DEXA Bone Density: What Your Number Changes About Your Treatment

What a DEXA Bone Density Score Actually Measures

A DEXA scan uses two low-dose X-ray beams to calculate bone mineral density (BMD) at the lumbar spine (L1-L4), femoral neck, and total hip. The machine outputs a value in grams per square centimeter, then compares it to two reference populations to produce two scores.

The T-score compares your BMD to a young adult (age 30) reference population of the same sex. The Z-score compares your BMD to an age-, sex-, and ethnicity-matched population. Both numbers matter clinically, but they answer different questions. The International Society for Clinical Densitometry (ISCD) 2019 Position Statement specifies that T-scores should drive diagnosis and treatment decisions in postmenopausal women and men over 50, while Z-scores guide diagnosis in premenopausal women and younger men.

T-Score: The Diagnostic Number

The WHO established the three-tier T-score classification in 1994 and it remains the global standard. The WHO Scientific Group on Osteoporosis (2003) defines normal as T-score at or above -1.0, osteopenia as T-score between -1.0 and -2.5, and osteoporosis as T-score at or below -2.5. A T-score of -2.5 at any of the three standard sites (femoral neck, total hip, or lumbar spine L1-L4) meets the diagnostic threshold for osteoporosis.

Each 1.0-SD decrease in femoral neck BMD is associated with approximately a 2.6-fold increase in hip fracture risk in women, based on data pooled from 11 prospective cohort studies (N=39,563) in the Study of Osteoporotic Fractures and related work compiled by Marshall et al. In BMJ (1996).

Z-Score: The Secondary-Cause Flag

A Z-score below -2.0 means your bone density is lower than expected for your age. This result should prompt workup for secondary osteoporosis, including vitamin D deficiency, hyperparathyroidism, celiac disease, or chronic corticosteroid use. The ISCD 2019 Statement specifies: "In premenopausal women, the diagnosis of osteoporosis should not be made on the basis of densitometric criteria alone." Reference the ISCD Position Statement here.

Which Sites the Machine Scans

The femoral neck T-score is preferred for fracture risk prediction. The lumbar spine T-score is preferred for monitoring treatment response because trabecular bone there turns over faster and shows measurable changes within 12-24 months. When the two sites disagree, the lower score is used for diagnosis per ISCD guidance.

Normal DEXA Bone Density Range

A T-score at or above -1.0 is classified as normal by WHO criteria. Clinically, this means your bone density is within 1 standard deviation of a healthy young adult. No pharmacotherapy is indicated. The treatment recommendation from the National Osteoporosis Foundation (NOF) Clinician's Guide (2014) is weight-bearing exercise, calcium 1,000-1,200 mg daily from food or supplement, and vitamin D 800-1,000 IU daily.

What Normal Still Does Not Rule Out

A normal T-score does not eliminate fracture risk. Bone quality, fall risk, and prior fragility fractures contribute independently of BMD. A woman who fractured a wrist after minimal trauma should be evaluated for treatment consideration even with a T-score above -2.5, per AACE/ACE 2020 Osteoporosis Clinical Practice Guidelines.

Rescreening Interval for Normal Results

The USPSTF 2018 Recommendation Statement on Osteoporosis Screening recommends screening all women 65 and older and younger postmenopausal women with elevated risk. For women with a normal baseline scan, rescreening every 15 years is reasonable if no new risk factors emerge, based on the Study of Osteoporotic Fractures cohort (N=4,957) published by Gourlay et al. In NEJM (2012).

What Osteopenia Means for Treatment

Osteopenia (T-score between -1.0 and -2.5) does not automatically trigger a prescription. The decision to treat depends on your 10-year absolute fracture risk, calculated using the FRAX tool developed by the WHO Collaborating Centre at the University of Sheffield. The FRAX methodology paper by Kanis et al. (2008) in Osteoporosis International describes how clinical risk factors are combined with BMD to estimate risk.

The FRAX Thresholds That Trigger Treatment

The NOF and AACE both recommend initiating pharmacotherapy in patients with osteopenia when the FRAX 10-year probability of major osteoporotic fracture reaches 20% or higher, or the 10-year probability of hip fracture reaches 3% or higher. AACE/ACE 2020 Clinical Practice Guidelines state: "We recommend pharmacological therapy for postmenopausal women and men age 50 and older with osteopenia and a 10-year probability of major osteoporotic fracture of 20% or higher or hip fracture of 3% or higher."

Below those thresholds, treatment remains optional and individualized. Some clinicians treat osteopenia at the lower spine in patients already on aromatase inhibitors or long-term glucocorticoids, where bone loss accelerates well beyond the baseline rate.

