Hypophosphatemia in the setting of metabolic bone disease: case reports and diagnostic algorithm

Osteoporosis: Investigations and Monitoring

Background

Osteoporosis is characterized by microarchitectural disruption of the bone, decrease in bone mineral density, and increased skeletal fragility and risk of fracture. Osteoporosis occurs due to the decoupling of bone formation and bone resorption, with a significant increase in resorption. This review article focuses on the role of laboratory investigations in the diagnosis and monitoring of treatment in patients with osteoporosis.

Methods

This review article collected literature from various databases using keywords such as 'Laboratory investigations', 'Osteoporosis', 'Diagnosis', 'Monitoring', and 'Bone turnover markers'.

Results and Discussion

Laboratory investigations, including serum calcium, alkaline phosphatase, vitamin D, and parathormone, are commonly performed tests to exclude secondary causes of osteoporosis and monitor the response to therapy. The biochemical markers of bone turnover are newly emerged tests for monitoring individual patients with osteoporosis. These markers are classified as bone formation and resorption markers, measurable in both serum and urine. The use of these markers is limited by biological and analytical variability. The International Federation of Clinical Chemistry and Laboratory Medicine and the International Osteoporosis Foundation recommend serum procollagen type 1 amino-terminal propeptide as the bone formation marker and β-form of C-terminal cross-linked telopeptide of type I collagen (β-CTx-1/β-CrossLaps) as the marker of choice, using standardized procedures. However, in specific cases, such as patients with chronic renal disease, CTx-1 is replaced by the resorption marker tartrate-resistant acid phosphatase 5b, as its levels are not affected by renal excretion.

Conclusion

Bone turnover markers have emerged as tools for the assessment of osteoporosis, using standardized procedures, and are useful in monitoring therapy and treatment compliance.

Misdiagnosed metabolic bone abnormality: a case report

BACKGROUND

Metabolic bone disease causes significant morbidity and mortality, especially when misdiagnosed. With genetic testing, multiple disease pathologies can be analyzed.

CASE PRESENTATION

A 5-year and 9-month-old otherwise healthy Yemeni girl presented to her Yemen physician for evaluation of inward bending of her right knee and short stature. After extensive medical testing, she was given a diagnosis of hypophosphatemic rickets and growth hormone deficiency and started on treatment. Despite appropriate treatment, however, her condition continued to progress, prompting her family to pursue additional workup including genetic testing outside of Yemen. Genetic testing ultimately revealed a variation of unknown significance associated with amelogenesis imperfecta.

CONCLUSIONS

Hypophosphatemic rickets secondary to renal tubular acidosis was the working diagnosis. However, the patient's condition did not improve. Further genetic testing revealed a variation of unknown significance associated with amelogenesis imperfecta. We aim to present this case, provide an overview of the causes, and diagnostic metabolic bone health evaluation.

A Case Report of Hypophosphatemia Leading to the Diagnosis of Mesothelioma

Hypophosphatemia can be commonly encountered as an electrolyte imbalance and is defined as a value less than 0.8 mmol/l (2.5 mg/dl). It can be an incidental finding, but it is not uncommon to see it presenting with varied symptoms. It is good to have a clear diagnostic approach to this so adequate treatment can be instated.

We present a 66-year-old gentleman who presented with hypophosphatemia. Investigations confirmed renal phosphate wasting secondary to fibroblast growth factor-23 (FGF-23). Imaging showed right pleural effusion, and pleural biopsy confirmed malignant mesothelioma. This may just be an association rather than the cause of his hypophosphatemia. It does however highlight the importance of further investigations for patients with tumor-induced osteomalacia.

FGF23-related hypophosphatemia in patients with low bone mineral density and fragility fractures: challenges in diagnosis and management

PURPOSE

Hypophosphatemia (HP) can be observed in patients evaluated for skeletal fragility. We investigated prevalence of HP among outpatients referred for low bone density or fragility fractures, HP-associated clinical and biochemical features and outcomes of recommended diagnostic algorithm in our cohort.

