Osteoporosis: Causes, Diagnosis, and Evidence-Based Treatment

What Is Osteoporosis and How Is It Defined?

Osteoporosis is a systemic skeletal disease in which bone mass falls and bone micro-architecture deteriorates, leaving bones fragile enough to fracture under forces that healthy bone tolerates easily. The World Health Organization sets the diagnostic threshold at a bone mineral density (BMD) T-score at or below -2.5 at the lumbar spine, femoral neck, or total hip on dual-energy X-ray absorptiometry (DXA). A T-score between -1.0 and -2.5 is classified as osteopenia, a state of lower-than-normal density that carries meaningful fracture risk but does not yet meet the osteoporosis threshold.

Bone is not static. Osteoclasts continuously resorb old bone while osteoblasts deposit new matrix, a remodeling cycle that completes roughly every 3 to 4 months. Peak bone mass is reached between ages 25 and 30; after that, resorption gradually outpaces formation. When the imbalance becomes severe, trabeculae (the internal scaffolding of cancellous bone) thin and perforate, making vertebral bodies and the femoral neck especially vulnerable. The National Institutes of Health Consensus Development Panel defined osteoporosis as "a disease characterized by low bone mass and microarchitectural deterioration of bone tissue, leading to enhanced bone fragility and a consequent increase in fracture risk" [1].

Approximately 10 million Americans have osteoporosis and another 44 million have low bone mass, according to the National Osteoporosis Foundation [2]. Osteoporotic fractures account for more than 2 million events annually in the United States, with total costs exceeding $19 billion per year.

Who Gets Osteoporosis? Risk Factors Across Populations

Risk divides into non-modifiable and modifiable categories, and understanding both informs how aggressively to screen and treat any individual patient.

Non-modifiable risk factors include female sex, advanced age, White or Asian ethnicity, personal history of fragility fracture after age 50, first-degree relative with hip fracture, and early menopause (before age 45). Genetic variants in genes encoding the vitamin D receptor and collagen type I alpha 1 (COL1A1) explain a substantial fraction of heritable BMD variation.

Modifiable risk factors cover tobacco use, excessive alcohol (three or more drinks per day), calcium and vitamin D deficiency, physical inactivity, low body weight (BMI <18.5 kg/m²), and long-term use of bone-depleting medications such as glucocorticoids, proton-pump inhibitors, and aromatase inhibitors.

The FRAX tool (developed at the University of Sheffield and endorsed by the WHO) integrates clinical risk factors with or without femoral neck BMD to generate a 10-year probability of major osteoporotic fracture and hip fracture. The American College of Rheumatology (ACR) and the National Osteoporosis Foundation recommend considering pharmacologic treatment when the 10-year hip fracture probability reaches 3% or the major osteoporotic fracture probability reaches 20% [3].

Postmenopausal Bone Loss: Why It Happens So Fast

Estrogen deficiency is the single largest driver of rapid bone loss in women, acting through multiple cellular pathways simultaneously. This makes the perimenopausal and early postmenopausal years the highest-risk window for BMD decline.

Estrogen suppresses osteoclast activity by down-regulating RANK ligand (RANKL) and up-regulating osteoprotegerin (OPG), a decoy receptor that blocks RANKL. When estrogen falls at menopause, the RANKL/OPG ratio shifts sharply toward resorption. Women lose 1 to 3 percent of spinal BMD per year in the first 5 years after the final menstrual period, a rate roughly five to ten times faster than age-related loss in men of the same age [4].

The USPSTF recommends BMD screening for all women aged 65 and older and for younger postmenopausal women whose 10-year fracture risk equals or exceeds that of a 65-year-old White woman with no additional risk factors [5]. That translates to a FRAX 10-year major osteoporotic fracture probability of approximately 9.3%.

Hormone therapy (HT) with estrogen, with or without progestogen, is the only intervention that addresses the root cause of postmenopausal bone loss. The Women's Health Initiative (WHI) trial (N=16,608 for the combined arm) showed that conjugated equine estrogen plus medroxyprogesterone acetate reduced hip fracture risk by 34% and vertebral fracture risk by 34% compared with placebo over a mean 5.6 years [6]. The Endocrine Society's 2022 postmenopausal hormone therapy guideline states: "In healthy women younger than 60 years or within 10 years of menopause onset, the benefits of HT for symptom relief and fracture prevention outweigh the risks for most women" [7].

For women who cannot or choose not to use HT, bisphosphonates remain the standard first-line pharmacological option (see Treatment section).

Male Osteoporosis: An Underdiagnosed Problem

One in four men over age 50 will sustain an osteoporotic fracture, yet men are screened and treated far less often than women. Male osteoporosis carries a worse prognosis per fracture: men have a hip fracture mortality roughly twice that of women at one year [8].

Testosterone deficiency is the male analogue of estrogen deficiency. Hypogonadism, whether primary or secondary, accelerates bone loss through the same RANKL/OPG pathway. The Osteoporotic Fractures in Men (MrOS) study (N=5,995 men aged 65 and older) demonstrated that low free testosterone was independently associated with increased fracture risk after adjustment for BMD [9].

