Hyperparathyroidism is a condition characterized by an excess of parathyroid hormone (PTH), with loss of the finely regulated homeostasis of calcium and phosphate and a direct effect on the skeleton through a pathological increase in bone remodeling. In this context, “metabolic bone disease” refers to the spectrum of abnormalities in turnover, mineralization and microarchitecture that may result from primary hyperparathyroidism, caused by autonomous parathyroid secretion, secondary hyperparathyroidism, representing an adaptive response typically associated with chronic kidney disease, and tertiary hyperparathyroidism, characterized by autonomy after prolonged stimulation. Clinical consequences range from a subtle reduction in bone mineral density and increased fracture risk to classical high-turnover forms such as osteitis fibrosa cystica and focal lesions known as brown tumors.
The central clinical issue is that hyperparathyroidism may cause skeletal damage even when the presentation appears relatively asymptomatic, because PTH-driven signaling cumulatively alters cortical and trabecular bone, impairs bone biomechanics and increases vulnerability to falls and fractures. Assessment must therefore integrate etiology, biological severity, duration of exposure, mineral reserve and comorbidities, explicitly distinguishing forms associated with CKD-MBD (chronic kidney disease-mineral and bone disorder) from “pure” endocrine forms, because their mechanisms, diagnostic tools and therapeutic priorities differ.
The epidemiology of bone disease associated with hyperparathyroidism strongly depends on the distribution of underlying causes and the extent to which the condition is detected early. In populations where serum calcium and PTH measurements are widely available, primary hyperparathyroidism frequently presents in a “non-classical” form, with biochemical abnormalities and measurable skeletal involvement rather than advanced bone pain and deformities. Under these circumstances, skeletal involvement tends to manifest as preferential loss of cortical bone, reduced bone density at the distal radius and an increased fracture risk. Historical presentations with osteitis fibrosa cystica are now less common but remain clinically relevant in settings of delayed diagnosis or more aggressive disease.
From a nephrological perspective, bone disease caused by secondary hyperparathyroidism is a central component of CKD-MBD and becomes increasingly important as glomerular filtration declines, accelerating in advanced stages and during dialysis. In this setting, the risk of skeletal damage depends not only on absolute PTH levels but also on the cumulative history of phosphate disturbances, calcitriol deficiency, inflammation and treatment. These factors may shift the phenotype toward excessive turnover or toward iatrogenic low turnover, which is equally hazardous in terms of fractures and bone pain.
The main risk factors for a severe skeletal phenotype in primary hyperparathyroidism include a longer duration of hypercalcemia, high PTH levels, vitamin D deficiency and conditions that amplify negative bone balance, such as menopause, muscle weakness and physical inactivity. Clinically, diffuse bone pain, previous fractures or a marked reduction in proximal muscle strength suggest prolonged PTH-driven activity with high turnover and loss of structural bone tissue.
In secondary hyperparathyroidism caused by CKD, the principal determinants include chronic phosphate retention, reduced calcitriol, relative hypocalcemia and increased phosphaturic signals such as FGF23, creating a vicious cycle that promotes parathyroid hyperplasia. Prolonged exposure to therapies that excessively suppress turnover, advanced age, diabetes and chronic inflammation may instead predispose patients to adynamic bone disease. This condition increases fracture risk even in the absence of markedly elevated PTH because it impairs the ability of bone to repair microdamage and adapt to mechanical loading.
Another epidemiological determinant is the quality of clinical management. Monitoring programs that include trends in PTH, phosphate, calcium and alkaline phosphatase, together with fracture-risk assessment and optimization of vitamin D status, reduce the likelihood of advanced presentations. Conversely, in patients referred late or receiving discontinuous care, bone disease may become evident only after substantial structural damage has developed, resulting in a greater clinical burden and more complex treatment.
