What are the key diagnostic criteria for confirming a diagnosis of SIADH in a primary care setting?

In a primary care setting, confirming a diagnosis of Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) typically involves initial assessment and subsequent referral, as hyponatraemia related to endocrine disorders is uncommon and requires specialist input for definitive diagnosis and treatment ¹.

Key considerations and actions in primary care include:

• Initial Assessment of Hyponatraemia: People with asymptomatic, mild hyponatraemia (serum sodium concentration of 130–135 mmol/L) may initially be managed in primary care ¹. It is crucial to ensure a repeat serum sodium measurement is taken to rule out a rapidly decreasing serum sodium concentration, which would necessitate hospital admission ¹.
• Investigating Underlying Causes: If the person is well, even with low serum sodium levels (especially if the fall was slow), it may be reasonable to investigate for the underlying cause in primary care ¹. This includes treating any acute illness that might be contributing to hyponatraemia and rechecking serum sodium after two weeks ¹. If a medication is contributing, it should be stopped if appropriate, and serum sodium rechecked after two weeks ¹. If the medication cannot be stopped, the appropriate specialist should be contacted to discuss management ¹.
• Referral for Specialist Input: Referral to an endocrinologist is recommended if SIADH or another endocrine cause is suspected, with the urgency depending on clinical judgement ¹. Specialist input is required to confirm the diagnosis and initiate treatment for endocrine-related hyponatraemia ¹. An urgent 2-week wait referral to the appropriate specialist should be arranged if malignant disease is suspected as an underlying cause of SIADH ¹. If the cause of hyponatraemia is not clear, referral to an endocrinologist is appropriate ¹.