Steve,
My herbal books show that St. Johns Wort is contraindicated in ITP. I could only find one web reference and it's a wiki source (really not the best source). However, it does jive with what my herb books are saying.
wiki.plantstemcells.net/index.php?title=St._Johns_Wort_%E2%80%93_Hypericum_Perforatum#Hematology
You might consider instead adding in Vit. D3 (5000mg) to your diet if you live in the northern hemisphere (I'm assuming you are stateside). Also liquid B complex vitamins can help as well. Neither is contraindicated in ITP and in fact, both can be very helpful with both mood and illness. NOW brand is a good quality brand that is not overly expensive.
www.luckyvitamin.com
is the best place I've found for stuff like this.
Hope this helps.
patti