A simple itch on the scalp that fades away even if you just ignore is not a thing to worry about, but if the itch persists for more than a couple of days, consulting a doctor may be necessity as it may signify underlying medical conditions.
There are actually 19 medical conditions which may be the cause of itching of the scalp and some of it the following:

• Dandruff.
  This is a very common condition characterized by flaking and itching of the scalp.
  This is basically not a severe condition but it may possibly lead to bacterial and fungal infections.
  
• Sunburn.
  This is a condition generally defined as an injury to the skin as a result of too much exposure to ultraviolet radiation.
  Unlike the skin on other parts of the body, sunburn on the scalp manifests itself with itching instead of pain and blisters.
  
• Psoriasis.
  Psoriasis is a lifelong medical condition characterized by flaking of the skin of the entire body due to the increased production of new skin tissues.
  This condition manifests itself first as an intense itching on scalp with thin glossy flakes falling off the head.
  The flaking will eventually start to go down passing through the hair line to the other parts of the body.
  
• Head lice.
  This is a very common condition throughout the world characterized by the infestation of a parasitic insect on the head.
  These parasites feed on the blood of its hosts by piercing its skin which causes the itchiness.
  The mere presence of the gray parasite on the hair and scalp confirms infestation.
  Other signs and symptoms include red piercing marks on the scalp and presence of white nits on the hair and scalp.
  
• Eczema.
  This condition is a result of the over sensitivity of the skin to certain substances.
  Eczema is long term medical condition in which the skin including the scalp becomes red, inflamed and itchy.
  It is a fairly widespread disorder affecting 10 to 20 percent of the world's population.
  

Itching, like pain is a mechanism of the body to tell you that something is wrong, so never underestimate a simple itch that might be caused by 19 medical conditions that has the capacity to endanger your life.