This paper explores the detection of vitamin D3 (VD3) using a fluorescent microcavity sensor. There is currently a high demand for measurement and analysis of this vitamin, whose deficiency may result in several different pathologies; however, the standard methods, predominantly based on mass spectroscopy and liquid chromatography require large equipment and must be done remotely. In contrast, measurement of solvent concentrations via microcavity-based optical systems can be sensitive to small changes and require miniscule analyte volumes. However, the refractive effect of VD3 (cholecalciferol) in different solvents has not been widely investigated. This paper shows that VD3 can be detected in an optofluidic setup consisting of a fluorescent capillary with a high-refractive-index quantum-dot coating. Detection and quantification were performed both in pure ethanol and food-grade soy oil solvents. While the refractive detection limits (~1 mg/mL) were higher than in current standards for D3 analysis, specific VD3 binding could lower these values. Since cholecalciferol is almost certainly too small (mass 384 Da) to be measured by attempting to treat the cavity surface for its -OH functionality, binding of the VD3 carrier protein is shown instead. This suggests practical routes toward specific microfluidic detection of VD3. ; Judy Chan, Torrey Thiessen, Stephen Lane, Peter West, Kirsty Gardner, Alexandre François, and Al Meldrum