I have been told by a very well respected consultant this- a few years ago the decision to proceed with hysteroscopy following a scan was to be determined by a cut off point based on clinical evidence relating to the likelihood of endometrial cancer which is what everyone fears on or off HRT. Prior to this everyone had a D and C if they had random bleeding patterns, and it was just that random and not very scientific.With the advent of excellent hysteroscopy techniques and equipment procedures are really accurate and can, following scans, check up on areas and biopsy them with pinpoint accuracy. A cut off had to be established so no one fell through the net, but of course averages are just that averages. If you took 10 ladies off the street with no symptoms some might have thickened linings with no pathology.
I have a 6mm lining and am not on HRT but am being monitored. My hysteroscopy for biopsy purposes could not be completed as my lining was too atrophic - so 6mm is not always thick it may just be how I am made to be. Also scans can indicate possible pathology, but it may not be actually there when a consultant looks with a scope. Hope this helps.