BMJ  2006;332:854 (8 April), doi:10.1136/bmj.332.7545.854

Letter

Disorders of sodium balance

Hypothyroidism and hyponatraemia: an old wives' tale?

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.

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.

Hull Royal Infirmary, Hull HU3 2JZ Eric.Kilpatrick{at}hey.nhs.uk

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Competing interests: None declared.

References

1. Reynolds RM, Padfield PL, Seckl JR. Disorders of sodium balance. BMJ 2006;332: 702-5. (25 March.)[Free Full Text]
2. Asami T, Uchiyama M. Sodium handling in congenitally hypothyroid neonates. Acta Paediatr 2004;93: 22-4.[ISI][Medline]
3. Croal BL, Blake AM, Johnston J, Glen AC, O'Reilly DS Absence of relation between hyponatraemia and hypothyroidism. Lancet 1997;350: 1402.[Medline]
4. Baajafer FS, Hammami MM, Mohamed GED Prevalence and severity of hyponatremia and hypercreatininaemia in short-term uncomplicated hypothyroidism. J Endocrinol Invest 1999;22: 35-9.[Medline]
5. Warner MH, Holding, S, Kilpatrick ES. The effect of newly diagnosed hypothyroidism on serum sodium concentrations: a retrospective study. Clin Endo 2006 (Published online, doi: 10.1111/j.1365-2265.2006.02489.x).

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Relevant Article

Disorders of sodium balance

Rebecca M Reynolds, Paul L Padfield, and Jonathan R Seckl
BMJ 2006 332: 702-705. [Extract] [Full Text] [PDF]