I started a thread last week about low free T and hypogonadism.
To summarise, there was a European study that found low free T was a better predictor of hypogonadism than TT. The lower bounds of what constitutes a normal range hasn’t been determined, but it’s probably somewhere between 6 and 7 (maybe as high as 8) ng/dL.
Low free T is also associated with depression and anxiety. (I’ve been meaning to write a post about this.) It’s not certain which is the cause, and which is the symptom though.