@sebutler you have a great doctor!

My understanding of these two markers is that while the P1NP/CTX (or CTX/P1NP) ratio is a vital 'balance' check during the anabolic phase, where P1NP is the dominant driver, it’s a bit of a different story once you switch to an antiresorptive like Fosamax.

In antiresorptive phase, CTX is the dominant mover - decreasing significantly to 'lock' in gains via suppress bone turnover. Because of coupling phenomenon, P1NP will eventually follow it down after a slight lag. And because both numbers drop so significantly, we don't necessarily look for a preferential 'ratio' like we do during the building/anabolic phase.

The graphs from the TRIO study illustrates the changes in CTX and P!NP beautifully with 3 different bisphosphonates, link here (fosamax, look for the attached photos): https://connect.mayoclinic.org/comment/1425205/
I actually used this same logic during my HRT treatment; monitoring CTX was incredibly helpful for me to figure out the right dosing. From my endo and another bone specialist's view, getting CTX down to lower half of the range for healthy premenopausal women is preferred. I'd be curious to hear what your endo's preference would be.