You are allowed to bring syringes on aircraft. I average about five air roundtrips a year an am diabetic and have both syringes and pen needles and have never had any problem. I have a "medical backpack" which carries my CPAP, Blood Pressure Meter, Blood Sugar tester, all my pills and liquid medications. And the liquids are in a Frio Cold PAck to keep them at cooler temeratures. And since it is all medical in the pack it does not count against any luggage limitations.
Additionally, I would suggest going to
www.coolerconcept.com and looking at the Frio Cold Packs. There are several people in this forum, myself included, who use them.
These are gel packs that are water activated. They will keep the internal temperature at insulin-safe levels for over 40 hours. As the water evaporates you can just soak it in water for a few minutes to reactivate it. They come in various sizes.
For example, I have a "double wallet". Depending on what I am doing it will hold my Byetta pen as well as either a syringe with insulin in it or two insulin vials. And I have never had any problems with TSA over it.