Sometimes the cause of an itch is obvious, such as an insect bite. However, in many cases, the cause of an itch is complicated and difficult to diagnose. Itching occurs when the nerves in the skin react to the release of chemicals such as histamine. The signals from these nerves are processed in the brain and perceived as itching.

Pruritus often results from an associated skin disease, such as chickenpox, hives, lichen planus, psoriasis or eczema. Skin infections and skin rashes are also common triggers. In some cases, itching results from dry skin (xerosis). Dry skin can be caused by atmospheric conditions (e.g., lack of humidity, cold air), aging or another source. Certain medications (e.g., hormones, narcotic pain relievers) and foods can also cause skin reactions that lead to pruritus. Pregnancy causes hormonal changes that may result in itching as well. This typically occurs in the second and third trimesters of pregnancy, and quickly subsides after delivery.

Internal diseases can also cause pruritus. Infestations of the skin with a parasite – such as in the case of scabies or head lice – may cause itching. When pigmented moles itch, they should be checked to ensure that the mole has not become cancerous. Other internal conditions that may cause pruritus include:

• Allergic reactions
  
  
• Shingles
  
  
• Blood disorders, such as anemia, polycythemia vera and multiple myeloma
  
  
• Kidney failure
  
  
• Liver disease, including hepatitis C and primary biliary cirrhosis
  
  
• Neurologic conditions, such as pinched nerves and strokes
  
  
• Thyroid disease, including both excessively high or excessively low thyroid hormone levels
  
  
• Cancers, such as adenocarcinomas, squamous cell carcinomas and Hodgkin’s disease

Other diseases, such as AIDS and diabetes, can also cause pruritus. Pruritus may also occur due to faulty processing of the itch sensation within the nervous system. In rare cases, pruritus is psychogenic (something that is mental or emotional rather than physiological in origin).