Parathyroid hormone assay: problems and opportunities

Changes in bone structure and the muscle-bone unit in children with chronic kidney disease

The impact of pediatric chronic kidney disease (CKD) on acquisition of volumetric bone mineral density (BMD) and cortical dimensions is lacking. To address this issue, we obtained tibia quantitative computed tomography scans from 103 patients aged 5-21 years with CKD (26 on dialysis) at baseline and 12 months later. Gender, ethnicity, tibia length, and/or age-specific Z-scores were generated for trabecular and cortical BMD, cortical area, periosteal and endosteal circumference, and muscle area based on over 700 reference subjects. Muscle area, cortical area, and periosteal and endosteal Z-scores were significantly lower at baseline compared with the reference cohort. Cortical BMD, cortical area, and periosteal Z-scores all exhibited a significant further decrease over 12 months. Higher parathyroid hormone levels were associated with significantly greater increases in trabecular BMD and decreases in cortical BMD in the younger patients (significant interaction terms for trabecular BMD and cortical BMD). The estimated glomerular filtration rate was not associated with changes in BMD Z-scores independent of parathyroid hormone. Changes in muscle and cortical area were significantly and positively associated in control subjects but not in CKD patients. Thus, children and adolescents with CKD have progressive cortical bone deficits related to secondary hyperparathyroidism and potential impairment of the functional muscle-bone unit. Interventions are needed to enhance bone accrual in childhood-onset CKD.

Long-term management of CKD-mineral and bone disorder

Chronic kidney disease-mineral and bone disorder (CKD-MBD) is the term used to describe the abnormalities of bone and mineral metabolism that occur in the setting of kidney disease. The spectrum of these abnormalities is wide, ranging from severe high-turnover bone disease on one end to marked low bone turnover bone disease on the other. Similarly, some patients have severe vascular calcifications while others do not, and the values for biochemistry determinations, including calcium, phosphorus, and parathyroid hormone, also may vary widely among patients. This variability may be influenced by such things as the chronicity of the particular kidney disease, effects of therapies such as corticosteroids on modifying the course of kidney disease, and comorbid conditions, such as diabetes, heart disease, age, and osteoporosis. The heterogeneity of CKD-MBD makes strict protocol-driven therapeutic approaches difficult; accordingly, considerable individualized therapy is required. Using a case history, we explore several of the variables and difficulties involved in patient management.

Prevention and control of phosphate retention/hyperphosphatemia in CKD-MBD: what is normal, when to start, and how to treat?

Phosphate retention and, later, hyperphosphatemia are key contributors to chronic kidney disease (CKD)-mineral and bone disorder (MBD). Phosphate homeostatic mechanisms maintain normal phosphorus levels until late-stage CKD, because of early increases in parathyroid hormone (PTH) and fibroblast growth factor-23 (FGF-23). Increased serum phosphorus, and these other mineral abnormalities, individually and collectively contribute to bone disease, vascular calcification, and cardiovascular disease. Earlier phosphate control may, therefore, help reduce the early clinical consequences of CKD-MBD, and help control hyperphosphatemia and secondary hyperparathyroidism in late-stage CKD. Indeed, it is now widely accepted that achieving normal phosphorus levels is associated with distinct clinical benefits. This therapeutic goal is achievable in CKD stages 3 to 5 but more difficult in dialysis patients. Currently, phosphate control is only initiated when hyperphosphatemia occurs, but a potentially beneficial and simple approach may be to intervene earlier, for example, when tubular phosphate reabsorption is substantially diminished. Early CKD-MBD management includes dietary phosphate restriction, phosphate binder therapy, and vitamin D supplementation. Directly treating phosphorus may be the most beneficial approach because this can reduce serum phosphorus, PTH, and FGF-23. This involves dietary measures, but these are not always sufficient, and it can be more effective to also consider phosphate binder use. Vitamin D sterols can improve vitamin D deficiency and PTH levels but may worsen phosphate retention and increase FGF-23 levels, and thus, may also require concomitant phosphate binder therapy. This article discusses when and how to optimize phosphate control to provide the best clinical outcomes in CKD-MBD patients.

Mysterious PTH values after parathyroidectomy in a patient on maintenance dialysis

A 25-year-old patient with end-stage renal disease on maintenance peritoneal dialysis underwent parathyroidectomy when his secondary hyperparathyroidism did not respond to medical management. However, over the subsequent months he developed extremely raised parathyroid hormone (PTH) levels. When surgical removal of the autotransplant was considered, preoperative work-up revealed a PTH level within the target range. It became apparent that the very high PTH values were due to the location of the blood draw close to the autotransplant, thus measuring a local rather than the systemic PTH value. The multiple causes of varying PTH measurements other than clinical and physiological reasons are reviewed.

The role of bone biopsy in patients with chronic renal failure

Renal bone disease is a heterogeneous group of metabolic bone diseases that requires quantitative bone histomorphometry to make the correct differential diagnosis. Included in this group is osteoporosis. However, osteoporosis in stage 4 to 5 chronic kidney disease cannot be diagnosed on the basis of bone mineral density criteria established by the World Health Organization or the presence of fragility fractures because patients with all forms of renal bone disease can demonstrate low bone mineral density and fragility fractures. Clinical cases in patients with either low bone mineral density and/or low-trauma fractures will be used to demonstrate the value of bone biopsy and quantitative histomorphometry in making a diagnosis of the specific renal bone disease and assisting with subsequent management decisions.