You can stay on it long term. Even if your thyroid doesn't handle it that's not a big deal. The kidneys just need a lot of monitoring and appropriate responses if anything changes. After about 10 years I developed something called diabetes insipidus which meant that my kidneys weren't pulling electrolytes from my urine so I was peeing a ton of water and losing electrolytes. Often they stop the lithium for that but it helped me so much that I stayed on it and took a med to help. When I stopped lithium that went away.