Finasteride doesn't cause atrophy. I was on the high dose of Fin alone for almost 2 years before I started HRT.
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I'm pretty sure the atrophy has to do with large Spiro dosages, and not due to the estrogen.
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As far as stopping HRT after 6 months, most sources I've read put 6 months as the 'point of no return' as far as fertility goes. If you're looking to try threading a needle of returning testosterone to normal but still removing fertility, I'd say that's a bit unlikely. Anything at that point of HRT length is a bit iffy. No one can tell you how your body will react to HRT, or if your body will be able to recover its testosterone levels to pre-HRT levels after such a drastic change in hormones. My guess is that you may be able to take T injections to return to old T levels, but I wouldn't bet on your body being able to do it without help. You're rolling the dice, and you won't know the results until after the 6 months are up. If you're not sure that you want to take HRT for the rest of your life, think long and hard before you start it.