The diagnosis of hyponatremia involves assessing levels of sodium in the blood. A healthy sodium level is between 135 and 145 mmol/l and a person is considered to be hyponatremic if the level falls to below 135 mmol/l. The hyponatremia is considered severe if this level falls to below 125 mmol/l.

While evaluating a patient, a doctor will also try to establish the type of hyponatremia. Osmolality and urine sodium tests are performed to help diagnose the underlying cause of the condition.

Types of hyponatremia

The different types of hyponatremia are described below:

Depletional hyponatremia

Depletional hyponatremia refers to reduced levels of fluid and salt, as seen in cases of gastrointestinal loss through recurrent vomiting or diarrhea, for example.

Dilutional hyponatremia

A hyponatremic state can also be caused by blood becoming too dilute. Examples of causes include the administration or intake of too much fluid and the excessive release of antidiuretic hormone by the pituitary gland, referred to as syndrome of inappropriate antidiuretic hormone (SIADH). SIADH can occur as a result of several underlying conditions including major infection, heart disease, respiratory disease and small-cell carcinoma of the lung. The use of certain medications can also cause this syndrome and, in turn, dilutional hyponatremia.

There are several biochemical tests that can help distinguish between these two types of hyponatremia. Serum potassium, urea and albumin levels are often reduced in dilutional hyponatremia and the blood pressure is often normal or high. In cases of depletional hyponatremia, the blood pressure is often low.

Some of the tests that may be run to help determine which form of hyponatremia a person has include:

• Blood urea, potassium, calcium, magnesium and phosphate
• Plasma and urine osmolality. In dilutional hyponatremia, the plasma osmolality is lower than normal.
• Blood glucose, which may be raised in cases of hyponatremia
• Thyroid function tests
• Urine sodium level
• Cardiac investigations may be performed to check for heart failure

SIADH

In order to check whether hyponatremia may be caused by SIADH, a person is assessed for the following features of the syndrome:

• Normal blood volume
• Normal kidney, adrenal and thyroid function
• Raised urine sodium
• Urine osmolality of less than 100
• The patient is not taking any medication that could cause hyponatremia such as diuretics