Using TIN to convert values in a shapefile (point)

Is there a function or command that would use a TIN to convert values in a point layer to ones that are relative to the TIN? I have a point shapefile that was generated from sediment core data that has the depth of the sediment core. I have a TIN that was used to generate bathymetry for the same area and would like to use that to convert the sediment core depths to depths relative to the datum of the TIN. But I can't find a function that would perform this calculation. Do I have to convert both to a grid format first?

Thanks - KB