it's never been proven that they desensitize, it's just thought this might happen by some doctors. they stay away from high doses just in case this happens. most of the effect would be short term and once you lowered the dose or came off, they would re-sensitize. but it's possible they wouldn't ever be quite as sensitive as they were before. no one really knows it's all just speculation.
if you stay on 500 iu's per injection or less and don't inject more than eod you should be fine even for long term hcg usage. that said most people report better results by combining hcg with trt. so i recommend getting on trt anyway. this is what the top trt practitioners advise.
what are you using hcg for? how long have you been on it and at what dosage?
and as always, bloodwork has the final word. if you're worried about low t many things could cause this so you'll need to get checked and maybe go on trt anyway.