High FSH

I am suffering from PFS. I have taken many blood tests over the years and one value is striking: My FSH is always either too high or right at the end of the range. What might be the reason for this? Perhaps this has to do with my PFS?

My LH is always ok (in the middle of the range). My Testosterone levels are also in the middle of the range (e.g. 18,1 nmol/l - range: 5,5-25,2 mol/l). Does anybody know what a too high FSH can cause?

Did you get tested for prolactin and a full thyroid panel?