METHODS

Chronic HP (phosphate ≤ 2.7 mg/dL over 6 months or longer) was retrospectively investigated among 2319 patients. In renal wasting-related HP, intact FGF23 was assessed; non-suppressed FGF23 prompted the performance of ⁶⁸Ga-DOTATOC PET/CT in the suspicion of tumor-induced steomalacia (TIO).

RESULTS

Renal wasting-related HP (median 2.2, range 1.6-2.6 mg/dL) was observed in 19 patients (0.82%). FGF23 levels were suppressed in two patients diagnosed with renal tubular disease, increased in one and within normal range in most patients. X-linked hypophosphatemic rickets was diagnosed in one woman. In the remaining 16 patients, highly prevalent fragility fractures (50%) and severely reduced bone mineral density were detected, though diagnostic criteria for osteomalacia were not fulfilled. ⁶⁸Ga-PET was performed in nine patients and was positive in four. While intact FGF23 levels alone failed to differentiate PET's outcomes (positive: FGF23 median 70.5 pg/mL; negative: 52 pg/mL, P = 0.462), the coexistence of multiple biochemical and radiologic alterations performed better in prediction of PET's positivity.

CONCLUSION

Mild, apparently unexplained HP is observed in 0.82% of patients with low bone density or fragility fractures. In asymptomatic patients with isolated mild hypophosphatemia, the probability of finding an underlying tumor disease is very low, and utility of extensive and expensive diagnostic workup should be carefully considered in this setting.

An intracranial mass causing tumor-induced osteomalacia (TIO): Rapid and complete resolution of severe osteoporosis after surgical resection

Tumor-induced osteomalacia (TIO) is a rare disease in which patients suffer from fractures and progressive disabling bone pain and muscle weakness. TIO is caused by the hypersecretion of Fibroblast Growth Factor 23 (FGF23) from rare neoplasms of mesenchymal origin. This case report describes a 29-year-old male with 2 years of low back/hip pain, gait changes, proximal muscle weakness, and multiple stress fractures. Bone densitometry was remarkable for severe osteoporosis, hypophosphatemia was seen on routine labs, and advanced labs demonstrated an "inappropriately normal" FGF23 level. A 68Ga-DOTATATE scan and MRI showed a 1.3 × 1.1 × 1.0 cm intracranial mass. The patient underwent tumor resection by Neurosurgery. Shortly after, laboratory levels normalized, and the patient's symptoms improved drastically. This case exemplifies the notion that TIO can be caused by FGF23 levels within normal limits, the role of 68-Ga DOTATATE imaging for establishing a diagnosis, and that these tumors can arise anywhere-even intracranially. We also review current surgical and nonsurgical treatment options, as well as emerging novel therapeutics.

Influence of a physical exercise until exhaustion in normothermic and hyperthermic conditions on serum, erythrocyte and urinary concentrations of magnesium and phosphorus

AIM

The aim of this study was to evaluate the effect of the performance of a maximal exercise test until exhaustion in normothermic and hyperthermic conditions on body concentrations of magnesium (Mg) and phosphorus (P).

METHODS

19 adult males (age: 22.58 ± 1.05 years) performed two maximum incremental exercise tests on a cycloergometer separated by 48 h. The first was performed in normothermia (22 ± 2 °C) and the second in hyperthermic conditions induced with a sauna (42 ± 2 °C). Blood and urine samples were taken before and after each test.

RESULTS

The tests in hyperthermia did not produce ergospirometric alterations or a noticeable cardiovascular drift. Serum Mg concentrations underwent a reduction after the stress test in hyperthermia (p > 0.05) but not in normothermia. Nevertheless, urinary and erythrocyte concentrations of Mg, and urinary, erythrocyte and serum concentrations of P did not undergo alterations in either conditions.

CONCLUSIONS

It seems that exercise in hyperthermic conditions induces a tissue redistribution of Mg in the body, a fact which was not observed in normothermic conditions.