First of all that's a rubbish reason not to be on hormone therapy if you were finding it beneficial.
It doesn't make medical sense - either you have a hormonally active pituitary lesion requiring treatment in it's own right, OR you had a slight elevation in prolactin and cortisol secondary to exogenous hormones, which is a known phenomenon, is benign and doesn't necessitate discontinuation.
If there was any suggestion that estroprogestins were hastening the growth of the pituitary tumour, the progestin desogestrel should be equally contraindicated.
So firstly I would establish exactly what is going on medically. Nothing you have said should contraindicate hormone replacement - if it did then they should be getting you booked in for bilateral oophorectomy or shutting you down with Zoladex, because you are still menstruating and hence producing the exact same hormones that you have been told to stop taking.
Regarding desogestrel, I have been on it for menstrual suppression - one 75mcg tablet did not work and I needed 150mcg.
This did the job nicely however this is a properly suppressive dose and will reduce your own ovarian estradiol production to a very low level.
Mine was 44pg/ml on this, and whilst I was not symptomatic I was not happy to remain on this long term and chose a combined pill instead.
In my opinion the desogestrel mini pill is neither use nor ornament as it doesn't shut ovarian function down properly at the usual dose and contains no estrogen, although I recognise it has a role in the therapeutic arsenal.
You may be better with either a mirena IUS that doesn't have the same systemic side effects, or a combined oral contraceptive pill taken continuously like me- I take Zoely.
Or you could go back onto the MHT you were taking before, which is unlikely to make a jot of difference regarding the pituitary tumour, but will also not provide menstrual suppression.