Researchers haven't unraveled the mystery of just how testosterone increases libido. It's normal for a man's sex drive to slowly decline from its peak in his teens and 20s, but libido varies widely between men. What one man might consider a low sex drive, another might not. Also, sex drive changes within each man over time and is affected by stress, sleep , and opportunities for sex. For these reasons, defining a "normal" sex drive is next to impossible. Usually, the man himself identifies a lack of sex drive as a problem. Other times, his partner may consider it to be an issue.
If you have symptoms and haven't already had multiple tests this is essential. Your tests should include SHBG and Albium so that free and bioavailable testosterone can be calculated in addition to total testosterone. Your tests should be done at 8am or as close to this as can be arranged and at a time when your life is a normal as possible (. not at a time of high stress or other transient medical conditions). I not a doctor but personally I would also request other HPA axis tests before making a decision about starting HRT. These include LH, FSH, GnRH, ATCH, 24h cortisol. This is especially the case if there are no obvious issues with your testicles.