On a supressive cycle (one with injectable androgens), nothing will keep your HPTA from shutting down. HCG will keep your testes working, but this still bypasses the pituitary's role.

You might be able to maintain most of your HPTA funtion if running a milder anabolic (Anavar, Tbol, Primo) with a SERM, but this is just speculation. I have never seen a study or anything on it.