Sodium and water balance disorders are closely related, as sodium plays an important role in the movement of water into and out of cells. Water balance disorders arise when there is an imbalance in the body’s water content, presenting problems with thirst or the production of too much urine. Although common, such disorders are often poorly controlled or undiagnosed.

Many thirst disorders are difficult to diagnose, particularly when they are not severe. An endocrinologist can help to differentiate between problems where the patient produces too much urine – diabetes insipidus – and is thirsty to compensate, and those where the patient drinks too much – primary polydipsia – and produces too much urine to compensate. 

Diabetes insipidus (DI) may require a supervised water deprivation test in hospital as it is a specialist area even within endocrinology. It is important to differentiate cranial DI – which responds to DDAVP – from nephrogenic DI, which does not. A very similar clinical picture can often occur from primary polydipsia (drinking too much), which some call dipsogenic DI.

Hyponatraemia – a low level of sodium in the blood – is also extremely common and again, very easy to misdiagnose and mistreat. In many patients it requires specialist attention much earlier than it is often received. Some patients may have transient low sodium alongside a critical illness, but if it persists, or is more severe, a thorough assessment will be needed to come to a diagnosis and plan treatment.

One of the causes of hyponatraemia is the syndrome of inappropriate ADH secretion (SIADH or SIAD). Diagnosis is essential because it can be associated with tumours in the brain or lung, or other diseases across the body. New treatments such as vaptans have revolutionised its treatment, but they must be used under specialist supervision.