I have a few sample letters I was provided by a nurse case manager from my insurance.
I'm not sure if they'll be any help, but I think they might be especially if your doctor is the one prescribing you hormones?
A letter should obviously be on letterhead and signed by the doc.
The keywords in a letter are Diagnosis, Recommendation for Surgery, and Medically Cleared for any procedures [or specific procedure, if you'd like the letter to be specific.
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To Whom it May Concern:
I am writing in regards to my patient [client name], date of birth 00/00/0000, who has asked me to write a letter in support of [his/her/their] interest in reconstruction surgery as part of [his/her/their] ongoing anatomical gender transition from [gender] to [gender]. I have seen [client name] in support of gender transition since 00/00/00 and have been responsible for managing [his/her/their] hormone therapy since that time. I strongly support [client name]'s desire for surgery, as it is the necessary and appropriate next step in [his/her/their] transition process and will make [his/her/their] body as congruent as possible with [his/her/their] preferred sex. Per the World Professional Association of Transgender Health's (WPATH) Standards of Care, [client name] meets full medical necessity criteria for [his/her/their] desired surgery. [He/She/They] has[have] no physical or medical barriers to surgery and I communicate [his/her/their] clearance for surgery to [his/her/their] surgeon, [surgeon's name], by delivery of this letter.
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Hopefully this helps, I'm not experienced with this as I've not gotten my letters and I'm just starting on my journey to medically transition with hormones and surgery.