Drug Options in the Osteopenia Zone

When FRAX scores cross the treatment threshold, alendronate 70 mg once weekly is the typical starting point due to its generic availability and the strength of the evidence base. Risedronate 35 mg weekly is an alternative for patients who cannot tolerate alendronate. The Fracture Intervention Trial (FIT, N=6,459) demonstrated that alendronate reduced vertebral fracture risk by 47% (RR 0.53, 95% CI 0.41-0.68) in women with low femoral neck BMD over a 3-year period.

Monitoring Interval in Osteopenia

For patients managed with lifestyle measures only, DEXA rescreening every 2-3 years is standard. Patients who start pharmacotherapy warrant a repeat scan at 1-2 years to confirm a treatment response.

What Osteoporosis Means for Treatment

A T-score at or below -2.5, or any prior fragility fracture (fracture from a standing height or lower), meets the criteria for osteoporosis and almost always indicates pharmacotherapy. The AACE/ACE 2020 Guidelines categorize patients with a prior hip or vertebral fracture as "very high risk" and specify that anabolic agents may be preferred over antiresorptive drugs in that subgroup.

First-Line Antiresorptive Therapy

Alendronate 70 mg orally once weekly remains the most prescribed first-line agent in the United States. Patients must take it with 8 ounces of plain water, remain upright for 30 minutes, and wait before eating. Risedronate 35 mg weekly or 150 mg monthly is used when upper GI tolerability is a concern.

Zoledronic acid 5 mg IV once yearly is preferred when adherence with oral therapy is poor or esophageal disease is present. The HORIZON Key Fracture Trial (N=7,765) showed that once-yearly zoledronic acid reduced the risk of morphometric vertebral fracture by 70% (RR 0.30, 95% CI 0.24-0.38, P<0.001) over 3 years versus placebo.

When to Use Anabolic Agents First

For patients classified as very high risk (T-score at or below -3.0, or prior hip or vertebral fracture, or two or more prior fractures), AACE/ACE 2020 Guidelines recommend initiating therapy with an anabolic agent such as teriparatide (recombinant PTH 1-34, 20 mcg SC daily) or romosozumab (210 mg SC monthly for 12 months) before transitioning to an antiresorptive.

Teriparatide is approved for up to 24 months lifetime use per FDA prescribing information. After completing a course of teriparatide, patients transition to a bisphosphonate or denosumab to preserve the BMD gains.

Denosumab as an Alternative

Denosumab 60 mg SC every 6 months is approved for postmenopausal women with osteoporosis at high fracture risk and for men receiving androgen deprivation therapy (ADT) for prostate cancer. The FREEDOM trial (N=7,808) showed denosumab reduced new vertebral fracture risk by 68% (RR 0.32, 95% CI 0.26-0.41, P<0.001) over 36 months. Discontinuation of denosumab without transitioning to a bisphosphonate carries a well-documented rebound fracture risk; this is not a drug to stop abruptly.

DEXA Monitoring on Therapy

Repeat DEXA scanning at 1-2 years after starting pharmacotherapy confirms response and guides continuation decisions. A meaningful response is generally defined as a BMD increase at the lumbar spine or no further significant loss. The ISCD 2019 Position Statement defines the least significant change (LSC) as the precision error multiplied by 2.77; a change smaller than the LSC at a given facility is not statistically meaningful.

How Hormone Therapy Intersects With DEXA Results

Menopausal hormone therapy (MHT) with estrogen preserves bone density and may prevent osteoporosis in perimenopausal and early postmenopausal women. The Women's Health Initiative (WHI) trial showed combined estrogen-progestin therapy reduced hip fracture incidence by 34% (HR 0.66, 95% CI 0.45-0.98) over a mean 5.2-year follow-up. MHT is not FDA-approved primarily as an osteoporosis treatment but is a reasonable option for women under 60, or within 10 years of menopause, who have vasomotor symptoms along with low BMD.

Testosterone and Bone in Men

In men receiving TRT for hypogonadism, testosterone aromatizes to estradiol, and estradiol drives most of the bone-protective effect. Finkelstein et al. (2013) in NEJM demonstrated in 198 men that estradiol, not testosterone per se, was the dominant hormonal determinant of bone density and that BMD at the lumbar spine declined significantly when estradiol was suppressed, even with adequate testosterone. Men on TRT with a T-score below -2.5 should receive concurrent bisphosphonate therapy rather than relying on testosterone alone.

Aromatase Inhibitors and Bone Loss

Women on aromatase inhibitors (AIs) for breast cancer, and men on ADT for prostate cancer, face accelerated bone loss. Eastell et al. In JAMA (2008) showed that letrozole therapy caused a 7.5% loss in lumbar spine BMD over 5 years without bone-protective co-treatment. DEXA at baseline and every 1-2 years is standard in these patients, with bisphosphonate or denosumab started earlier than in the general population.

