Consider this item from the GLAAD Media Reference Guide:

PROBLEMATIC: "transgendered"
PREFERRED: "transgender"

The word transgender never needs the extraneous "ed" at the end of the word. In fact, such a construction is grammatically incorrect. Only verbs can be transformed into participles by adding "-ed" to the end of the word, and transgender is an adjective, not a verb.

I hesitate to criticize what is in general a very useful guide, but on this issue, the guide is simply incorrect: 'transgendered' is clearly grammatically correct and 'transgender' is the term whose grammatical status is in question. It is certainly true that some transgendered people use 'transgender' as an adjective to describe themselves or others, but a review of the above will show that this is, strictly speaking, grammatically incorrect.

There are adjectives that we use to describe people that do not end in 'ed,' including 'Norwegian' or 'Chinese,' 'masculine' or 'feminine,' and 'gay' or 'lesbian,' not to mention 'bisexual.' But to point this out is simply to point out the diversity of adjectival constructions in the English language; in itself, it does not constitute an objection to the use of 'transgendered' as an adjective.