@tillymack, CTX (C-terminal telopeptide) is a bone turnover marker that reflects how much bone is being broken down (resorbed), so it indirectly measures how active osteoclasts are.

When you are on antiresorptives like Evista or Reclast, your CTX levels usually goes down. Because Reclast is a stronger antiresorptive than Evista, the drop in CTX is typically much greater with Reclast.

Measuring bone turnover markers isn't standard practice everywhere yet, but some endos do them to help guide treatment.