Other common causes of secondary male osteoporosis include chronic glucocorticoid use, alcohol use disorder, hypogonadism from androgen-deprivation therapy (ADT) for prostate cancer, hyperparathyroidism, and celiac disease. The Endocrine Society recommends DXA screening for men aged 70 and older, and for men aged 50 to 69 with clinical risk factors or prior fragility fracture [10].

Treatment evidence in men is more limited than in women, but alendronate 70 mg weekly, risedronate 35 mg weekly, and zoledronic acid 5 mg IV annually each have demonstrated fracture risk reduction in male populations. The FIT-extension and Health Outcomes and Reduced Incidence with Zoledronic Acid Once Yearly (HORIZON) trials included male cohorts and showed similar relative risk reductions to those seen in women [11].

Steroid-Induced Osteoporosis: Prevention and Treatment

Glucocorticoid-induced osteoporosis (GIOP) is the most common cause of secondary osteoporosis and the leading drug-induced cause of fracture worldwide. Fracture risk rises within the first 3 months of starting oral corticosteroids at prednisone-equivalent doses as low as 5 mg/day, and the risk is at least partially reversible upon discontinuation.

Glucocorticoids suppress bone formation by inducing osteoblast and osteocyte apoptosis while simultaneously increasing osteoclast lifespan. They also reduce intestinal calcium absorption and increase renal calcium excretion, driving secondary hyperparathyroidism that further accelerates resorption.

The ACR 2022 Guideline for the Prevention and Treatment of Glucocorticoid-Induced Osteoporosis stratifies patients into low, medium, and high fracture-risk groups [3]. Key protocol points:

• All patients starting glucocorticoids at 2.5 mg/day or higher for 3 or more months should receive calcium (1,000 to 1 to 200 mg/day total) and vitamin D (600 to 800 IU/day, titrating to a serum 25-OH vitamin D of 40 to 60 ng/mL).
• Patients at medium or high fracture risk should start oral bisphosphonate therapy, typically alendronate 70 mg weekly or risedronate 35 mg weekly, at the same time as the glucocorticoid.
• Patients at very high fracture risk (prior fragility fracture, T-score <-2.5, or FRAX 10-year major osteoporotic fracture probability above 20%) may qualify for teriparatide 20 mcg/day subcutaneously, which showed superiority over alendronate in reducing new vertebral fractures in a head-to-head trial in glucocorticoid-treated patients (N=428, P<0.001 for new vertebral fracture reduction at 18 months) [12].

Baseline DXA and a FRAX calculation are recommended before or within 6 months of starting chronic glucocorticoid therapy, with repeat DXA every 1 to 2 years while on therapy.

Diagnosing Osteoporosis: DXA, FRAX, and Lab Work

A proper diagnostic workup goes beyond a single T-score. Fracture risk is a composite of BMD, bone quality, fall risk, and comorbid conditions, and treating one number in isolation misses the clinical picture.

DXA scanning measures BMD at the lumbar spine (L1-L4) and proximal femur. Results are expressed as both a T-score (comparison to a young adult reference population) and a Z-score (comparison to age-matched peers). A Z-score below -2.0 in a premenopausal woman or a man under 50 suggests secondary osteoporosis and warrants workup. DXA delivers a radiation dose of approximately 1 to 6 microsieverts, less than a dental X-ray.

FRAX (available at shef.ac.uk/FRAX) accepts 12 clinical risk factors plus optional femoral neck BMD to output 10-year fracture probabilities. For clinical decision thresholds the National Osteoporosis Foundation Clinician's Guide (2014) [13] remains a widely used reference.

Laboratory workup for suspected secondary osteoporosis typically includes: complete metabolic panel, serum calcium, phosphorus, 25-OH vitamin D, PTH, CBC, serum protein electrophoresis (to exclude multiple myeloma), TSH, and in men, total and free testosterone. In women, FSH and estradiol help characterize menopausal status. Bone turnover markers, specifically serum C-terminal telopeptide (CTX) for resorption and procollagen type 1 N-terminal propeptide (P1NP) for formation, are used to monitor treatment response rather than for diagnosis.

FDA-Approved Treatments for Osteoporosis

Treatment selection depends on fracture risk level, sex, route-of-administration preference, renal function, and tolerability. Several drug classes with distinct mechanisms are available.

Bisphosphonates

Bisphosphonates inhibit osteoclast-mediated resorption by accumulating in bone mineral and inducing osteoclast apoptosis. They are the most widely prescribed osteoporosis drugs globally.

• Alendronate (Fosamax): 70 mg orally once weekly. The Fracture Intervention Trial (FIT, N=2,027) showed a 47% relative risk reduction (RRR) in hip fracture and 55% RRR in vertebral fracture over 3 years in women with prior vertebral fracture [14].
• Risedronate (Actonel): 35 mg orally once weekly or 150 mg once monthly.
• Zoledronic acid (Reclast): 5 mg IV once annually. The HORIZON Key Fracture Trial (N=7,765) showed 41% RRR in hip fracture and 70% RRR in morphometric vertebral fracture over 3 years [15].
• Ibandronate (Boniva): 150 mg orally monthly or 3 mg IV every 3 months, with evidence limited to vertebral fracture reduction.

