Aetiology

Understanding of Porphyria cutanea tarda has been hampered by the existence of several forms of the disease and by confusion in earlier studies with Variegate porphyria.

Two broad categories are now agreed upon:

• PCT symptomatic or sporadic - resulting from exposure to certain drugs and chemicals
  
  
• PCT hereditary - those individuals who have other family members with the disease and in which there is no obvious environmental exposure. Probably inherited as an autosomal dominant

Uroporphyrinogen decarboxylase is deficient in both the liver and erythrocytes in the hereditary form of PCT but is deficient only in the liver in the non-hereditary form.