Hi walking the dog
You have been given excellent information and advice already by Dancginggirl.
You may not be progesterone intolerant as such (ie were fine with their own prog) - but many women can't tolerate taking the high doses of oral progesterone needed to keep the lining thin. The reason it has to be at such high doses is that progesterone ( ie the bio-identical one) is very unstable and breaks down quickly in the body, and oral prog has much lower bio-availability than when used vaginally. Also oral prog as it is digested and goes through the liver - has a greater number of what are known as metabolites (different compounds it breaks down to) some of which cause some of the troublesome side effects. (I've just been reminding myself of all this with a great scientific paper by Kuhl - if anyone wants the info I can post the details but is v technical and involving some unfamiliar biochemistry so somewhat hard going in places!).
Dancinggirl is right about the difficulty in determining how long prog stays in your system! From what I read actually it is eliminated quicker when taken orally but I need to read the paper again (which takes ages!). Also that progesterone like oestrogen acts at two levels - at the cell membranes so external to the cells - and probably responsible for some of the immediate effects as with oestrogen) and intracellular though the nucleus and synthesis of proteins etc. I imagine some of these take a bit longer to reverse.
All the progestogens vary in how long they take to be eliminated and some also differ whether they are taken orally or transdermally/vaginally, not only that there is variation amongst different women in this too so no wonder there cannot be a definitive answer!
I'm sorry I'm probably too late for your doctor's visit - but you don't actually have to take Utrogestan to thin the womb. You could take a short course of a synthetic progestogen every day - one that is more effective - such as norethisterone (which also can have side effects), or Provera as Dancinggirl says. Once your lining is thin you could then go back to Utrogestan for 12 days per month if you preferred.
When you say you were peri 6 years ago - what was your cycle doing and if you had skipped periods, how long had this been going on before you started HRT?
Hope this helps a bit
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Hurdity x