The relationship between hyperparathyroidism and metabolic bone disease arises from the role of PTH as an endocrine regulator of extracellular calcium and phosphate availability and as a direct modulator of skeletal remodeling. Under physiological conditions, minimal variations in ionized calcium rapidly modify PTH secretion, producing effects on the kidneys, including calcium reabsorption and phosphate excretion, and on the production of calcitriol, which in turn regulates intestinal absorption and feedback on the parathyroid glands. Hyperparathyroidism disrupts this finely tuned regulation, transforming an adaptive signal into a chronic stimulus that remodels bone and alters mineral metabolism in a manner no longer proportionate to the body's physiological requirements.
In primary hyperparathyroidism, the cause is autonomous PTH secretion by one or more parathyroid glands, frequently associated with benign lesions or hyperplasia. The pathophysiological result is a persistent increase in the secretory set point, with hypercalcemia and often a relative reduction in phosphate levels. In bone, chronic stimulation by PTH increases turnover through pathways that enhance the availability of RANKL and reduce the activity of regulatory systems such as osteoprotegerin, promoting osteoclast differentiation and activity. This produces an imbalanced remodeling process that is particularly damaging to cortical bone, where porosity increases and mechanical strength declines even before clear symptoms appear.
In secondary hyperparathyroidism caused by chronic kidney disease, the etiology is different and more complex. The reduced ability of the kidneys to eliminate phosphate and produce calcitriol, together with changes in FGF23, acid-base balance and inflammation, creates persistent stimulation of the parathyroid glands. PTH secretion becomes progressively higher and less suppressible, accompanied by glandular hyperplasia and, in long-standing cases, possible progression to functional autonomy, known as tertiary hyperparathyroidism. In this context, bone disease is not solely a hyperparathyroid disorder but part of a systemic syndrome that includes vascular calcification, abnormalities of mineral metabolism and frailty, with clinical effects extending beyond the skeleton.
Skeletal pathophysiology may develop along a continuum encompassing high-turnover conditions such as osteitis fibrosa, mixed forms and low-turnover conditions such as adynamic bone disease. The histomorphometric classification of renal osteodystrophy uses the TMV system, which evaluates turnover, mineralization and volume, because uremic bone may exhibit not only changes in turnover but also abnormalities in mineralization and bone volume. This distinction is clinically decisive. A patient with CKD may experience pain and fractures either because of excessive PTH-driven resorption or because of an inability to form and mineralize bone adequately, and the therapeutic approach differs substantially between these conditions.
Classical high-turnover manifestations include subperiosteal bone resorption, cortical rarefaction, cystic lesions and the formation of brown tumors. These are not neoplasms but reactive lesions caused by extreme remodeling, with fibrous tissue and hemosiderin deposits. Brown tumors may mimic metastases or multiple myeloma, requiring a rigorous differential diagnosis. At the systemic level, chronic PTH excess also impairs neuromuscular function and contributes to proximal muscle weakness and postural instability, further increasing the risk of falls and fractures.
Finally, the pathophysiology of hyperparathyroid bone disease interacts with vitamin D deficiency, which is common in both primary hyperparathyroidism and CKD and worsens mineral balance while potentially increasing the magnitude of the secretory stimulus. Correction of vitamin D deficiency and interpretation of biochemical markers must therefore be integrated into a dynamic model in which an isolated measurement is less informative than its trajectory over time and its consistency with clinical and imaging findings.
The clinical presentation of hyperparathyroid bone disease is variable and often insidious because structural damage may accumulate before clear symptoms develop. In primary hyperparathyroidism detected at an early stage, patients may report nonspecific symptoms such as fatigue, reduced physical performance and vague muscular pain, while skeletal involvement emerges through reduced bone mineral density, vertebral microfractures or fragility fractures. In more advanced forms, bone pain becomes more characteristic and localized, worsens with weight-bearing and limits function. It may also be associated with deformities or loss of height when vertebral compression fractures are present.
