BACKGROUND Diffusion tensor imaging (DTI) is an advanced magnetic resonance imaging (MRI) method used to identify changes in microstructures in the brain's white matter. Severe brain injuries after trauma are associated with disorders of consciousness (DOC) and may result in hyponatremia due to damage to the hypothalamus. This case-control study aimed to use DTI to evaluate the hypothalamus in 36 patients with hyponatremia and DOC due to severe brain injuries. MATERIAL AND METHODS Thirty-six patients with DOC after traumatic brain injury (TBI) and 36 healthy control subjects were enrolled in this study. The diagnosis of DOC was based on the coma recovery scale-revised (CRS-R). The 36 patients were divided into 2 groups: Group A (18 with hyponatremia, serum sodium level <135 mmol/L) and group B (18 without hyponatremia). The DTI scans were conducted using a 6-channel head coil on a 1.5T Philips Gyroscan Intera scanner. Among the DTI data, fractional anisotropy (FA) and the apparent diffusion coefficient (ADC) of the hypothalamus were analyzed. RESULTS Patient group A had a lower FA value (P=0.044) and higher ADC value (P=0.004) of the hypothalamus and showed a longer length of hospital stay (P=0.03), lower CRS-R score at discharge (P=0.01), and less change in CRS-R score (P=0.004) compared to patient group B. The improvements in the CRS-R score revealed a moderate negative correlation (r=-0.467) with the severity of the hyponatremia (P=0.004). CONCLUSIONS Post-traumatic hyponatremia was associated with hypothalamic injury and the presence and severity of hyponatremia were associated with poor clinical outcomes in DOC patients.