My wife is a critical care and nephrology (kidney) nurse practitioner and she gave me some detail of it beyond what you might get in a google search.
Spiro is a diuretic that is one drug that can be used to reduce blood pressure even though it is not the preferred drug to use for that purpose. Kidney function is important to keep up with because that is where the process occurs. Reduction of the testosterone level is a side affect of Spiro and not its primary purpose.
Both biological men and women have estrogen and testosterone in their system, just at different levels, testosterone being stronger. If a biologic male has the normal amount of testosterone in their system then the estrogen that we take will have less of an effect since testosterone will be countering it, therefore the reason that testosterone is intentionally reduced. The desire is to reduce testosterone to a level of what would be normal for a biologic female. So the Endocrinologist tests for both hormones and getting the correct dosage is a matter of trial and error. The level for one person is not the same as for another since the dose is not as important as the cells ability to transfer the hormone across the cell wall, which varies for each individual.
If a MTF trans person is unable to take Spiro due to kidney function and their testosterone level is high the urologist can remove the testicles. I had a vasectomy 15 years ago and the urologist cut too much and my testosterone went to almost zero. I had no thoughts of transitioning 15 years ago and so I had to take testosterone for 12 years until I decided to start HRT.
So if you can get by without Spiro that is best but it is not like it will cause you severe issues as long as you have good kidney function.
Like some others have said the worst normal side affect of Spiro is having to go pee every hour.