Which condition is characterized by low sodium levels in the blood?

The condition characterized by low sodium levels in the blood is hyponatremia. This condition occurs when the sodium concentration in the serum falls below the normal range, which is typically around 135-145 milliequivalents per liter (mEq/L). Hyponatremia can result from various factors, including excessive fluid intake, certain medications, or underlying medical conditions affecting the body's balance of sodium and water.

Understanding hyponatremia is essential because it can lead to neurological symptoms due to the brain's response to swelling, as sodium plays a crucial role in maintaining fluid balance and cellular function. Symptoms can range from mild (nausea and headache) to severe (confusion, seizures, and coma) depending on how rapidly the sodium levels decline.

In contrast, hypernatremia refers to elevated sodium levels in the blood, hyperkalemia involves high potassium levels, and hypocalcemia is characterized by low calcium levels—all distinct conditions unrelated to sodium levels.