Teriparatide

• Severe Osteoporosis
• Glucocorticoid-Induced Osteoporosis
• High-Risk Fractures
• Bone Loss Treatment

Teriparatide (Forteo) is a recombinant fragment of human parathyroid hormone (PTH 1-34) used to treat severe osteoporosis in patients at high risk of fracture. Unlike bisphosphonates and other antiresorptive agents that slow bone breakdown, teriparatide is anabolic — it actively stimulates new bone formation, increasing both mineral density and microarchitectural integrity. It is delivered by daily subcutaneous injection from a multi-dose pen and is reserved for patients whose fracture risk justifies its complexity, cost, and lifetime two-year limit. The drug occupies a specific niche where the magnitude of need warrants the intensity of treatment.

Mechanism of Action

Endogenous parathyroid hormone has a paradoxical relationship with bone: when secreted continuously, as in primary hyperparathyroidism, it drives net resorption and bone loss; when delivered in brief intermittent pulses, it favors bone formation. Teriparatide exploits this duality. A once-daily subcutaneous injection produces a sharp peak of PTH 1-34 activity that lasts a few hours and then falls back to baseline, the classic intermittent exposure pattern. PTH receptors on osteoblasts and on osteoblast precursors are activated, increasing osteoblast number and survival, prolonging the bone formation phase of the remodeling cycle, and recruiting quiescent bone-lining cells back into active osteoblast service. Wnt signaling and IGF-1 pathways both contribute to the anabolic response.

The net result over months is increased trabecular thickness, restored connectivity, improved cortical thickness in the spine and hip, and improved bone strength beyond what BMD alone captures. Vertebral fracture risk falls by roughly 65 percent and nonvertebral fracture risk by about 50 percent in the pivotal trial. The early bone formation effect outpaces resorption for the first six to twelve months, producing a net anabolic window. Beyond about two years the formation advantage narrows, which is the rationale for the 24-month lifetime cap. After completing teriparatide, patients require transition to an antiresorptive — usually a bisphosphonate or denosumab — to lock in the bone gained, because stopping anabolic therapy without follow-on therapy allows rapid bone density loss. Calcium absorption from the gut increases modestly through PTH-driven activation of vitamin D, which supports the formation effect but also explains the transient hypercalcemia that some patients experience. The NIH bone health resources provide accessible patient information.

Clinical Use

Within the osteoporosis treatment algorithm, oral bisphosphonates such as alendronate and risedronate remain first-line for most patients because of cost, oral dosing, and decades of outcome data. Teriparatide is positioned for patients at very high fracture risk: those with a prior vertebral or hip fracture, those with multiple low-trauma fractures, those with T-scores at or below -3.0, those who have failed antiresorptive therapy with new fractures or continued bone loss, and those with glucocorticoid-induced osteoporosis at high risk. Comparative effectiveness data from the VERO trial showed teriparatide outperformed risedronate in reducing new vertebral and clinical fractures in postmenopausal women with severe osteoporosis.

It is approved for postmenopausal women with osteoporosis, men with primary or hypogonadal osteoporosis, and patients of either sex with glucocorticoid-induced osteoporosis. Patient selection considers ability to handle daily injections, baseline serum calcium, history of skeletal radiation, and contraindicating malignancies. The romosozumab/anabolic option romosozumab is an alternative anabolic agent with a different mechanism and a 12-month course; choice between teriparatide, abaloparatide, and romosozumab depends on cardiovascular history, prior treatment, and access. Patients can read more in our overview of osteoporosis and bone strength at any age and our broader bone health after 50 guide. Background on osteoporosis from the Endocrine Society supports patient education. Sequence matters in osteoporosis treatment: starting with anabolic therapy followed by antiresorptive consolidation produces greater BMD gain than the reverse sequence in patients with severe disease.

