Losartan is an angiotensin II receptor blocker (ARB) widely prescribed to manage high blood pressure (hypertension). It is also used to treat heart failure and protect the kidneys in patients with type 2 diabetes and kidney disease. Hyponatremia is defined as a low concentration of sodium in the blood, specifically below 135 milliequivalents per liter (mEq/L). Sodium is an electrolyte essential for balancing fluid inside and outside the body’s cells and for proper nerve and muscle function. When sodium levels drop too low, water moves into cells, causing them to swell, which is particularly dangerous for brain tissue.
Losartan and the Clinical Likelihood of Hyponatremia
Hyponatremia is considered an infrequent or rare adverse effect when Losartan is used as a single medication. Clinical trials and post-marketing surveillance have not identified it as a common side effect associated with this specific angiotensin II receptor blocker. Most evidence linking Losartan directly to low sodium levels comes from isolated case reports detailing a probable relationship between the drug and the condition in specific individuals.
One case report documented severe hyponatremia in a 73-year-old diabetic patient taking Losartan monotherapy for a few months. The patient’s sodium level dropped significantly, and the condition resolved after the medication was discontinued. These reports underscore that while the risk is low for the general population, the potential for Losartan-induced hyponatremia does exist.
The risk is notably higher when Losartan is combined with a diuretic, especially a thiazide diuretic, which is a common combination for treating hypertension. Thiazide diuretics are well-known to increase the risk of hyponatremia by causing the body to excrete more sodium and water. A study involving a Losartan/hydrochlorothiazide combination reported cases of severe hyponatremia, highlighting the compounded risk of the two drugs working together. Combination therapy requires increased monitoring, though Losartan alone is a minimal concern.
How Losartan Affects Fluid and Electrolyte Balance
Losartan works by targeting the Renin-Angiotensin System (RAS), a complex hormonal cascade that regulates blood pressure and fluid balance. The active component is Angiotensin II, a hormone that normally causes blood vessels to constrict and stimulates the release of other hormones that promote sodium and water retention. Losartan specifically blocks the Angiotensin II Type 1 (\(\text{AT}_1\)) receptor, preventing Angiotensin II from binding and exerting its effects.
By blocking the \(\text{AT}_1\) receptor, Losartan causes blood vessels to relax, lowering blood pressure. This blockade also prevents Angiotensin II from stimulating the adrenal glands to release aldosterone. Aldosterone typically tells the kidneys to retain sodium and water. The resulting reduction in aldosterone activity leads to decreased sodium and water reabsorption by the kidneys, effectively promoting their excretion.
This mechanism helps lower blood pressure but creates the physiological potential for low sodium levels. Losartan’s action also inhibits the Angiotensin II-induced release of Antidiuretic Hormone (ADH), also known as vasopressin. ADH helps the body conserve water by increasing its reabsorption in the kidneys. Disrupting this balance can lead to the kidneys being unable to manage water excretion effectively, which indirectly contributes to the dilution of sodium in the blood.
Identifying Symptoms and Related Risk Factors
The symptoms of hyponatremia can range from subtle to life-threatening, depending on the severity and speed of the sodium drop. Mild hyponatremia (130–134 mEq/L) may cause non-specific symptoms such as headache, nausea, and general fatigue or malaise. As the sodium level drops further, symptoms become more pronounced, including confusion, drowsiness, and muscle weakness or cramps.
Severe hyponatremia, typically defined as below 125 mEq/L, can lead to serious neurological symptoms. These include seizures, loss of consciousness, and potentially coma, resulting from the dangerous swelling of brain cells. Patients taking Losartan, particularly those at higher risk, should be vigilant for new or worsening symptoms like persistent headache or increased confusion.
Risk Factors
Several factors significantly increase the risk of developing hyponatremia while taking Losartan.
- Advanced age: Older adults often have health conditions and age-related changes that affect sodium balance.
- Pre-existing conditions: Heart failure, kidney disease, or liver failure impair the body’s ability to regulate fluid and sodium levels.
- Concurrent diuretic use: Thiazides actively promote sodium loss, representing the most significant medication-related risk.
- Fluid loss: Conditions causing significant fluid loss, such as severe vomiting or diarrhea, can exacerbate the risk.
- Other medications: Certain other medications, such as some antidepressants, can also increase risk.
Regular monitoring of serum electrolyte levels is a simple but important action, especially when starting Losartan or adjusting the dosage, to detect low sodium levels early.