@2young4bones, P1NP is the fragment left in the bone after specific protein-cutting enzymes cleave off extension peptides from the procollagen molecule before it forms collagen fibers. The N-terminal propeptide (P1NP) is one of these removed fragments, released into the bloodstream. Measured they stand to represent the action of the cells that build bone, just as CTX represents the action of cells that break down bone.
They are both sourced through a simple blood draw that should be done fasting, early in the morning after an interlude from biotin and collagen.
The standard use is CTX before dosing with an antiresorptive and again after 2 months; P1NP before use and after a month of use. Primarily it is used to ascertain that the medication is effective.
If the original plan was to take Tymlos, P1NP would be (have been) useful.
I think that they should always be measured together. But, I haven't done the long arduous studies required to be a doctor.