I’m 61 this month and last October I had a mirena fitted whilst having a biopsy due to post menopausal bleeding on continuous oestrogel and utrogestan. At my 6 week coil check the nurse at my surgery couldn’t find the mirena threads so referred me for a scan to check it was still in place or if the strings had been cut too short. I had the scan the week before Xmas and had a call from the drs the next day to say that the scan had shown that my endometrium had pretty much doubled in thickness in three months, from 4.5mm when the coil was fitted to 10mm. I was referred for a hysteroscopy and another biopsy, which was carried out at the beginning of January. The consultant showed me vascular synachiae, which are apparently adhesions, which had wrapped themselves around the arms of the coil, he also removed a polypoidal area. I was also told that although my previous biopsy result was negative, the lab had said there was insufficient tissue, so he took a more thorough biopsy. He also replaced the previous coil as the consultant that had fitted it had cut the threads too short. My biopsy results all came back negative, but there was no reason given for the endometrium thickening other than the fitting of the mirena can trigger it in some people. I hope that goes some way to reassuring you a little. I know I felt exactly the same as you when I found out, I did post on here at the time as I was very concerned.