What is the mechanism of action of teriparatide, the new parathyroid hormone (PTH) analog being used for the treatment of osteoporosis? Since PTH causes bone resorption, how can an analog cause formation of new bone? — MATTHEW G. BECKWITH, MD, Hagerstown, Md.

Teriparatide is a synthetic polypeptide hormone that represents the 1-34 amino-acid fragment (N-terminal) of endogenous human parathyroid hormone. Its actions mimic those of the endogenous hormone. When teriparatide is administered as a continuous IV infusion, it produces bone loss. When administered cyclically, however, teriparatide has been shown to increase bone mass, particularly in the vertebral spine. Cyclic administration, therefore, is the reason for its beneficial therapeutic effect. The manufacturer recommends 20 µg by injection daily. — Walter N. Kernan, MD (149-8)