Wendyi, NTX is N-Telopeptide (in my case was urine test; can also be blood test). It was mentioned in The Whole Body Approach to Osteoporosis by Dr Keith McCormick. It is a bone biomarker test that looks at the amount of cross-liked N-telopeptides of bone type 1 collagen. Elevated levels of NTx indicate increased bone resorption. My understanding now is that CTX biomarker (similiar to NTX) is the more accepted test now. CTx may be more accurate, not sure. CTx is also used to monitor the effectiveness of anti-resorption drug therapies.