Hi Gale,
PTSD and Depression are two distinct disorders. Depression is a brain chemistry issue where some neurotransmitters become overused and deplete the brain of their use. This is why reuptake-inhibitor medications work.
PTSD is an entirely different brain issue and is not impacted by SSRI's.
The fact that they occur comorbidly is simply because trauma causes them both. In light of this, I doubt your psychiatrist will write that letter.
Nancy
Psychotherapist
ABD for a PhD in Psychology, Psychotherapist for over 20 years