Unexplained Hyponatremia Revealing Vitamin D Deficiency: A Clinical Case Report

"SIAD" (syndrome of inappropriate antidiuresis) is frequently encountered in clinical practice, often necessitating the exclusion of various differential diagnoses and involving expensive treatments. In cases of SIAD concomitant with vitamin D deficiency, treatment with vitamin D supplementation may not only correct hyponatremia but also improve accompanying clinical findings, offering a cost-effective alternative for clinicians. A 52-year-old Caucasian Turkish white male presented to the emergency department with symptoms of nausea, weakness, and altered consciousness. The patient was euvolemic. Laboratory results showed severe hyponatremia with low serum osmolality and high urine osmolality. Serum ADH levels were elevated. Extensive workup excluded other causes of SIAD. No significant pathology was detected, except for vitamin D deficiency. Following high-dose vitamin D supplementation, the patient's sodium levels normalized without requiring further vasopressin receptor antagonist treatment. Long-term follow-up revealed that sodium levels remained within normal limits. Vitamin D deficiency is not traditionally considered among the common causes of SIAD. It may contribute to dysregulation of ADH. However, correcting this deficiency may resolve hyponatremia, making it a potentially overlooked, yet reversible, contributor. In patients with unexplained hyponatremia, vitamin D levels should be evaluated and corrected before initiating costlier treatment options.