Feline hyperparathyroidism: pathophysiology, diagnosis and treatment of primary and secondary disease

Assessment of alendronate and dietary treatment in the management of feline idiopathic ionised hypercalcaemia and ionised hypercalcaemia associated with chronic kidney disease: 29 cases (2016-2022)

OBJECTIVES

This retrospective cohort multicentre study aimed to characterise the signalment, clinicopathologic data, complications and the association between treatment and outcome (the reduction in ionised calcium concentration) in cats with idiopathic or chronic kidney disease-associated ionised hypercalcaemia managed with alendronate and standard treatment or standard treatment alone, and to compare the outcome between the two groups.

MATERIALS AND METHODS

Medical records for cats diagnosed with idiopathic or chronic kidney disease-associated ionised hypercalcaemia were retrospectively reviewed. Cats treated with alendronate and standard treatment were assigned to the alendronate group and cats treated with standard treatment alone were assigned to the control group. The standard treatment was defined as dietary modification and/or monitoring of ionised calcium concentrations and management of complications secondary to hypercalcaemia. The follow-up period was selected as 6 months.

RESULTS

Twenty-nine cats were enrolled in the study. Nine cats were included in the control group and 20 cats in the alendronate group. A significant reduction in serum ionised calcium was observed in both groups between the baseline and the follow-up visit; however, this reduction did not differ significantly between both groups (the mean change in the ionised calcium concentration in alendronate and control group was -0.18 and -0.17, respectively). Suspected bisphosphate-related osteonecrosis of the jaw was reported in one cat receiving alendronate.

CLINICAL SIGNIFICANCE

In this study, similar reduction in serum ionised calcium concentration was observed in cats with ionised hypercalcaemia treated with diet alone and in cats treated with diet and alendronate. These results should be interpreted with caution, as the study was underpowered for meaningful statistical comparison. Cats receiving alendronate should be monitored for the development of adverse reactions, including osteonecrosis of the jaw.

Exploring the relationship between calcitonin, ionized calcium, and bone turnover in cats with and without naturally occurring hypercalcemia

Objectives

This case-control study aimed to evaluate calcitonin response in naturally occurring hypercalcemia in cats and assess the relationships between calcitonin and ionized calcium (iCa) and examine relationships between calcitonin, iCa and bone turnover.

Methods

Hypercalcemic cats (persistently increased iCa concentration [>1.40 mmol/l]) were identified retrospectively via a medical database search; additional hypercalcemic and normocalcemic cats were recruited prospectively. Data regarding routine biochemical and urine testing, diagnostic imaging and additional blood testing were obtained. Serum alkaline phosphatase (ALP) activity was used as a marker of bone turnover. Serum calcitonin concentration was analyzed using a previously validated immunoradiometric assay. Hypercalcemic cats with an increased calcitonin concentration (>0.9 ng/L) were termed responders. Group comparisons were performed using a Mann-Whitney test for continuous variables and a χ2 test for categorical variables. Spearman's correlation coefficient was used to examine the relationships between calcitonin, iCa and ALP.

Results

Twenty-six hypercalcemic and 25 normocalcemic cats were recruited. Only 5/26 (19.2%) of the hypercalcemic cats were identified as responders, and all were diagnosed with idiopathic hypercalcemia. There was no significant correlation between the concentrations of calcitonin and iCa (p = 0.929), calcitonin and ALP (p = 0.917) or iCa and ALP (p = 0.678) in hypercalcemic cats, however, a significant negative correlation was observed between calcitonin and ALP (p = 0.037) when normocalcemic and hypercalcemic cats with an elevated calcitonin concentration were analyzed together.

Discussion

The expected increase in calcitonin concentration was present in only a small subset of hypercalcemic cats; no correlation was found between iCa and calcitonin concentration. The inverse relationship between calcitonin and ALP in cats with increased calcitonin concentrations suggests that the ability of calcitonin to correct hypercalcemia may be related to the degree of bone turnover.

Nutritional secondary hyperparathyroidism in a kitten, supported by immunoenzymatic measurement of feline intact parathyroid hormone

A 6-mo-old, intact male, domestic shorthair cat was referred with a history of poor growth, reluctance to move, and deformation of the nasal profile. The kitten had been fed a diet composed almost exclusively of a complementary pet food and tuna, which was similar to an all-meat diet. We detected osteopenia and hypocalcemia associated with severe parathyroid hormone (PTH) and calcitriol increases; we measured PTH concentrations with an immunoenzymatic method that has been validated in cats. Dietary correction, consisting of a complete and balanced wet pet food formulated for growth, resulted in normalization of calcium and PTH concentrations within 2 mo.

