Osteoporosis is commonly evaluated using dual-energy X-ray absorptiometry (DXA) for bone mineral density (BMD). Non-contrast computed tomography (CT) scans provide an alternative for opportunistic osteoporosis assessment. This study aimed to evaluate screening thresholds for osteoporosis based on CT attenuation values in Hounsfield units (HU) of L1-L4 vertebrae from abdominal CT scans, compared to DXA assessments of the lumbar spine and hips. Conducted retrospectively over about two years, the analysis included 109 patients who had both CT and DXA scans within 12 months, excluding those with metal artifacts affecting the vertebrae. CT attenuation values in the trabecular region of the vertebrae were measured and compared among three groups based on the lowest T-score from DXA. In a predominantly female cohort (mean age 66.3 years), significant correlations were found between the CT HU values of L1-L4 vertebrae and the lowest T-score. CT HU values differed significantly among normal (17.4%), low bone mass (46.8%), and osteoporosis (35.8%) groups (P < 0.001). A HU threshold of ≤142 at the L1 vertebra showed 91.9% sensitivity and 48.4% specificity, while a threshold of ≤160 HU showed 97.3% sensitivity and 31.3% specificity for screening osteoporosis. In conclusion, this study supports the utility of non-contrast CT with these HU thresholds as an opportunistic osteoporosis assessment.