Oral bisphosphonates require fasting administration with 8 oz plain water and 30 minutes of upright positioning to reduce esophageal irritation. After 3 to 5 years of therapy, a "drug holiday" may be appropriate for lower-risk patients given the theoretical risk of atypical femoral fracture with long-term use, though that risk is very low (3 to 50 per 100,000 person-years) [16].

Denosumab

Denosumab (Prolia) is a human monoclonal antibody against RANKL, given as 60 mg subcutaneously every 6 months. The FREEDOM trial (N=7,808) demonstrated 68% RRR in vertebral fracture, 40% RRR in hip fracture, and 20% RRR in non-vertebral fracture at 36 months [17]. Denosumab is particularly useful in patients with impaired renal function (eGFR <35 mL/min/1.73 m²) where bisphosphonates are cautioned.

A critical point: discontinuing denosumab without transitioning to a bisphosphonate causes rapid rebound bone loss and multiple vertebral fractures within 12 to 18 months. Clinicians must plan the transition strategy before starting denosumab.

Teriparatide and Abaloparatide

Both are anabolic agents that stimulate bone formation. Teriparatide (Forteo) is recombinant human PTH(1-34), 20 mcg/day subcutaneously; abaloparatide (Tymlos) is a PTHrP analogue at 80 mcg/day subcutaneously. In the ACTIVE trial (N=2,463), abaloparatide reduced major osteoporotic fracture incidence by 70% vs. placebo at 18 months [18]. Duration is limited to 24 months total across a lifetime. After stopping, patients must transition to an antiresorptive to preserve gains.

Romosozumab

Romosozumab (Evenity) is a monoclonal antibody against sclerostin that simultaneously increases bone formation and decreases resorption. It is given as two 105 mg subcutaneous injections monthly for 12 months only. The ARCH trial (N=4,093) compared romosozumab followed by alendronate against alendronate alone and showed 48% RRR in new vertebral fracture and 19% RRR in hip fracture over 24 months [19]. Because the ARCH trial showed a small imbalance in cardiovascular events in the romosozumab arm, the FDA added a boxed warning; romosozumab should be avoided in patients with prior myocardial infarction or stroke within 12 months.

Raloxifene

Raloxifene (Evista) is a selective estrogen receptor modulator (SERM), 60 mg/day orally. It reduces vertebral fracture risk by about 30 to 50% but does not reduce hip fracture risk and increases risk of venous thromboembolism. It remains an option for postmenopausal women who need concurrent breast cancer risk reduction.

Calcium, Vitamin D, and Lifestyle: The Non-Negotiable Foundation

No pharmacological intervention works optimally on a foundation of calcium and vitamin D deficiency.

Calcium: The Institute of Medicine recommends 1 to 000 mg/day for women aged 19 to 50 and men 19 to 70, rising to 1 to 200 mg/day for women over 50 and men over 70. Dietary sources are preferred. Supplemental doses above 500 mg at one time are poorly absorbed; split doses accordingly. Evidence from the Women's Health Initiative Calcium/Vitamin D trial does not support large supplemental doses in women already meeting dietary requirements, and excess supplemental calcium (not dietary) has been associated with modest cardiovascular signal in some meta-analyses [20].

Vitamin D: The Endocrine Society recommends maintaining serum 25-OH vitamin D at 30 ng/mL or above, with 40 to 60 ng/mL as a reasonable target in osteoporosis patients receiving pharmacotherapy. Typical supplemental doses range from 1,500 to 2 to 000 IU/day of vitamin D3 to reach that range. The VITAL trial (N=25,871) found that vitamin D3 2 to 000 IU/day did not reduce fracture incidence in the general population but did not include patients selected for osteoporosis or vitamin D deficiency [21].

Weight-bearing exercise: Resistance training and impact exercise (brisk walking, jogging, stair climbing) stimulate osteoblast activity via mechanotransduction. A Cochrane review of exercise trials found that impact exercise improved femoral neck BMD by approximately 1% vs. controls in postmenopausal women [22]. Fall-prevention programs, including tai chi and balance training, reduce falls by 21 to 23%, which is arguably as important as BMD gains for reducing fracture incidence.

Smoking cessation and alcohol moderation are standard recommendations without specific fracture trial data but are supported by large observational datasets.

Monitoring Treatment Response

DXA should be repeated 1 to 2 years after starting pharmacotherapy, then every 2 years once stable. A significant increase in BMD is defined as greater than the least significant change (LSC) of the specific DXA machine, typically 2 to 3% at the lumbar spine.

Bone turnover markers offer earlier signals. Serum CTX should fall by at least 25 to 35% from baseline within 3 to 6 months on an antiresorptive, and P1NP should rise within 1 to 3 months on an anabolic agent. If markers do not respond appropriately, the clinician should evaluate adherence, calcium/vitamin D adequacy, and whether secondary causes have been missed.

Treatment duration remains individualized. After 5 years on oral bisphosphonate or 3 years on zoledronic acid, a formal reassessment of fracture risk determines whether to continue, take a drug holiday, or switch to an alternative agent.