Hi Dollydream - are you post-menopausal? I imagine once the Mirena has been in a while and the endometrium has settled - if you are post-menopausal it would end up being thinner than 11 mm, but if you are still having a cycle of some sort then there will still be some ebbing and flowing (of the thickness) as the cycle comes and goes each month or so, but may not be enough to cause a proper period?
ann58 - I definitely would not be overly worried by 5 mm but would be happy to have any investigations offered on NHS! Mine's usually around this measurement or 4.5 mm - on HRT, when it's been scanned post-bleed. In fact with the Mirena - it's better that it is slightly thicker because sometimes if the oestrogen dose is not high enough and you are post-meno, it can thin too much (with Mirena) causing endometrial atrophy, ulceration and spotting/bleeding.
Let us know how you get on ...
Hurdity x