Disorders of sodium balance

Editor—Like many other sources, Reynolds et al include hypothyroidism as a secondary cause of hyponatraemia.¹ However, the evidence supporting this association is extremely poor. Studies in babies with severe congenital hypothyroidism have found no change in serum sodium concentrations after treatment with levothyroxine.² This has repeatedly been found to be the case in adults too.³,⁴ A study performed by our group found no difference in sodium concentrations between 999 patients with a new diagnosis of hypothyroidism from their family doctor and 4875 controls with normal thyroid function.⁵ Indeed, none of the hypothyroid patients had a serum sodium concentration < 120 mmol/l at diagnosis, while this was present in two of the controls.

Figure 1.

Credit: FRANCIS LEROY/BIOCOSMOS/SPL

Hypothyroidism is a common condition, and so is hyponatraemia in acutely unwell patients admitted to hospital. However, when hyponatraemia and hypothyroidism are found to co-exist, the hyponatraemia is not necessarily a consequence of the hypothyroidism, and so other causes of low sodium concentrations should still be sought. Future guidelines could be helpful in clarifying the misconception of this association.