Unlike clinical scanners, micro-CT scanners are often not calibrated (well), so you cannot rely on absolute image intensity ranges. Of course, density of bone and teeth depends on the individual and the imaging is never perfect (there is always some noise, blurring, partial volume effect, etc.), so if structures have similar intensity then you may not be able to segment based on only a global threshold. You may then need to use region-growing based approaches (e.g., Grow from seeds, Watershed), or just manually segment a few slices and interpolate between them using “Fill between slices” effect.