I’ve heard of some doctors starting their patients on only spiro (or other antiandrogens) for a period of a few months, before adding estrogen. I happened to start on both at once. Everything I’ve heard suggests that spiro-only would be a bad thing on a permanent basis, as sex hormones (either estrogen or testosterone) are supposedly needed for bone health, and taking only spiro would leave you without either.
This is not entirely accurate. The male body has naturally occurring estrogen in it, and spiro doesn’t affect that. Spiro also doesn’t halt T production and absorption entirely; instead, it’s used to bring T down to female levels. Essentially, high dose spiro alone would lead to Male level estrogen and Female level T. Not exactly a medically positive situation, but not life threatening. You’d basically have the hormone levels of a eunuch.