SAT-053 The role of hyponatremia in severe hypothyroidism

Abstract Disclosure: J. Beck: None. E. Stasi: None. M. Christ-Crain: None. 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. REFERENCES1 Spasovski et al, Clinical practice guideline on diagnosis and treatment of hyponatraemia, 2014, Eur.J. Endocrinol., doi.org/10.1530/EJE-13-10202Robert C. Hawkins, Age and gender as risk factors for hyponatremia and hypernatremia, 2003,Clinica Chimica Acta, doi:10.1016/j.cccn.2003.08.001 Presentation: Saturday, July 12, 2025