Its very hard with spiro alone to reduce T levels, but... It should not really matter since even with higher than female range T levels, since its not binding with T receptors, you'd still get demasculinasation.
How Spiro works is that it binds to T receptors (its the steroid aldactone btw).
In binding to the receptors, there's a lot of free unbinded T floating around,
the pituary sees this and says Whooaa too much sex hormones in the body
and instructs the testes to produce less (they're not shut down, they just
produce less). If you continue taking spiro, the body keeps reducing
the teste's output over time through iterations until the level of free T
becomes normal.
The reason this doesn't work perfectly in the case of spiro is that its
short lived, so unless you split your dose and take a sufficient dose
(which is different for everybody) to bind all receptors all the time,
some of the free T produced by the testes will be binded to T receptors
and so the retroactive shutdown of the testes can plateau depending
on dosage and when you take the pills.
When inserting a sex steroid in the system like E, your introducing
hormones than can actually be bound and the testes can be quickly
shut down even if some T receptors are still unbound.
The main reason for using spiro is that allows for a smaller dose of E
to effectively shut down the testes, with the added bonus of
immediate demasculinisation you don't get with estrogen
(you would have to wait till E shuts down the testes to get that).
For the pituary, there's no difference between T and E, that's why the testes
are shutdown by the insertion of exogenous E in the body. It is the same
reason the testes are shutdown if you're using steroids (bodybuilders
cycle them to prevent this and even use E for this reason!).
