Some months ago, I read a study which showed that finasteride decreased the level of free testosterone in all the subjects! I've used a couple of hours now, trying to find it again - but without any result. However, I did come across another study, and it shows exactly the same thing! Take a look at page 6!
DHT has greater affinity for SHBG, and finasteride decreases the level of DHT by 60 - 70%. The hormone which is next in line is testosterone, and that's probably the reason.