Harry, my answer is a qualified yes. Sewing thread comes in many flavors including rayon, nylon, polyester, cotton, and blends of these.

"Standard" sewing machine thread is quite a bit larger in diameter than the 6/0 or 8/0 thread commonly used for tying flies. So it wouldn't be easy tying small flies. I have used various nylon and rayon threads as body material and ribbing. It might be useful for larger flies as tying thread, but I personally haven't used it that way.

You can find sewing threads in somewhat smaller diameters at specialty fabric stores. Serger thread, bobbin thread, and "extra-fine" are all a little smaller diameter.

I've never seen a sewing thread designated light Cahill, though.

Good luck!

Jim