Key elements in the medical history include low-energy fractures, persistent bone pain, muscle cramps and proximal weakness, together with an assessment of factors that amplify fragility, such as menopause, immobility, protein malnutrition and the use of medications that affect bone and mineral metabolism. In patients with CKD, it is essential to reconstruct the nephrological history, duration of dialysis, trends in PTH and phosphate, and exposure to phosphate binders, vitamin D analogues and calcimimetics, because these factors help distinguish high-turnover from low-turnover phenotypes.
On physical examination, metabolic bone disease may present with tenderness over skeletal segments, joint restriction secondary to pain, an unstable gait and reduced proximal muscle strength, particularly in the pelvic and shoulder girdles. More severe cases may show skeletal deformities, especially in the presence of multiple fractures or malunions. In advanced CKD, frailty is often multifactorial and includes neuropathy, sarcopenia and anemia, which increase the risk of falls and make skeletal disease clinically important even when it is not immediately evident.
Focal lesions such as brown tumors may present with localized pain, swelling or an incidental radiological finding and may occasionally cause pathological fractures. Their clinical significance is twofold. They indicate prolonged PTH excess and high bone turnover, but they also require differential diagnosis from neoplastic lesions, particularly when multiple lesions or increased uptake on functional imaging are present. In these circumstances, the biochemical context and parathyroid assessment become integral components of the diagnostic evaluation of an apparent focal bone disorder.
Overall, the clinical presentation should be interpreted as the expression of a remodeling disorder that reduces bone quality and increases the risk of fractures, with direct consequences for independence, chronic pain and functional prognosis. Assessment must therefore extend beyond an isolated calcium or PTH value and quantify both the extent of skeletal damage and its evolution over time.
Metabolic bone disease associated with hyperparathyroidism should be suspected whenever skeletal fragility, bone pain or fractures cannot be adequately explained by primary osteoporosis or by trauma of proportionate severity. A typical clinical warning sign is the occurrence of low-energy fractures at characteristic sites, particularly when associated with proximal weakness and reduced muscle mass, because excess PTH may affect bone and muscle simultaneously, increasing both intrinsic skeletal fragility and the risk of falls.
From an endocrine perspective, the combination of even mild hypercalcemia, elevated or inappropriately normal PTH and reduced bone mineral density, particularly in cortical bone, suggests primary hyperparathyroidism with skeletal involvement. Suspicion should increase in the presence of persistent bone pain, incidental vertebral fractures, loss of height and a history of nephrolithiasis or hypercalciuria, because these findings increase the pre-test probability of a clinically significant parathyroid disorder.
From a nephrological perspective, any patient with moderate to advanced CKD or receiving dialysis who develops bone pain, fractures, suggestive radiological abnormalities or marked postural instability should be evaluated within the framework of CKD-MBD. In this population, persistently elevated or rapidly rising PTH levels, together with increased alkaline phosphatase and abnormalities in calcium and phosphate, increase the probability of a high-turnover phenotype. Conversely, low or excessively suppressed PTH in a patient receiving intensive treatment suggests a risk of low turnover and adynamic bone disease.
A particularly error-prone clinical scenario is the presence of multiple or focal osteolytic lesions, especially when accompanied by significant pain or increased uptake on functional imaging. In such cases, an oncological diagnosis is often considered immediately, but hyperparathyroidism with brown tumors must remain in the differential diagnosis because correct recognition completely changes management and may prevent inappropriate procedures. Clinical suspicion must therefore lead to an integrated diagnostic pathway, with urgency proportionate to the fracture risk and to the potential severity of alternative diagnoses.
The diagnosis of hyperparathyroidism associated with metabolic bone disease requires two logical steps: confirmation of the parathyroid disorder and definition of the skeletal phenotype in terms of turnover, structural damage and fracture risk. When primary hyperparathyroidism is suspected, the cornerstone is documentation of persistent hypercalcemia with elevated or inappropriately non-suppressed PTH, integrating phosphate, creatinine, 25-hydroxyvitamin D and urinary calcium measurements when indicated. In patients with CKD, interpretation is more complex because calcium and phosphate may fluctuate and PTH must be evaluated as a trend in relation to disease stage, treatment and bone markers, avoiding conclusions based on a single blood sample.
