I got this info from two of my doctors, and from my bone marker tests. Also "Great Bones" by McCormick... and anecdotally (see threads below). Radius, the manufacturer, only provides assistance for 18 months. (My copy of "Great Bones" is in storage at the moment or I would give you a citation!).
https://www.inspire.com/groups/bone-health-and-osteoporosis/discussion/taking-tymlos-longer-than-2-years/
https://connect.mayoclinic.org/discussion/disappointing-results-would-you-continue-or-skip-2nd-year-of-tymlos/