My thoughts are:
1. Do you have photo evidence proving to yourself that you are thinning even on sides and back?
2. Are you experiencing other bad signs, such as oilier skin, higher libido, itchy scalp, etc.?
If so, I would advise going down on a lwee dose, or possibly stop taking it for a while then restart on a low dose and see how it goes. Temporarily thinner hair itself is not enough evidence as it is common for finasteride starters. If you experience clear signs of higher androgen activities then it's a different story. Otherwise you should not arrive at the belief that you have reflex hyperandrogenesity so soon. Besides, taking a irregular dose can be really bad as if sends your body into panic mode and you will be hard to stablize health and hair wise. Just keep that in mind.