According to Kidney Disease: Improving Global Outcomes (KDIGO) guidelines for CKD-MBD and international recommendations for primary hyperparathyroidism, assessment should combine biochemical testing, skeletal imaging and estimation of fracture risk. Invasive procedures should be reserved for cases in which the bone phenotype cannot be defined using noninvasive methods or when treatment depends on distinguishing high from low turnover.
Diagnostic assessment of hyperparathyroid bone disease
Assessment of bone damage must consider that hyperparathyroidism characteristically affects cortical bone by increasing porosity and reducing strength, although trabecular bone may also be altered depending on the duration and severity of disease. DXA provides a quantitative estimate but does not constitute a complete assessment because bone quality abnormalities and vertebral fractures may be underestimated when evaluation is limited to bone density alone. In advanced forms, radiographic signs of high turnover, such as subperiosteal resorption and characteristic changes in the skull or phalanges, support a dominant hyperparathyroid component. In CKD, however, these findings must be interpreted in conjunction with treatment history and biochemical markers.
When focal lesions are present, the diagnostic objective is to establish whether they are reactive manifestations of hyperparathyroidism or neoplastic disease. Consistency between elevated PTH, the turnover pattern and radiological findings is fundamental, but biopsy may be required in selected scenarios to prevent diagnostic errors, particularly when imaging is nondiscriminatory and an oncological diagnosis remains plausible. In patients with CKD, bone biopsy retains a specific role when antiresorptive treatment or more intensive suppression of PTH could worsen low-turnover disease, making accurate definition of the phenotype essential.
The diagnostic process should culminate in a clinical synthesis linking the etiology of hyperparathyroidism, the skeletal phenotype and fracture risk, because this integration guides treatment. A correct diagnosis is not limited to assigning the label of “hyperparathyroidism” but requires precise determination of how extensively bone has already been compromised, the mechanisms responsible and the urgency of intervention to reduce fractures and disability.
The most useful classification is primarily etiological, because effective treatment removes or controls the factor driving PTH secretion. The main forms are primary hyperparathyroidism, secondary hyperparathyroidism, predominantly associated with CKD, and tertiary hyperparathyroidism, characterized by autonomous secretion after prolonged stimulation, typically in long-standing CKD or following kidney transplantation. Each form has a different mineral profile and probability of skeletal damage and, most importantly, a different therapeutic objective: correcting autonomous excess in primary disease, reducing the adaptive stimulus in secondary disease and controlling acquired autonomy in tertiary disease.
A second classification focuses on the skeletal phenotype of metabolic bone disease. In primary hyperparathyroidism, the typical phenotype is high turnover, with cortical bone loss and increased fracture risk, although clinical expression ranges from subclinical involvement to severe classical disease. In CKD, the TMV classification of renal osteodystrophy distinguishes abnormalities of turnover, mineralization and volume, because uremic bone may be high-turnover and predominantly PTH-driven, low-turnover and adynamic, osteomalacic or mixed. These differences have direct implications for the safety of treatment.
Clinical severity may be stratified according to three dimensions: documented structural damage, including markedly reduced bone mineral density, vertebral or nonvertebral fractures and deformities; biological activity, with PTH and alkaline phosphatase acting as proxies for turnover when interpreted as trends; and functional impact, including pain, impaired mobility and fall risk. This stratification is particularly important in frail patients and those with advanced CKD, in whom the same degree of biochemical abnormality may result in markedly different clinical risks depending on age, comorbidities and muscle reserve.
