Thyroxine is the synethetic stuff, but yes, it replaces what your body should be producing. The natural stuff (Armour in the US) is dessicated pig thyroid - ew, but meant to be better. But then, those Americans, they're terribly fussy.
The only side effects with thyroxine are if your dose is too high for you, which will make you hypERthyroid instead of hyPO - lots of energy on the plus side, but heart palpitations and other bad stuff on the negative side.
I find hypothyroidism a real grab-bag of symptoms, some you get, some you don't - I really had the lack of energy and tendency to depression, constipation (TMI), but not others like lack of sweating (unfortunately), and I don't think my weight gain has ever been unusual (for someone who likes cake a LOT, and drinks cider by the pint).
My energy levels are difficult to assess on thyroxine - I don't think I have dramatically more energy, but then I think if I wasn't on it, I wouldn't be able to cope with my life (part-time job and study, small child) - ie, I DO have more energy, but my life saps it more than it used to!
Oh, and if you do get diagnosed, you have free prescriptions for ever, for everything, not just your thyroxine, yay! (this was my GP - 'the bad news, there is something wrong, the good news, it's easily treated and you get free prescriptions!')