The SSA policy now permits a transgender person to change their gender marker upon submission of
one of the following types of documents:
A letter from the person's doctor confirming that the person has undergone "appropriate clinical treatment for gender transition";
A full-validity, 10-year U.S. passport that includes the person's correct gender;
A state-issued birth certificate that shows the correct gender; or
A court order recognizing the person's gender.
More at <
http://transgenderlawcenter.org/archives/8339>