Finally, certain manifestations should be considered indicators of advanced disease. Brown tumors, pathological fractures, severe bone pain and deformities indicate prolonged hyperparathyroidism or extremely high turnover and require a more rapid and coordinated therapeutic strategy. In CKD, excessive suppression of PTH with development of low turnover also represents severe disease because it increases fractures and may worsen pain and disability despite the apparent “normalization” of laboratory values.
Treatment must be based on a central principle: reducing fracture risk and preventing progression of skeletal damage by addressing the PTH driver and correcting the components of mineral metabolism that sustain pathological remodeling. In primary hyperparathyroidism, the most effective treatment for removing the cause is parathyroidectomy, which normalizes or substantially reduces PTH and calcium levels and permits a progressive recovery of bone mineral density. The clinical benefit is greater when skeletal disease is active and fracture risk is high. The decision to perform surgery is guided by established clinical and biochemical criteria in international recommendations, including age, serum calcium, renal function, bone density and the presence of fractures, because the objective is not merely to correct a laboratory abnormality but to prevent skeletal events and reduce morbidity.
When surgery is not feasible or is not chosen, medical management of primary hyperparathyroidism may pursue two separate objectives: control of hypercalcemia and skeletal protection. Calcimimetics reduce calcium and PTH levels by acting on the calcium-sensing receptor, while antiresorptive agents may improve bone mineral density in selected patients, particularly when fracture risk is high. Treatment must remain pathophysiologically coherent. In the presence of high turnover, reducing resorption may be beneficial, but this strategy must be integrated with correction of the parathyroid disorder and careful monitoring of mineral metabolism and vitamin D status.
In secondary hyperparathyroidism caused by CKD, the strategy is necessarily multidimensional because the underlying cause is systemic. KDIGO guidelines recommend an approach based on PTH trends and progressive correction of the components of the disorder. This includes control of phosphate through dietary measures and phosphate binders, attention to calcium balance, use of vitamin D analogues or active vitamin D according to disease stage and phenotype, and calcimimetics, particularly in dialysis when PTH remains persistently elevated and its trajectory suggests uncontrolled parathyroid hyperplasia. The clinical objective is to reduce excessive turnover without pushing the patient toward iatrogenic low turnover, because both extremes increase the risk of fractures and pain.
In tertiary hyperparathyroidism, where secretion becomes autonomous after prolonged stimulation, treatment may require a combination of medical strategies and, frequently, surgical assessment. In this setting, balancing PTH control, calcium and phosphate stability and skeletal protection is particularly delicate because bone may rapidly take up minerals after abrupt correction of PTH excess, increasing the risk of prolonged postoperative hypocalcemia.
Management of vitamin D status and nutrition is a fundamental component of treatment because vitamin D deficiency amplifies the secretory stimulus and worsens bone quality. Correction must be individualized, particularly in CKD, to avoid hypercalcemia or an excessive increase in calcium-phosphate burden. In parallel, treatment should include fall-risk reduction, restoration of muscle function and optimization of protein intake and mechanical loading, because fracture prevention depends both on bone quality and neuromuscular stability.
Finally, in patients with focal lesions and substantial pain, treatment of the underlying cause promotes biological regression, but orthopedic or rehabilitative support may be required to manage fractures, instability and chronic pain. Effective treatment is therefore an integrated pathway that reduces pathological PTH secretion, corrects mineral metabolism and gradually restores locomotor function, with constant attention to iatrogenic risks and patient frailty.
Follow-up must assess two dimensions: control of hyperparathyroidism and recovery or stabilization of the skeletal phenotype. In primary hyperparathyroidism, after surgery or medical treatment, surveillance includes monitoring of calcium, PTH and vitamin D, with particular attention to the early post-parathyroidectomy period in patients with high turnover. Transition from chronic stimulation to normalization may induce rapid skeletal uptake of minerals and a clinically significant fall in serum calcium. Surveillance should therefore be more intensive in patients who previously had elevated alkaline phosphatase, advanced bone lesions or radiological signs of osteitis fibrosa.
