- The electrolyte imbalance characterized by a decreased concentration of sodium in the bloodstream is called Hyponatremia.
- Sodium is a key electrolyte that helps maintain fluid balance, nerve function, and muscle contraction.
- When sodium levels fall below the normal range (less than 135 mEq/L), it leads to hyponatremia.

- Hypernatremia refers to an elevated sodium concentration in the blood, typically above 145 mEq/L, and is the opposite condition.
- Hyperkalemia indicates an increased potassium level, while hypocalcemia describes a decreased calcium level. Neither of these involves sodium imbalance.

Therefore, the correct answer is Hyponatremia, as it specifically describes a low sodium concentration in the bloodstream.

Reference: Harrison's Principles of Internal Medicine, 20th Edition, Volume 1, Chapter 28: Disorders of Water and Electrolyte Balance