I Am A 56 Year Old Male, On Testosterone Supplements

I am a 56 year old male, on testosterone supplements (implanted pellets), take metformin, lisinopril, and I've had graves disease. I also take levothyroxine and liothyronine. I am sexually active, but require a small dose of sildenofil to achieve a reliable erection and lately, I have great difficulty achieving an orgasm or can't at all. What can cause this?