In Part>> The disorders that most commonly induce thrombocytopenia (platelet
deficiency) are drug reactions (cephalosporins can cause these), cancers,
infectious illnesses — especially ehrlichiosis , systemic lupus
erythematosus, rheumatoid arthritis and von Willebrand’s disease. It is
important to consider all of these possibilities in a dog thrombocytopenia.

If no primary cause can be identified then the condition is referred to as
idiopathic thrombocytopenia (ITP). Since this diagnosis is made by ruling
out disorders that are sometimes hard to find, it is always suspect.
Rechecks of previous tests are often important as diseases or disorders
that lead to thrombocytopenia are sometimes easier to find at one time
than another.

Several case histories and treatments on the above site.
References :
dog breeder since 1968