Internationally, I've flown to the Republic of Panama and Serbia while on T. I've also gone on several domestic fights. I never had an issue with the T or needles/syringes. I kept a letter from my prescribing doctor (basically saying my T was a legal prescription) with my supplies. I usually ask whatever agent is running the baggage scanner to hand-screen my T rather than running it through the X-ray machine; just a personal preference. The T will need to be screened in its own quart-size clear plastic bag since it is a liquid. They don't care about needles, but make sure everything is capped--nobody wants to get accidentally poked. I've never been asked to show identification to prove the T is mine, but it would still be a good idea to make sure the name on your passport matches the name on your prescription bottle.
There's really no point in putting off taking T so you don't have to travel with it--presuming T is something you'll be taking for the rest of your life, are you going to avoid traveling internationally for your entire life?
You might have an issue if you have to use one of those full-body scanners, though. Packers show up on those scans as 'foreign objects.' I managed to avoid the body scanners for my international flights but did have to use one on a domestic flight. I kept my packer/STP in my carry-on and there was no problem with the full-body scan. Happily, nobody watching the baggage scanner has ever freaked out upon seeing my plastic dick on their screen. If you need to get a pat-down (or opt out of the scan and request an 'enhanced' pat-down), be aware that they will tap your groin area until they feel resistance. If you pass as male (you'll know because you will be assigned a male agent), that could get awkward. I've heard that the best policy is to disclose anything unusual prior to the pat-down so they don't think you are trying to get something past them.
