More fields in letter?

When sending GP letter and further referrals, I need some additional fields inserted into my GP Letter template.

At the moment the only information that can be added is patients name, DOB and address and GP name and address. I would like to add information from my case record for a particular patient into these letters.

i.e. the date the case record was created, I also have a couple of custom fields on the case records

area_of_treatment
diagnosis

I would like this information to be displayed in the letters also? Please help.