Which of the following is an early sign of lithium toxicity?

Polyuria is considered an early sign of lithium toxicity due to the drug's effects on the renal system, specifically its influence on kidney function and the ability to concentrate urine. When lithium levels rise, it can interfere with the kidneys' ability to handle water, leading to increased urine output. This occurs because lithium has a similar ion exchange mechanism to sodium, which can disrupt the body's normal sodium and water balance.

In the context of lithium therapy, monitoring for signs of toxicity is crucial since elevated levels can lead to more severe complications. Increased thirst and subsequent polyuria are often among the first indicators that a patient may have elevated lithium levels, allowing for timely intervention before progressing to more severe symptoms of toxicity. Recognizing polyuria early can guide healthcare providers to assess lithium levels and potentially adjust the dosage or initiate other appropriate care measures.