MANAGEMENT OF ENDOCRINE DISEASE: Hypothyroidism-associated hyponatremia: mechanisms, implications and treatment.

BACKGROUND Patients with moderate to severe hypothyroidism and mainly patients with myxedema may exhibit reduced sodium levels (<135 mmol/L). SUMMARY The aim of this short review is the presentation of the mechanisms of hyponatremia and of the available data regarding its implications and treatment in patients with hypothyroidism. Hypothyroidism is one of the causes of hyponatremia, thus thyroid-stimulating hormone determination is mandatory during the evaluation of patients with reduced serum sodium levels. The main mechanism for the development of hyponatremia in patients with chronic hypothyroidism is the decreased capacity of free water excretion due to elevated antidiuretic hormone levels, which are mainly attributed to the hypothyroidism-induced decrease in cardiac output. However, recent data suggest that the hypothyroidism-induced hyponatremia is rather rare and probably occurs only in severe hypothyroidism and myxedema. Other possible causes and superimposed factors of hyponatremia (e.g. drugs, infections, adrenal insufficiency) should be considered in patients with mild/moderate hypothyroidism. Treatment of hypothyroidism and fluid restriction are usually adequate for the management of mild hyponatremia in patients with hypothyroidism. Patients with possible hyponatremic encephalopathy should be urgently treated according to current guidelines. CONCLUSIONS Severe hypothyroidism may be the cause of hyponatremia. All hypothyroid patients with low serum sodium levels should be evaluated for other causes and superimposed factors of hyponatremia and treated accordingly.