This case-sensitive code system http://hl7.org/fhir/us/cdmh/CodeSystem/pcornet-gender-identity defines the following codes:

Code	Display	Definition
M	Man	Gender identity is a Man.
F	Woman	Gender identity is a Woman.
TM	Transgender male/Transman/Female-to-male	Gender identity is Transgender male or Transman or Female-to-Male.
TF	Transgender female/Transwoman/Male-tofemale	Gender identity is Transgender female or Transwoman or Male-to-female.
GQ	Genderqueer/Non-binary	Gender identity is GenderQueer/Non-binary.
SE	Something else	Gender identity is Something else.
MU	Multiple gender categories	Gender identity is Multiple gender categories.
DC	Decline to Answer	Person has declined to answer about their gender identity.
NI	No information	There is no information about the Person's gender identity
UN	Unknown	Person's gender identity is unknown.
OT	Other	Person's gender identity is Other.