Sodium and volume depletion due to any conditions like vomiting, diarrhoea, febrile illness, renal insufficiency, excessive exercise, water restriction, excessive sweating, low sodium diet, and congestive heart failure may enhance lithium reabsorption in the kidneys. Furthermore, drugs reducing the glomerular filtration rate might inflict chronic toxicity. The kidneys treat lithium and sodium similarly which is the reason sodium depletion can significantly elevate lithium reabsorption. Serum lithium concentrations are monitored to assure patient compliance and to avoid intoxication. Early symptoms of intoxication include drowsiness, speech difficulties and twitching.

According to the Irish Medication Safety Network, Best Practice Guidelines on Lithium Prescribing, they advise that a serum sample should be taken 12 hours after the last dose to get a more accurate serum level. They also recommend that the minimum effective plasma level for prophylaxis is 0.4mmol/L with the optimal range being 0.6-0.75 mmol/L and that toxic effects reliably occur at levels >1.5mmol/L. Levels should be checked every 3 months routinely, with increased testing at initiation and if any dose adjustments are made to ensure levels are within normal range.

https://imsn.ie/wp-content/uploads/2019/11/imsn-lithium-guidelines-may-2012.pdf