Other
While inherited deficiencies in uroporphyrinogen decarboxylase often lead to the development of PCT, there are a number of risk factors that can both cause and exacerbate the symptoms of this disease. One of the most common risk factors observed is infection with the Hepatitis C virus. One review of a collection of PCT studies noted Hepatitis C infection in 50% of documented cases of PCT. Additional risk factors include alcohol use disorder, excess iron (from iron supplements as well as cooking on cast iron skillets), and exposure to chlorinated cyclic hydrocarbons and Agent Orange.It can be a paraneoplastic phenomenon. Exacerbating factors
Alcohol
Estrogen
Iron
Hepatitis C virus

Pathogenesis
Porphyria cutanea tarda is primarily caused by uroporphyrinogen decarboxylase deficiency (UROD). Uroporphyrinogen decarboxylase occurs in nature as a homodimer of two subunits. It participates in the fifth step in heme synthesis pathway, and is active in the cytosol. This enzymatic conversion results in coproporphyrinogen III as the primary product. This is accomplished by the clockwise removal of the four carboxyl groups present in the cyclic uroporphyrinogen III molecule. Therefore, a deficiency in this enzyme causes the aforementioned buildup of uroporphyrinogen and hepta-carboxylic porphyrinogen, and to a lesser extent hexa-carboxylic porphyrinogen, and penta-carboxylic porphyrinogen in the urine, which can be helpful in the diagnosis of this disorder.The dermatological symptoms of PCT that include blistering and lesions on sun-exposed areas of the skin are caused by a buildup of porphyrin compounds (specifically uroporphyrinogen) close to the surface of the skin that have been oxidized by free radicals or sunlight.
Genetics
Inherited mutations in the UROD gene cause about 20% of cases (the other 80% of cases do not have mutations in UROD, and are classified as sporadic). UROD makes an enzyme called uroporphyrinogen III decarboxylase, which is critical to the chemical process that leads to heme production. The activity of this enzyme is usually reduced by 50% in all tissues in people with the inherited form of the condition.Nongenetic factors such as excess iron or partially genetic factors such as alcohol use disorder and others listed above can increase the demand for heme and the enzymes required to make heme. The combination of this increased demand and reduced activity of uroporphyrinogen decarboxylase disrupts heme production and allows byproducts of the process to accumulate in the body, triggering the signs and symptoms of porphyria cutanea tarda.The HFE gene makes a protein that helps cells regulate the absorption of iron from the digestive tract and into the cells of the body. Certain mutations in the HFE gene cause hemochromatosis (an iron overload disorder). People who have these mutations are also at an increased risk of developing porphyria cutanea tarda.In the 20% of cases where porphyria cutanea tarda is inherited, it is inherited in an autosomal dominant pattern, which means one copy of the altered gene is sufficient to decrease enzyme activity and cause the signs and symptoms of the disorder.