Background: Hypothyroidism is referred to be a rare but possible cause of hyponatremia. However, there is only poor evidence supporting this association. Since hyponatremia and hypothyroidism are both common conditions themselves, co-occurrence does not have to be causal.
Methods: To address a potential relationship, a retrospective analysis of data from the Division of Endocrinology of the Medical University of Vienna from April 2004 to February 2016 was performed. A total of 8053 hypothyroid patients (48 ± 18 years of age; 71% female) with thyrotropin >4.0 μIU/mL and available blood tests for free thyroxine and sodium (Na+) within maximal ± seven days were included and screened for hyponatremia. Patients' records were searched for concomitant disease and medication when Na+ concentration was <135 mmol/L.
Results: Hyponatremia was present in 448/8053 (5.56%) patients. Analysis of medical history revealed potential alternative causes of hyponatremia in 442/448 (98.88%) patients (i.e., side effects of medication, concomitant underlying disease, or other endocrine disorders). This distribution did not differ between patients suffering from clinical or subclinical hypothyroidism. No case of clinically relevant hyponatremia (Na+ < 130 mmol/L), present in 111/448 (24.78%) patients could be attributed only to hypothyroidism. There was a very weak but statistically significant trend toward a positive association between thyroid function and serum Na+ levels (Na+/thyrotropin: R = 0.022, p = 0.046; Na+/free thyroxine: R = -0.047, p < 0.001).
Conclusion: The results suggest that hypothyroid patients with moderate to severe hyponatremia often have other potential explanations for their low serum Na+ concentrations in routine care.
Keywords: electrolyte imbalance; myxedema; sodium disturbances; thyroid disorder.