclosed public places)
  • low self-esteem 

It's not clear whether the increased risk of developing these conditions is related to physical problems, such as a reduction in the neurotransmitters (brain chemicals) that affect mood, or psychological problems, such as worrying about PKU and the low-protein diet. 

Read more about treating phenylketonuria.

Untreated PKU

A wide range of symptoms can occur if treatment for PKU isn't started or is delayed. The most common is a learning disability. Usually, the more treatment is delayed, the more severe the disability becomes.

Even if PKU isn't diagnosed during newborn screening, it's worth starting dietary treatment later in life, because it's sometimes possible to reverse some of the decline in intelligence.

For example, in one reported case, a child’s treatment was delayed until they were two years old. They went from having a moderate learning disability to having average intelligence.

Other symptoms of untreated PKU include:

  • behavioural difficulties – such as frequent temper tantrums and episodes of self-harm
  • fairer skin, hair and eyes than siblings without the condition (phenylalanine is involved in the body's production of melanin, the pigment responsible for skin and hair color)
  • eczema 
  • recurrent vomiting
  • jerking movements in arms and legs
  • tremors 
  • epilepsy
  • musty smell on the breath, skin and urine

Page last reviewed: 02/10/2014

Next review due: 02/10/2016