The Department of State, where passport applications are processed, recently made a change to allow for applicants to choose F or M as their gender marker with no additional "proof." What this means is that when applying for a US passport, you will designate your gender marker without having to show a medical professional's letter. This is a great step forward, but it's only a start.
USCIS and Gender Markers
USCIS applicants, whether family-based or business-based filings, are still required to present a professional's letter if their gender marker on the immigration application is not the same as seen on the birth certificate. No medical procedures are required, nor should they every be asked about, but a letter from a designated professional will be required in an application to have your gender appropriately designated on your final immigration documents. The professionals able to write such a letter consist of medical doctors, nurse practitioners, licensed social workers and therapists, among a few others.
Third Gender Choice
Ideally, USCIS will expand their gender options beyond the traditional and outdated F and M designations. The US passport application will eventually add a third choice, but the Department of State has not yet committed to a date to do so. It is certainly a step in the right direction, but these efforts towards equality are not complete.