My therapist and I discussed the letter and this is what we came up with.
______________________
From: VP of division
To: Department staff
Subject: important team update
Date: Wednesday June 17, 2015
Team,
Our colleague who we have known as <old name> is undergoing gender transition and starting Thursday June 18 will identify and present as female at work.
Her new name is Kate. As of June 18 it would be appropriate that we use feminine pronouns when referring to her, such as she, her and hers.
Kate welcomes your support but personal questions of a medical nature are not appropriate.
I hope you all join me in supporting her in her journey.
Sincerely,
Boss
VP of stuff and other stuff
rest of signature
goes here
------------------------------
So that's what we're going with. HR wanted it short and simple. This is not going to be a big coming out party just a note that I am changing.