Lithium Toxicity
Lithium is handled by the kidney like sodium and has a narrow therapeutic index, so anything that promotes sodium and volume conservation drives lithium retention and pushes levels into a self-perpetuating, neurotoxic range.
First principles
Renal handling explains why levels rise so easily
Lithium is a small cation that is freely filtered at the glomerulus and, like sodium, substantially reabsorbed in the proximal tubule. Because the kidney cannot distinguish lithium from sodium at this site, any state that increases proximal sodium and water reabsorption (dehydration, low salt intake, or diuretic use) increases lithium reabsorption in parallel, raising plasma levels even without any change in dose. This single fact is why lithium's narrow therapeutic index makes it so vulnerable to everyday physiological stress.
Educational content pending clinical review. Not medical advice.