This is such a shame, the normative cut off value of 4mm for endometrial thickness in postmenopausal women was derived from an UNTREATED population.
Women on hormone therapy are being subjected to inappropriate investigations left and right because of bone-headed failure to understand the physiology of hormones making everything thicker and healthier such as your bones, your skin and yes your endometrium too.
Additionally, vaginal atrophy from low estrogen can also present with spotting.
The solution to bleeding or spotting is never to reduce a therapeutic dose of estrogen - this simply results in an undertreated patient and frequently doesn't resolve the issue. Instead, assuming adequate estrogen, the progestogen should be tweaked to better control the endometrium.
However you are where you are now, and if you want to keep your prescription you could argue for a change in progestogen (alongside a return to the lowest EFFECTIVE dose of estradiol as per guidelines) and a follow up scan in 6 months, however you may well be forced into a hysteroscopy.
The best thing you can do in this situation is advocate for sedation, cervical local anaesthetic and the insertion of an IUS at the same time, which will at least net you some clinical benefit from the procedure and provide good endometrial control going forward.
