[A case of hyponatremia due to central hypothyroidism associated with senile hypopituitarism].

Hyponatremia is one of the most common ion disorders encountered in everyday practice. It is usually caused by sodium loss or hypervolemic dilution, and side effects of drugs may also contribute. Less well known is the hyponatremia associated with hypothyroidism. The mechanism of its development is not well understood, but it is mainly explained by a decrease in myocardial pump function and secondary arginine vasopressin stimulation-induced fluid retention as wells as a decrease in renal fluid excretion. Although hyponatremia is not a typical and uncommon symptom of hypothyroidism, it may be important to be aware of the possibility of hypothyroidism underlying ion imbalance. In our case report, an elderly patient with hyponatremia was found to have pituitary insufficiency, which caused hypothyroidism and consequent hyponatremia by a central mechanism. Thyroid-stimulating hormone levels in the normal range did not initially raise suspicion of the disease, but free hormone determination revealed an underlying hypothyroidism, the central origin of which was confirmed by imaging. Our case may serve as a useful lesson for everyday practice. Orv Hetil. 2025; 166(20): 783–787.