Anika, have you gone for a physical and had a full panel of bloodwork done? Lithium can mess with your thyroid and with your kidneys. And any time you have unexplained weight loss, that's a bad thing, and should get checked out by a doctor.

Go to your GP and ask for a full physical. You may need a referral to an internist to figure out what is wrong with you. But don't just attribute this to meds. You could have a serious underlying illness. Last time I was losing weight for no reason, turned out I had gallstones and pancreatitis. I ended up in the ICU, because it was so advanced when I found out. Don't delay in getting properly checked out by your doctor. When we are mentally ill, we tend to attribute everything either to our mental illness or to the meds we are on. But sometimes it's neither of those things, and it's a physical illness instead.

Anyway, at the least, you should have your kidneys and thyroid checked out since you say your health has declined since going on the lithium. Lithium can cause both kidney and thyroid failure. It's absolutely vital that you rule those two conditions out.