Can hypothyroidism (Thyroid Stimulating Hormone (TSH) elevation) cause hyponatremia (low sodium levels)?

Can Hypothyroidism Cause Hyponatremia?

Yes, severe hypothyroidism (particularly with TSH levels as high as 43 mIU/L) can cause hyponatremia, though this occurs primarily in moderate to severe cases and myxedema, not in mild hypothyroidism. 1

Mechanism of Hyponatremia in Hypothyroidism

The primary mechanism involves impaired free water excretion due to inappropriately elevated antidiuretic hormone (ADH) levels, which result from hypothyroidism-induced decreased cardiac output 1. This creates a syndrome similar to SIADH, where:

• Plasma ADH levels become inappropriately high relative to low plasma osmolality 2
• The kidneys retain free water while maintaining sodium excretion
• Hyponatremia develops through a pure renal mechanism 3

Clinical Context and Severity

Recent evidence suggests hypothyroidism-induced hyponatremia is actually relatively rare and occurs predominantly in severe hypothyroidism and myxedema, not in mild to moderate cases. 1 A prospective study of 212 thyroid cancer patients with acute severe hypothyroidism (mean TSH 141.6 mU/L) found that clinically important hyponatremia was uncommon, with only 1.9% developing moderate hyponatremia (≥120 mEq/L) 4.

Key Risk Factors Beyond Hypothyroidism

When evaluating a patient with hypothyroidism and hyponatremia, you must actively exclude other causes and superimposed factors 1:

• Medications: Diuretics (23% of hyponatremic patients were on diuretics vs. 1% of eunatremic patients) 4
• Renal impairment: Elevated creatinine (36% of hyponatremic vs. 13% of eunatremic patients) 4
• Adrenal insufficiency: Must be excluded as hypothyroidism can mask concurrent adrenal disease 5
• Pre-existing hyponatremia: 9% of hyponatremic patients had baseline hyponatremia 4
• Age and sex: Older age and female gender associated with lower sodium levels 4

Diagnostic Approach

When encountering hyponatremia with TSH of 43 mIU/L:

1. Confirm true SIADH-like picture 5:

   • Serum sodium <135 mEq/L
   • Plasma osmolality <275 mosm/kg
   • Inappropriately high urine osmolality (>500 mosm/kg)
   • Urinary sodium >20 mEq/L
   • Absence of volume depletion

2. Exclude other causes 5, 1:

   • Check cortisol/ACTH to rule out adrenal insufficiency
   • Review medication list (diuretics, SSRIs, carbamazepine)
   • Assess for infections, malignancy (particularly lung cancer with SIADH)
   • Evaluate renal function

3. Assess severity of hyponatremia 5:

   • Severe symptoms (mental status changes, seizures): Requires urgent treatment
   • Mild symptoms (nausea, headache, confusion): Less urgent approach
   • Asymptomatic: Can proceed with conservative management

Treatment Algorithm

For Mild to Moderate Hyponatremia (Na 120-134 mEq/L) Without Severe Symptoms

Fluid restriction (<1 L/day) combined with thyroid hormone replacement is usually adequate 1:

• Initiate levothyroxine replacement (dose based on age, weight, cardiac status)
• Restrict free water intake to <1 L/24 hours 5
• Add sodium supplementation if needed 2
• Monitor sodium levels every 2-4 days initially

For Severe Symptomatic Hyponatremia (Na <120 mEq/L) or Acute Symptoms

Treat as a medical emergency with hypertonic saline while addressing the underlying hypothyroidism 5:

• Transfer to ICU for close monitoring 5
• Administer 3% NaCl to correct 6 mEq/L over 6 hours or until severe symptoms resolve 5
• Critical caveat: Total correction should not exceed 8 mEq/L over 24 hours to avoid osmotic demyelination syndrome 5
• Check sodium every 2 hours during acute correction 5
• Once symptoms improve, transition to fluid restriction and thyroid replacement 5

Thyroid Hormone Replacement Considerations

• Serum sodium levels may increase drastically once levothyroxine is initiated 2
• In severe hypothyroidism with hyponatremia, start with lower doses of levothyroxine in elderly or those with cardiac disease to avoid precipitating cardiac complications
• Monitor sodium levels closely during the first 1-2 weeks of thyroid replacement

Common Pitfalls

1. Assuming all hyponatremia in hypothyroidism is due to the thyroid disorder: Most patients with mild to moderate hypothyroidism and hyponatremia have other contributing factors 1, 4

2. Over-correcting sodium too rapidly: Risk of osmotic demyelination syndrome when correction exceeds 8 mEq/L in 24 hours 5

3. Missing concurrent adrenal insufficiency: Always exclude this before attributing hyponatremia solely to hypothyroidism 5

4. Not monitoring sodium during acute severe hypothyroidism: While clinically important hyponatremia is uncommon in uncomplicated acute severe hypothyroidism, patients with impaired renal function or on diuretics require sodium monitoring 4

5. Fluid restriction alone without thyroid replacement: Both interventions are typically needed for successful management 1