Health Topics

Urea Cycle Disorder (UCD)

A urea cycle disorder (UCD) is an inherited disease caused by the lack of an enzyme needed to break down ammonia in the body. Ammonia is a waste product that must be removed by the body.

Everyone needs protein, which is found in foods like dairy products, meat and fish. When a person eats food that contains protein, the body breaks it down into amino acids (the building blocks of protein that are used by the body for growth and tissue repair) and uses only what it needs. It changes the rest into ammonia, which must then be removed by the body.

In a healthy person, the liver supplies several enzymes to break down the ammonia into urea, which is then removed from the body in urine. This entire process is called the urea cycle, and it occurs in liver cells.

The liver in a person with urea cycle disorder is missing an enzyme necessary to convert ammonia into urea. As a result, ammonia, a highly toxic substance, builds up in the bloodstream and is not removed from the body. Untreated, the high amounts of ammonia can cause brain damage, coma and eventually death.

These urea cycle disorders are named based on the initials of the missing enzyme. They are:

  • OTC – ornithine transcarbamylase 
  • ASD – argininosuccinic acid synthetase (citrullinemia) 
  • AG – arginase 
  • ALD – argininosuccinase acid lyase (argininosuccinic aciduria) 
  • CPS – carbamoyl phosphate synthetase 
  • NAGS – N-acetylglutamate synthetase.

Urea cycle disorders occur in about one in 30,000 newborns.

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Causes of UCD

Signs and Symptoms of UCD

Diagnosis of UCD

Treatment for UCD

Long-Term Outlook


Last Updated: 07/2012