What causes blood platelets to be low?

My son has broken blood vessels in his legs which looks speckled and discolored. He went to the doctor and got several blood test done. They determined he has hepatitis c and low blood platelets and is taking Levothyroxin 50 MCG to help raise his platelets. They just doubled it from 25 MCG. He has just gotten another blood test and is waiting for the results.