To change the name listed on a passport only, no letter is required. Go to the US Passport Help Guide to learn about correct forms to complete and mail in.
To change the gender marker listed on a passport, a doctor's letter stating the person has undergone “clinically appropriate treatment” for gender transition must be provided to a location authorized to accept passport applications. Please see the attached template letter. The letter must be the original, on the doctor’s office letterhead, and signed (or co-signed) by a MD-level provider. This letter, with a completed form (DS-11) must be presented to the Passport Office in person, even if they would otherwise be eligible to renew by mail. Find a Passport Office near you.
For detailed information on how the process works, review the Transgender Law Center's Guide, ID Please.