One of the editors at one of the pubs I copy edit asked me to research whether it's best used as an adjective (a transgender person), a noun (Chris is a transgender), or whether it's "transgendered."
Webster's New World College Dictionary, which this newspaper group follows, say it's only an adjective and that "transgender" is preferred but "transgendered" is also okay. Merriam-Webster online agrees.
Dictionary.com gives it first billing as a noun but also lists it as an adjective. Plus it allows "transgendered."
American Heritage (via Dictionary.com) doesn't allow the spelling "transgender." It's "transgendered," say they, and it's only an adjective.