I did read somewhere about the difference between MS and HG being about how much weight loss you experience (as a % of body weight) but I don't know, having been through it I would think there is more too it that just weight loss. I think it was if you lost more than 5% of your body weight you had HG???....I lost over 7% in 1 week, I put my shoes on and could feel they were looser. That is just plain wrong!

But as the others have said hydration is very key to keeping yourself on the right side of OK. Things like hydralyte, gastrolyte or gatorade are good (frozen if you can't keep it down as a liquid, I think it's hydralyte that comes as in iceblock) and going to get hydrated at your hospital. Go and see a GP and work out a plan for how to manage it. With no. 2 I had a plan of start with maxalon, then if going X hours without keeping anything down either into GP or call locum for after hours. Next step was going to the local ER, my GP gave me a letter to present to ER to explain the situation and that I'd already done the first several steps.
But if you have gotten really bad it is probably too late for the at home hydration methods, go and get hydrated in hospital and then once you get back to a better level of hydration the hydralite etc. can be used to try and maintain that good level. If you don't get properly hydrated you end up in a hole that you just can't dig your way out of.

Good luck, it is so horrible