This was adapted from a report I wrote as a final exam for my genetics class.
Porphyrias are a group of diseases which are all deficiencies of any of the eight enzymes involved in heme synthesis. One of the major causes of symptoms in patients with porphyria is the accumulation of heme precursors. To make heme, the body needs some raw materials in addition to the eight enzymes. Heme procursors collect mainly in the marrow and liver. The main heme precursors that cause problems in porphyria are toxic metabolites: delta-aminolevulinic acid and porphobilinogen, which are neurotoxins, and porphyrins. These precursors are transferred from the tissues where they accumulate into the blood plasma and from there they are excreted in urine and stools. Accumulation of these precursors can cause photosensitivity and nerve damage, even paralysis.
The most common porphyrias are porphyria cutanea tarda (PCT), acute intermittent porphyria, and eryhtropoietic protoporphyria. These are very different diseases, with distinct symptoms, tests needed for proper diagnosis, and treatments. Additionally, they all share some symptoms with rarer porphyrias. Thus, porphyria is often misdiagnosed. Test results from patients with porphyria will be markedly abnormal, but one needs to select and interpret tests with porphyria in mind.
All porphyrias except for one are hereditary. PCT is not inherited directly. However, suspectibility to PCT is often inherited, in which case it is called familial (type II) PCT.
There are several systems for classifying porphyria. The preferred system is to classify by specific enzyme deficiency. Deficiencies of different enzymes are responsible for different porphyrias. There is also a system which breaks porphyrias down into acute porphyrias, which manifest themselves primarily via neurologic symptoms, and cutaneous porphyrias, which cause photosensitivity. In addition, there is a third system which splits up the diseases into hepatic porphyrias, where the heme precursors originate in the liver, and erythropoietic porphyrias, where they originiate in the marrow.
See my Porphyria Cutanea Tarda writeup for bibliography.