Analytical Validation of a New Immunoenzymatic Method for the Measurement of Feline Parathyroid Hormone in Cats with Chronic Kidney Disease

Simple Summary

Parathyroid hormone (PTH) is involved in many metabolic diseases, such as chronic kidney disease (CKD) and calcium disorders, and its measurement could be of clinical utility. However, available methods for the measurement of feline PTH are limited and not widely accessible. The aim of this study was to perform the analytical validation of a new method for PTH measurement in cats. Thirty-eight cats affected with CKD were included. The analytical protocol provided an evaluation of the precision, accuracy, and storage stability at different temperatures. The method investigated showed good precision and accuracy and good stability for 1 week of storage at freezing temperatures. The method was validated in cats, allowing its future use in diagnostic procedures.

Abstract

The determination of parathyroid hormone (PTH) in cats could be of clinical utility in many metabolic disorders, such as renal diseases, hypercalcemia, or nutritional imbalances. However, the available methods for the measurement of feline PTH are limited, not widely available, and need radioimmunoassays. The aim of this study was to perform the analytical validation of a new immunoenzymatic method for the measurement of feline PTH. Thirty-eight cats affected with chronic kidney disease (CKD) were included. PTH was measured using a two-site immunoenzymatic method validated in humans and dogs (ST AIA-PACK^® Intact PTH, Tosoh Bioscience, Tessenderlo, Belgium). The analytical validation provided the evaluation of precision (intra-assay and inter-assay), accuracy (linearity under dilution (LUD) and spike recovery test (SRT)), and the storage stability of serum samples at 20 °C, 4 °C, and −20 °C. The method showed good precision (intra-assay CVs (coefficient of variations) 3.19–9.61%; inter-assay CVs 9.26–15.28%). In both the intra- and inter-assays, the highest imprecision was found with the low concentration pool (9.61% and 15.28%) and accuracy (LUD and SRT r² = 0.99, p < 0.001), while the stability was optimal up until 7 days at −20 °C (−7.7%). The method was successfully validated in cats, allowing its future use in diagnostic procedures.

The Role of Vitamin D in Small Animal Bone Metabolism

Dogs and cats have differences in vitamin D metabolism compared to other mammalian species, as they are unable to perform vitamin D cutaneous synthesis through sun exposure. Therefore, they are dependent on the dietary intake of this nutrient. The classic functions of vitamin D are to stimulate intestinal calcium and phosphate absorption, renal calcium and phosphate reabsorption and regulate bone mineral metabolism. Thus, it is an important nutrient for calcium and phosphorus homeostasis. This review highlights the evidence of the direct and indirect actions of vitamin D on bone mineral metabolism, the consequences of nutritional imbalances of this nutrient in small animals, as well as differences in vitamin D metabolism between different size dogs.

Severe calcification of systemic blood vessel walls caused by continuous hypercalcemia in a cat with congenital hypothyroidism

A 97-day-old male Japanese domestic cat was diagnosed as congenital hypothyroidism. During the treatment, continuous hypercalcemia was detected. Although fluid therapy was performed, the cat died at the age of 1785 days. At autopsy, both parathyroid glands were enlarged, and elastic arterial walls were increased in thickness and hardness. Histopathological examination revealed hyperplasia of both parathyroid glands and interstitial fibrosis of bilateral kidneys. Severe calcification of the tunica media and tunica externa in systemic elastic and muscular arteries were also observed. These calcifications were considered to be due to renal secondary hyperparathyroidism. In the present case, hypothyroidism might have caused hyperparathyroidism through renal failure. In veterinary medicine, this is the first reported case of hypothyroidism accompanied with hyperparathyroidism.

Unusual presentation of vitamin D3-dependent rickets type II in a kitten

Case summary

A 15-week-old kitten presented with a 1-month history of intermittent generalised tremors and abdominal distension. Hypocalcaemia associated with increased 1,25-vitamin D₃ was consistent with vitamin D₃-dependent rickets type II. The bone appearance on CT scan was most consistent with the changes typically seen with nutritional secondary hyperparathyroidism and less typical of the changes seen with rickets. Our patient had a positive response to high vitamin D₃ therapy as it remained normocalcaemic 16 months after diagnosis, supporting the diagnosis of rickets.

