Lamictal has less of a potential for weight gain. As for the effects of specific medications on kidney function you'd have to ask your doctor about that. That is one of the side effects monitored for with lithium with the monthly blood test and not as much of a problem with other mood stabilizers but every medication has its specific set of side effects. Lithium does have a fair amount of side effects but its also highly effective. I personally found Lamictal quite effective for rapid cycling but every person has their own experience. We have good websites posted for information purporses on the various mood stabilizers clinically available. There are no natural remedies that are effective mood stabilizers. The one you will read about fish oil can be administered by psychiatrists as a supplement (in addition to medication) but the FDA approved anti-cholesterol medication Lovaza works exactly like it and is used experimentally for bipolar. We have this posted but to make it easier here's a full list of what is available and FDA approved:
http://www.psycheducation.org/depression/meds/moodstabilizers.htm
I suffer the same concerns and I also have the same side effects as you.
For this reason I have studied lithium intensely and I now know that lithium has a vast and overwhelming scientific study advantage over all other drugs in this field.
Lithium requires diligent monitoring and periodic organ function testing (any deterioration will happen slowly)
Using "natural" frontline treatments is irresponsible in my view.
There are strategies to deal with lithium side effects and I am happy to forward info if you pm me.
I don't know about weight neutral meds, but there are other mood stabilizers out there like depakote, lamictal and others that you can look up on WebMd or other bipolar sites.
I also am on Lithium for BP. I have been on it for about 5 years and have had no problems with either kidney or liver functions. If it works for you, don't give it up. It is one of the best things that I take. It does cause me to shake a little but I put up with it because it works so well.