Can hypothyroidism cause hyponatremia?

Hypothyroidism as a Cause of Hyponatremia

Yes, hypothyroidism can cause hyponatremia, particularly in cases of severe hypothyroidism or myxedema, though it is relatively uncommon as the sole cause of hyponatremia in mild to moderate hypothyroidism. 1

Mechanisms of Hypothyroidism-Induced Hyponatremia

• The primary mechanism is decreased capacity for free water excretion due to elevated antidiuretic hormone (ADH) levels, which occur mainly due to hypothyroidism-induced decrease in cardiac output 1
• Hypothyroidism leads to reduced glomerular filtration rate, which contributes to impaired water excretion 1
• Severe hypothyroidism may alter renal sodium handling through a pure renal mechanism, not solely attributable to inappropriate ADH secretion 2

Clinical Significance and Prevalence

• Hyponatremia is more likely to occur in severe hypothyroidism and myxedema rather than in mild or moderate hypothyroidism 1
• Recent evidence suggests hypothyroidism-induced hyponatremia is relatively rare - a retrospective analysis of 8,053 hypothyroid patients found hyponatremia in only 5.56% of cases 3
• Among hypothyroid patients with hyponatremia, 98.88% had other potential causes for their low sodium levels (medications, concomitant diseases, or other endocrine disorders) 3
• In a small case series of patients with extreme TSH elevations (>100 μU/mL), none had serum sodium levels below the normal range (<135 mmol/L) 4

Diagnostic Considerations

• When evaluating hyponatremia, thyroid-stimulating hormone (TSH) determination is mandatory as part of the workup 1
• Hypothyroidism must be excluded when diagnosing Syndrome of Inappropriate Antidiuretic Hormone (SIADH) 5
• Diagnostic criteria for SIADH specifically include "absence of hypothyroidism or adrenal insufficiency" 5
• Other common causes of hyponatremia should be ruled out, including:
  • Medications (diuretics, antidepressants, antipsychotics) 1
  • Heart failure 5
  • Cirrhosis 5
  • Adrenal insufficiency 5
  • Renal disease 1

Management Approach

• Treatment of the underlying hypothyroidism with thyroid hormone replacement is essential 1
• For mild hyponatremia in hypothyroid patients, fluid restriction and thyroid hormone replacement are usually adequate 1
• In cases of severe hyponatremia with neurological symptoms (hyponatremic encephalopathy), urgent treatment according to current guidelines for hyponatremia is necessary, regardless of the hypothyroid status 1
• Correction of severe hyponatremia should not exceed 10 mmol/L/day to prevent osmotic demyelination syndrome, especially in chronic cases 5
• Monitor sodium levels closely after initiating thyroid hormone replacement, as rapid correction can occur 6

Clinical Pearls and Pitfalls

• Hyponatremia in patients with mild/moderate hypothyroidism should prompt evaluation for other causes, as hypothyroidism alone is rarely sufficient to cause significant hyponatremia in these cases 3, 4
• Case reports exist of severe hypothyroidism with inappropriately elevated ADH causing significant hyponatremia that improved with thyroid hormone replacement 6
• In patients with multiple potential causes of hyponatremia, treating the hypothyroidism may still contribute to resolution of the electrolyte abnormality 1
• The correlation between thyroid function tests and serum sodium levels is statistically significant but very weak, suggesting other factors are more important in determining sodium levels in most hypothyroid patients 3