How to Take It

Teriparatide is administered as 20 mcg subcutaneously once daily into the thigh or abdominal wall. The Forteo pen contains 28 daily doses; the first injection from each new pen should be performed sitting or lying down because transient orthostatic hypotension and dizziness can occur within four hours of dosing, particularly during the first several days. Injection sites are rotated to prevent local irritation. The pen is refrigerated at 36 to 46 degrees Fahrenheit and must not be frozen. Once in use, it can remain refrigerated for up to 28 days before discarding. Missed doses are taken the same day if remembered; if a full day has passed, the dose is skipped — doubling up is not done. Calcium and vitamin D supplementation should continue throughout treatment to support the increased bone formation, typically targeting 1000 to 1200 mg of elemental calcium and 800 to 1000 IU of vitamin D daily through diet and supplements combined. The first week of therapy commonly produces mild leg cramps, transient dizziness, or nausea that improve over time. Travel logistics include keeping the pen cool with portable cold packs and informing TSA when flying.

Monitoring and Follow-Up

Baseline labs include serum calcium, 25-hydroxyvitamin D, creatinine, alkaline phosphatase, and a urine calcium if there is any history of stones. Bone mineral density by DEXA scan establishes baseline T-scores at the spine, total hip, and femoral neck. Serum calcium is rechecked at one month — transient mild rises are common and usually require no action. Persistent hypercalcemia warrants evaluation for excessive supplementation, dehydration, or other contributing causes. DEXA is repeated at one to two years to document gains. Markers of bone turnover such as serum P1NP and CTX rise within weeks and confirm biochemical response, but are optional in routine practice. Numbers worth attention include serum calcium above 11 mg/dL, alkaline phosphatase rising significantly above baseline, or persistent orthostatic symptoms despite the four-hour rest window. After the maximum 24-month lifetime course, patients transition to an antiresorptive within weeks to preserve gains. A repeat DEXA after one year on follow-on therapy confirms maintenance. Ongoing fall prevention assessment, vision check, and home safety review are integral to fracture reduction beyond the medication itself, particularly important for older adults at risk.

Special Populations

Teriparatide is contraindicated in pediatric and young adult patients with open epiphyses because of theoretical osteosarcoma risk extrapolated from rat studies. It is also contraindicated in patients with Paget's disease, prior skeletal radiation therapy, bone metastases, hypercalcemia, or unexplained alkaline phosphatase elevation. Pregnancy data are limited; use is generally not recommended. Lactation excretion is unknown. In renal impairment with creatinine clearance below 30 mL/min, use is approached cautiously because of limited safety data and possible accumulation. Hepatic impairment requires no specific adjustment. Older adults tolerate the medication well but require attention to hydration and orthostatic symptoms. Patients with active urolithiasis are managed cautiously because of transient hypercalciuria. Concurrent digoxin therapy warrants monitoring because hypercalcemia can predispose to digoxin toxicity. Patients with prior multiple myeloma or other bone-affecting malignancies should not receive teriparatide because of theoretical concerns about stimulating malignant osteoblastic activity. The FDA label details boxed warnings and contraindications.

When to Contact Your Doctor

Call promptly for severe nausea or vomiting, persistent confusion, muscle weakness, increased thirst, or excessive urination — these may signal hypercalcemia. Persistent dizziness or fainting, especially after the first injections, deserves evaluation even after the four-hour rest window. New flank pain, blood in the urine, or symptoms suggesting a kidney stone warrant prompt assessment. Severe injection site reactions, persistent leg cramps that do not improve, or signs of allergic reaction such as rash or swelling should be reported. Any new bone pain, particularly in the hip or thigh, deserves attention. New symptoms suggestive of digoxin toxicity in patients on that medication — nausea, visual halos, slow or irregular pulse — require immediate review. A new fracture, particularly with low-energy trauma, warrants reassessment of the overall osteoporosis treatment plan and evaluation of potential causes for breakthrough fracture.

If you have severe osteoporosis or have not responded to first-line therapy and would like to discuss anabolic options, contact us or schedule a visit at our St. Petersburg internal medicine practice for individualized planning.