Skeletal assessment requires a longer time frame than biochemical normalization. DXA and assessment for vertebral fractures should be repeated according to an individualized plan based on risk and treatment because recovery of bone mineral density and strength is gradual and may require months or years. Follow-up should also include muscle function and postural stability because reducing fracture risk requires restoration of strength and prevention of falls, particularly in older adults and patients with CKD.
In secondary hyperparathyroidism caused by CKD, monitoring is fundamentally trend-based. PTH, phosphate, calcium and alkaline phosphatase must be interpreted as trajectories in relation to treatment and disease stage. The objective is to avoid extreme fluctuations and to recognize early progression toward autonomous hyperplasia or iatrogenic low turnover. In dialysis, adjustments to calcimimetics and active vitamin D should be guided by consistency between laboratory values and clinical findings, recognizing that reducing PTH to excessively low levels may be as harmful as allowing it to remain persistently elevated.
In patients with focal lesions or fractures, follow-up includes targeted imaging to document stabilization or regression and to guide orthopedic decisions. In the case of brown tumors, regression is expected after PTH is controlled but may be slow. Persistent pain or radiological progression requires diagnostic reassessment to exclude alternative diagnoses and confirm that hyperparathyroidism is genuinely controlled.
Effective follow-up is also measured by the prevention of iatrogenic complications, including prolonged postoperative hypocalcemia, excessive suppression of bone turnover in CKD, treatment-related hypercalcemia and worsening vascular calcification in the setting of mineral disorders. Monitoring must therefore be scheduled, understandable to the patient and directed toward concrete clinical objectives, including reduction of fractures, chronic pain and loss of independence.
Prognosis depends on the promptness of diagnosis, the reversibility of structural damage and the ability to achieve stable control of pathological PTH secretion without producing iatrogenic phenotypes. In primary hyperparathyroidism, correction of the underlying cause through parathyroidectomy is associated with progressive improvement in bone mineral density and a reduced risk of adverse skeletal events, particularly when hyperparathyroidism was biologically active and signs of high turnover were present. However, previous fractures, deformities and marked loss of bone architecture may leave residual risk, requiring secondary prevention strategies and prolonged follow-up.
In secondary hyperparathyroidism caused by CKD, prognosis is influenced by the systemic nature of CKD-MBD and the relationship between mineral metabolism, the skeleton and the cardiovascular system. Fracture risk is high and multifactorial, involving renal osteodystrophy, sarcopenia, neuropathy and postural instability. Control of PTH and phosphate reduces the risk of extreme phenotypes but does not eliminate fragility, making an integrated approach that includes fall prevention, rehabilitation and management of comorbidities necessary.
The principal skeletal complications include fragility fractures, chronic bone pain, reduced mobility and, in advanced high-turnover disease, osteitis fibrosa with focal brown tumors and pathological fractures. Functional complications include proximal weakness and reduced physical performance, which increase the risk of falls and perpetuate the cycle of fragility, fracture and disability.
Among treatment-related complications, one of the most significant is hungry bone syndrome after parathyroidectomy in patients with high-turnover bone disease. The abrupt reduction in PTH causes a rapid transition to intense mineralization, with prolonged hypocalcemia requiring supplementation and close monitoring. In CKD, the corresponding complication is progression to low turnover because of excessive therapeutic suppression of PTH, which increases fractures and pain and reduces the ability of bone to repair itself. Treatment-related hypercalcemia or hyperphosphatemia may also worsen mineral balance and increase systemic risks, requiring cautious, trend-based management.
Overall, hyperparathyroidism associated with metabolic bone disease offers substantial potential for clinical improvement when the underlying cause is treated correctly and therapy is tailored to the turnover phenotype. The quality of prognosis depends on translating biochemical control into a genuine reduction in fractures, pain and loss of independence, an objective that requires continuity of care and structured monitoring.