How does hypothyroidism cause hyponatremia?

How Hypothyroidism Causes Hyponatremia

Hypothyroidism causes hyponatremia primarily through decreased cardiac output leading to elevated antidiuretic hormone (ADH) levels, which impairs free water excretion by the kidneys. 1

Pathophysiological Mechanisms

Primary Mechanism: Impaired Free Water Excretion

• Decreased Cardiac Output: Hypothyroidism reduces cardiac contractility and heart rate, leading to decreased cardiac output 2
• Compensatory ADH Release: The body responds to reduced effective arterial blood volume by increasing ADH secretion 1, 3
• Renal Water Retention: Elevated ADH activates vasopressin 2 receptors in renal tubules, increasing water reabsorption and diluting serum sodium 4

Secondary Mechanisms

1. Reduced Glomerular Filtration Rate (GFR)

   • Hypothyroidism decreases renal blood flow and GFR
   • This further impairs the kidney's ability to excrete free water 1

2. Altered Renal Sodium Handling

   • Some evidence suggests a direct renal mechanism for sodium loss 5
   • Decreased tubular sodium reabsorption may occur in severe hypothyroidism

Clinical Presentation and Diagnosis

When to Suspect Hypothyroidism-Induced Hyponatremia

• Typically occurs in moderate to severe hypothyroidism or myxedema 1
• Serum sodium levels usually <135 mmol/L
• Often accompanied by other symptoms of hypothyroidism:
  • Fatigue, muscle cramps, constipation
  • Cold intolerance, hair loss
  • Voice changes, weight gain, intellectual slowness 2

Diagnostic Approach

1. Laboratory Assessment:

   • Serum TSH (elevated) and free T4 (decreased)
   • Serum sodium, osmolality
   • Urine sodium and osmolality 4

2. Volume Status Assessment:

   • Hypothyroid patients with hyponatremia are typically euvolemic
   • Urine sodium typically >20-40 mEq/L 4

3. Differential Diagnosis:

   • Rule out SIADH: Similar laboratory findings but different underlying cause
   • Exclude adrenal insufficiency (often coexists with hypothyroidism)
   • Consider other causes of hyponatremia (medications, heart failure) 4, 1

Important Clinical Considerations

Severity and Prevalence

• Hyponatremia is more common in severe hypothyroidism and myxedema 1
• Recent evidence suggests hypothyroidism-induced hyponatremia may be less common than previously thought 6
• In a small study of patients with extreme TSH elevations (>100 μU/mL), none had serum sodium below normal range 6

Treatment Implications

• Primary Treatment: Thyroid hormone replacement therapy corrects the underlying cause 1, 3
• Supportive Measures: Fluid restriction may be needed initially 4
• Monitoring: Regular sodium level checks during treatment to avoid rapid correction
• Caution: Avoid increasing serum sodium by >10 mmol/L/day to prevent osmotic demyelination syndrome 4

Pitfalls to Avoid

1. Overlooking other causes: In mild/moderate hypothyroidism, consider other causes or contributing factors for hyponatremia 1
2. Misdiagnosis as SIADH: Similar laboratory findings but different treatment approach
3. Overaggressive correction: Rapid correction of chronic hyponatremia can lead to neurological complications 4

Conclusion

Hypothyroidism-induced hyponatremia results primarily from decreased cardiac output leading to elevated ADH levels and impaired water excretion. While this mechanism is well-established, recent evidence suggests it may be clinically significant only in severe hypothyroidism. Treatment with thyroid hormone replacement and appropriate fluid management typically resolves the electrolyte abnormality.