Glucocorticoid-Induced Osteoporosis: A Special Protocol

Chronic oral glucocorticoid use (prednisone 5 mg per day or more for 3 months or longer) triggers a separate treatment algorithm from the ACR. The 2017 ACR Guideline on the Prevention and Treatment of Glucocorticoid-Induced Osteoporosis recommends that patients on chronic glucocorticoids with a T-score below -2.5, or with moderate-to-high FRAX risk, receive oral bisphosphonate therapy within the first 6 months of glucocorticoid initiation.

Oral Bisphosphonate Protocol in GIO

Alendronate 70 mg weekly or risedronate 35 mg weekly are first-line. For patients who cannot take oral agents, zoledronic acid 5 mg IV yearly is the preferred alternative. DEXA rescreening at 1 year is recommended to confirm response.

Teriparatide in Severe GIO

Patients with T-score at or below -3.0 on glucocorticoids, or those who sustain fractures while on bisphosphonate therapy, may qualify for teriparatide. The ACR 2017 Guideline conditionally recommends teriparatide over oral bisphosphonate for high-risk patients in this context.

Bisphosphonate Drug Holiday: When to Stop and When to Restart

After 5 years of oral bisphosphonate or 3 years of IV zoledronic acid, guidelines support a reassessment. Patients at low-to-moderate risk (T-score above -2.5 at the hip, no prior hip or vertebral fracture) may take a drug holiday of 2-3 years. Patients at high risk should continue therapy without interruption.

Black et al. (FLEX trial, N=1,099) showed that women who discontinued alendronate after 5 years had a modestly elevated clinical vertebral fracture risk compared to those who continued, but no significant difference in nonvertebral fractures over the subsequent 5 years. This supports the idea of selective continuation rather than universal cessation.

Repeat DEXA at the end of the drug holiday determines whether to restart. A T-score that has fallen back below -2.5, or a new fragility fracture during the holiday, signals the need to resume antiresorptive therapy.

How GLP-1 Receptor Agonists Affect Bone Density

GLP-1 receptor agonists (semaglutide, liraglutide, tirzepatide) produce substantial weight loss, and rapid weight loss is independently associated with BMD decline. An analysis from the STEP-1 trial (N=1,961) published as part of extended metabolic outcomes data noted that semaglutide 2.4 mg produced 14.9% mean body weight loss at 68 weeks; the primary STEP-1 paper in NEJM by Wilding et al. (2021) documented this weight loss alongside changes in lean mass.

Patients losing more than 10% of body weight on GLP-1 therapy warrant baseline DEXA before or within 6 months of starting treatment, especially postmenopausal women over 55 and men over 65. Resistance exercise and adequate protein intake (at least 1.2 g per kg body weight per day) should accompany GLP-1 therapy to minimize lean mass and BMD loss.

Reading Your DEXA Report: Line by Line

A standard DEXA report lists BMD in g/cm2, T-score, and Z-score for each site. The lowest T-score across the femoral neck, total hip, and lumbar spine determines your diagnosis. Some reports also include a trabecular bone score (TBS), which estimates bone microarchitecture independently of BMD. Silva et al. In JBMR (2017) showed that TBS below 1.230 predicts fracture risk independently of T-score, adding incremental information in patients with diabetes or obesity who may have higher-than-expected fracture risk for their measured BMD.

Precision Error and What Changes Are Real

Every DEXA machine has a precision error, typically 1-1.5% at the lumbar spine and 1.5-2% at the hip. A reported BMD change that does not exceed the facility-specific least significant change (LSC) may be measurement noise. Ask your imaging center for their published LSC values before interpreting serial scans.

When to Get Scans at the Same Facility

Serial DEXA scans should be performed on the same machine, or at minimum the same manufacturer's platform, when possible. Cross-calibration errors between different machines can exceed the actual biological BMD change being measured, making trend interpretation unreliable.

Secondary Causes of Low Bone Density: The Workup

Before starting pharmacotherapy, a brief lab panel rules out secondary causes. Standard workup includes serum calcium, 25-hydroxyvitamin D, parathyroid hormone (PTH), TSH (thyroid excess suppresses bone), complete blood count, serum protein electrophoresis (to exclude multiple myeloma), and 24-hour urine calcium and creatinine. AACE/ACE 2020 Guidelines list at least 15 conditions that can cause secondary osteoporosis, including celiac disease, which requires serum tissue transglutaminase IgA testing if there is any clinical suspicion.

Vitamin D Deficiency Correction Before Bisphosphonate Start

Starting a bisphosphonate while vitamin D is deficient increases the risk of hypocalcemia. Correct 25-hydroxyvitamin D to at least 30 ng/mL before the first bisphosphonate dose. Supplementation with 50,000 IU ergocalciferol weekly for 8-12 weeks typically corrects moderate deficiency; recheck serum 25(OH)D at the end of the loading course.