Relevance and novel information

This case report is an unusual and interesting presentation of rickets in a kitten. Despite the characteristic vitamin D₃ disturbance for rickets type II, the atypical radiographic changes have not been previously reported. In the literature, a positive response to treatment is not commonly seen or follow-up is short. Our case responded well to treatment and was followed for 16 months from the time of diagnosis. This emphasises that the pathophysiology of the condition is not well understood, and that different types of vitamin D₃-dependent rickets type II may exist. Although the genetic defects responsible for some cases of rickets type I have been identified, this has still not been determined for rickets type II.

The Interplay Between the Renin-Angiotensin-Aldosterone System and Parathyroid Hormone

The renin-angiotensin-aldosterone system (RAAS) is the regulatory system by which renin induces aldosterone production. Angiotensin II (Ang II) is the main effector substance of the RAAS. The RAAS regulates blood pressure and electrolyte balance by controlling blood volume and peripheral resistance. Excessive activation of the RAAS is an important factor in the onset of cardiovascular disease and the deterioration of this disease. The most common RAAS abnormality is primary aldosteronism (PA). Parathyroid hormone (PTH) is a peptide secreted by the main cells of the parathyroid gland, which promotes elevated blood calcium (Ca²⁺) levels and decreased blood phosphorus (Pi) levels. Excessive secretion of PTH can cause primary hyperparathyroidism (PHPT). Parathyroidism is highly prevalent in postmenopausal women and is often associated with secondary osteoporosis. PA and PHPT are common endocrine system diseases. However, studies have shown a link between the RAAS and PTH, indicating a positive relationship between them. In this review, we explore the complex bidirectional relationship between the RAAS and PTH. We also point out possible future treatment options for related diseases based on this relationship.

Primary hyperparathyroidism due to a cystic parathyroid adenoma in a cat

A 15-year-old neutered female domestic shorthair cat was presented for weight loss, polydipsia/polyuria, and lethargy. A large fluctuant mass was palpated in the ventral right cervical region. Biochemistry results were consistent with primary hyperparathyroidism. Parathyroid hormone level in the fluid was higher to that observed in the plasma, consistent with a cystic parathyroid lesion. Right parathyroidectomy and thyroidectomy were performed without complications. Ionized calcium normalized within a few hours. Histopathology yielded a diagnosis of cystic parathyroid adenoma. Follow-up showed complete recovery of clinical signs and normalization of ionized calcium. This case shows an uncommon presentation of feline primary hyperparathyroidism secondary to a cystic parathyroid adenoma and is, to our knowledge, the first case presented with a large palpable mass in which parathyroid hormone concentration was measured. This report highlights the value of selective hormonal analyses of the cystic fluid to confirm the origin of the cystic lesion pre-operatively.

Perioperative characteristics, histological diagnosis, and outcome in cats undergoing surgical treatment of primary hyperparathyroidism

OBJECTIVE

To report perioperative characteristics, complications, histopathologic diagnosis and outcome in cats undergoing surgical treatment for primary hyperparathyroidism (PHPT).

STUDY DESIGN

Multi-institutional, retrospective case series.

ANIMALS

Thirty-two client-owned cats.

METHODS

Medical records of cats treated with surgical removal of 1 or more parathyroid gland(s) with confirmed histopathologic evaluation were reviewed. Cats were divided into preoperative ionized calcium (iCa) groups corresponding to the 33rd, 67th, and 100th percentiles of the preoperative iCa results of the study population. Follow-up consisted of phone conversation with owners or primary veterinarian.

RESULTS

Ionized calcium was above reference range in all cats (median 1.8 mmol/L [interquartile range, 1.5-1.9]). Abnormal tissue was excised after cervical exploration in all cats. The most common histopathologic diagnoses were parathyroid adenoma in 20 of 32 (62.5%) cats and parathyroid carcinoma in 7 of 32 (21.9%) cats. At discharge, 6 of 32 (18.8%) cats had hypercalcemia, 5 of 32 (15.6%) had hypocalcemia, and 21 of 32 (65.6%) were normocalcemic. Preoperative iCa did not correlate with postoperative iCa. The median follow-up time was 332 days (range, 7-3156). Overall median survival time was 1109 days (95% CI, 856-1332). Survival time was not associated with preoperative iCa group, hypocalcemia at discharge, hypercalcemia at discharge, or diagnosis of carcinoma.

CONCLUSION

In this cohort of cats, parathyroid adenoma was the most common cause of PHPT, and surgical treatment resulted in very good median survival time. Preoperative iCa was not predictive of postoperative hypocalcemia.

CLINICAL SIGNIFICANCE

Surgical parathyroidectomy for treatment of PHPT in cats provides a favorable prognosis.

