Physicians use teriparatide (Forteo and others), the active part of the parathyroid hormone (PTH) molecule, to treat severe osteoporosis. They treat patients who:
They use Forteo as once per day injections. As high PTH is associated with osteoporosis, many people, including me, find it illogical that giving PTH injections could help osteoporosis.
PTH and vitamin D are the primary regulators of calcium metabolism in bone, but the half-life of PTH is only about 5 minutes, meaning the Forteo is probably gone 10-15 minutes after the injection. PTH increases serum calcium, partially accomplishing this by increasing activated vitamin D levels. Thus, a PTH injection should increase activated vitamin D levels, at least for a few minutes.
For unknown reasons, chronic intermittent intravenous Forteo activates bone-building cells (osteoblasts) more than bone destroying cells (osteoclasts). Thus, injections of Forteo have a net effect of stimulating new bone formation, and effectively treating osteoporosis. Forteo is the only FDA approved agent for the treatment of osteoporosis that stimulates new bone formation.
Dr. Felicia Cosman of Columbia University and colleagues recently discovered some details of how Forteo works. Their discovery implies that it is crucial for patients who take Forteo be sure they take vitamin D supplements as well.
In this randomized controlled trial, Dr. Cosman and colleagues studied 1200 patients whose baseline vitamin D levels were rather high for Americans, around 30 ng/ml. The authors gave half of the patients Forteo and half placebo for a year, but all the patients took vitamin D supplements of varying dosages (between 400 and 1200 IU/day), along with 1,000 mg/day of calcium. The authors then measured both 25(OH)D and activated vitamin D levels periodically during the year of the study.
Remember, no one really knows how Forteo works. Generally, the higher your PTH, the worse your bones (and general health) and Forteo is, in effect, PTH. How can it help? Surprisingly, the authors found that Forteo significantly increased activated vitamin D levels in the blood, but significantly decreased 25(OH)D levels. Thus, it appears that Forteo works via a vitamin D effect, ramping up activated vitamin D levels, but depleting vitamin D reserves.
The authors were concerned about Forteo’s effect on vitamin D. They said, “The use of teriparatide (Forteo) . . . should be delayed until the correction of vitamin D deficiency.” Therefore, if you take Forteo, make sure you take 5,000 IU/day of vitamin D as well.