Does hyperthyroidism (an overactive thyroid) or hypothyroidism (an underactive thyroid) cause hyponatremia (low sodium levels)?

From the Guidelines

Both hypothyroidism and hyperthyroidism can cause low sodium (hyponatremia), though it's more commonly associated with hypothyroidism. In hypothyroidism, decreased cardiac output and reduced glomerular filtration rate lead to impaired water excretion and inappropriate antidiuretic hormone secretion, causing water retention that dilutes sodium levels 1. Severe hypothyroidism can decrease sodium levels to 120-130 mEq/L. Hyperthyroidism can also cause hyponatremia through increased renal blood flow and glomerular filtration rate, which may enhance sodium excretion. Additionally, increased metabolism in hyperthyroidism leads to greater fluid intake and potentially dilutional hyponatremia. Some key points to consider include:

• Decreased cardiac output in hypothyroidism contributes to hyponatremia
• Increased renal blood flow in hyperthyroidism can lead to enhanced sodium excretion
• Treatment focuses on correcting the underlying thyroid disorder rather than directly addressing sodium levels
• For hypothyroidism, levothyroxine replacement usually normalizes sodium levels as thyroid function improves
• For hyperthyroidism, treatments like methimazole, propylthiouracil, radioactive iodine, or surgery will help restore normal sodium balance as thyroid function normalizes 1. It is essential to prioritize treating the underlying thyroid condition to manage hyponatremia effectively. Some considerations for treatment include:
• Starting levothyroxine at 25-50 mcg daily for older adults or those with cardiac issues
• Starting levothyroxine at 75-100 mcg daily for others
• Monitoring sodium levels and adjusting treatment as needed to ensure normalization of thyroid function and sodium balance.

From the Research

Relationship Between Thyroid Disorders and Low Sodium Levels

• Hypothyroidism has been associated with hyponatremia (low sodium levels) in some studies, with the primary mechanism thought to be due to impaired free water excretion and changes in sodium handling in the kidney 2, 3.
• However, other studies suggest that hypothyroidism-induced hyponatremia is rather rare and probably occurs only in severe hypothyroidism and myxedema 2, 4.
• A retrospective analysis of 8053 hypothyroid patients found that hyponatremia was present in 5.56% of patients, but potential alternative causes of hyponatremia were found in 98.88% of these patients 4.
• Another study found that even in patients with extreme TSH elevations, none had a serum sodium level below normal (<135 mmol/L), suggesting that hypothyroidism may be a less clinically relevant cause of hyponatremia in otherwise healthy ambulatory patients 5.

Hyperthyroidism and Low Sodium Levels

• There is limited evidence to suggest a direct relationship between hyperthyroidism and low sodium levels.
• Hyperthyroidism is a state of thyroid hormone excess, which increases the metabolic rate and causes symptoms including anxiety and tremor, but its relationship to sodium levels is not well established 6.

Clinical Implications

• Thyroid-stimulating hormone determination is mandatory during the evaluation of patients with reduced serum sodium levels to rule out hypothyroidism as a potential cause 2.
• Patients with hypothyroidism and low serum sodium levels should be evaluated for other causes and superimposed factors of hyponatremia and treated accordingly 2, 4.