Endocrine Abstracts

JOINT975

Background: Current guidelines recommend excluding hypothyroidism as a differential diagnosis for euvolemic hyponatremia¹. However, there is a growing debate on hypothyroidism-induced hyponatremia, with poor evidence on its clinical significance. Especially data in severe hypothyroidism and myxedema is lacking.

Aim: To assess the prevalence of hyponatremia in patients with severe hypothyroidism and identify potential comorbidities associated with its development.

Design and Methods: Retrospective observational study, conducted at the University Hospital of Basel in Switzerland. Data were collected over a 10-year period (01/2014-11/2024) from in-and outpatients, who presented with free thyroxine (fT4) levels <8pmol/l with and without the diagnosis of myxedema, as documented in patients’ chart.

Results: A total of 820 cases were analyzed, showing a mean sodium level of 138±5 mmol/l, TSH level of 56±62 mU/l and fT4 level of 5±2 pmol/l. The prevalence of hyponatremia in the cohort was 19% (n = 155). No significant association was found between fT4 and sodium levels. Within this cohort, severe hypothyroidism, clinically defined by the treating physician, was identified in 45 cases (76% female, mean age of 63±15 years), including 12 cases of myxedema. The prevalence of hyponatremia in this subgroup was 22%, which was comparable to the overall cohort (P = 0.7). Further characterization of the hyponatremic population showed a higher frequency of comorbidities compared to the normonatremic group, including malignancy (30% vs 9%), cardiovascular disease (60% vs 43%), heart failure (20% vs.14%), infectious disease (40% vs. 20%) and kidney disease (30% vs. 43%). Presence of malignancy was a significant predictor of lower sodium levels (-5mmol/l, P = 0.04), while no significant correlation was found between fT4 levels and sodium levels (P = 0.6) or development of hyponatremia (P = 0.9). Also, the mortality rate within index hospitalization was higher in the hyponatremic group compared to the normonatremic group (20% vs 6%).

Conclusion: Our analysis suggests that the prevalence of hyponatremia in severe hypothyroidism is comparable to the general hospitalized population, which is estimated to be around 20%². Hyponatremia in our cohort cannot solely be explained by the presence of severe hypothyroidism. Instead, presence of comorbidities such as malignancy seem to play a major role in the development of hyponatremia in severe hypothyroidism.

References: 1. Spasovski et al, Clinical practice guideline on diagnosis and treatment of hyponatraemia, 2014, Eur. J. Endocrinol., doi.org/10.1530/EJE-13-1020.

2. Robert C. Hawkins, Age and gender as risk factors for hyponatremia and hypernatremia, 2003, Clinica Chimica Acta, doi:10.1016/j.cccn.2003.08.001.