With Lithium, you need periodic bloodwork,pdoc's will or should include thyroid function, while I was on Lithobid (is great stuff)my thyroid, as I kept saying yrs before I went on this med, that I was hypothyroid, always was told "results are within normal range" bla!bla! But during one of the workups, it finally showed I was not imagining the syymptoms, sooooooooo I take Synthroid, in fact even though I switched to Lamictal, which doesn't cause weight gain, or thyroid stuff, I need to have my GP bump up my thyroid med., my symptoms besides the menopause ones are back. Sorry, got off track, while on Lithium, have the doc check for hypothyroidism. When I first was DXed with Bipolar-II,in 2000, they gave me Depkote, that made me get fat instantly and so tired I could barely function, the Lithium weight gain was minimal, and after being on the Synthroid it worked out fine. If for some reason Lamictal should ever no longer work for me, I'll go back to Lithium any day.
I hope you have luck with this.
Take care,
DE