Great advice from
@helli. I too have travelled quite a lot to the US and never had any problems or questions about my diabetes supplies. Always taken them in carry on (too cold in the hold with checked in baggage for insulin, plus you need easy access to it during the flight).
I carry all my diabetes supplies in my camera backpack, along with camera gear, which I take as carry on, maybe make sure your insulin pens, needles and BG meter (if you’re using one instead of the Libre) are in a small pouch or wallet that you can tuck into the seat pocket to save having to rummage through the overhead lockers. And don’t forget something like glucose tablets for hypo treatments just in case.
I even had to buy a new finger pricker on one visit as mine broke on a US trip and that was so easy just getting it over the counter in a Walgreens pharmacy. Now travelled with the Libre and that too was fine, just remember to mention it at airport security and ask for an alternative to a full body scan and for your spares not to go through the X Ray with your other cabin baggage.
I have never been asked for a letter as proof of having diabetes and needing the supplies with me, but as
@helli says, it is good insurance and free from your hospital DSN / Consultant. One time rushing through Dubai airport for a connecting flight I had airport security open up my insulin pen wallet and check through it, but nothing was said and they let me through without me having to explain anything.
Good luck on your travels!