The Effect of Moderate Dietary Protein and Phosphate Restriction on Calcium-Phosphate Homeostasis in Healthy Older Cats

Background

Dietary phosphate and protein restriction decreases plasma PTH and FGF‐23 concentrations and improves survival time in azotemic cats, but has not been examined in cats that are not azotemic.

Hypothesis

Feeding a moderately protein‐ and phosphate‐restricted diet decreases PTH and FGF‐23 in healthy older cats and thereby slows progression to azotemic CKD.

Animals

A total of 54 healthy, client‐owned cats (≥ 9 years).

Methods

Prospective double‐blinded randomized placebo‐controlled trial. Cats were assigned to test diet (protein 76 g/Mcal and phosphate 1.6 g/Mcal) or control diet (protein 86 g/Mcal and phosphate 2.6 g/Mcal) and monitored for 18 months. Changes in variables over time and effect of diet were assessed by linear mixed models.

Results

A total of 26 cats ate test diet and 28 cats ate control diet. There was a significant effect of diet on urinary fractional excretion of phosphate (P = 0.045), plasma PTH (P = 0.005), and ionized calcium concentrations (P = 0.018), but not plasma phosphate, FGF‐23, or creatinine concentrations. Plasma PTH concentrations did not significantly change in cats fed the test diet (P = 0.62) but increased over time in cats fed the control diet (P = 0.001). There was no significant treatment effect of the test diet on development of azotemic CKD (3 of 26 (12%) test versus 3 of 28 (11%) control, odds ratio 1.09 (95% CI 0.13–8.94), P = 0.92).

Conclusions and Clinical Importance

Feeding a moderately protein‐ and phosphate‐restricted diet has effects on calcium‐phosphate homeostasis in healthy older cats and is well tolerated. This might have an impact on renal function and could be useful in early chronic kidney disease.

Does Secondary Renal Osteopathy Exist in Companion Animals?

Secondary renal hyperparathyroidism is an inevitable consequence of chronic kidney disease. In human patients, the disease is associated with decreased bone quality and increased fracture risk. Recent evidence suggests that bone quality is also decreased in companion animals, more pronouncedly in cats compared with dogs, likely because of a longer disease course. The clinical significance of these findings is yet to be determined. However, clinicians should keep in mind that animals with chronic kidney disease have decreased bone quality and increased fracture risk.

Hypercalcaemia in cats: The complexities of calcium regulation and associated clinical challenges

PRACTICAL RELEVANCE

Calcium is essential for many normal physiological processes within the body. Aberrations in calcium homeostasis leading to hypercalcaemia can result in clinical signs such as polyuriav and polydipsia, lethargy and weakness due to depressed excitability of muscle and nervous tissue, and gastrointestinal (GI) signs due to effects on GI smooth muscle. Hypercalcaemia in cats is mostly idiopathic, with chronic kidney disease and neoplasia also being common causes.

CLINICAL CHALLENGES

Hypercalcaemia can be a diagnostic challenge and a good understanding of the regulation of calcium homeostasis can aid in interpreting results of diagnostic tests. Furthermore, the management approach may depend on the underlying cause of hypercalcaemia, and also its severity and chronicity.

AUDIENCE

This review offers a comprehensive discussion of the regulation of calcium homeostasis, with a focus on the normal response to hypercalcaemia. It also discusses the diagnostic approach to, and management of, hypercalcaemia in cats, as well as specific aetiologies. This is relevant to all clinicians working with feline patients.

EVIDENCE BASE

The review draws evidence from peer-reviewed publications and also the author's own clinical experience.

Oral Manifestations of Chronic Kidney Disease and Renal Secondary Hyperparathyroidism: A Comparative Review

Recent epidemiological studies have demonstrated that significant associations exist between oral disease and diseases involving non-oral tissues. Occasionally, the roles may be reversed and the oral cavity can be severely affected by systemic disease originating in another part of the body. Renal secondary hyperparathyroidism is a common endocrinopathy that occurs as a consequence of chronic azotemic kidney disease. Renal osteodystrophy, the most dramatic clinical consequence of renal secondary hyperparathyroidism is uncommon, but can result in demineralization of maxillofacial bones, loosening of teeth, and pathological jaw fractures. The purpose of this report is to update the current understanding of the pathophysiology of this endocrine disease and to compare the oral manifestations of renal secondary hyperparathyroidism in humans and companion animals. A 50-year review of the veterinary literature was undertaken to examine the clinical presentation of renal osteodystrophy in dogs, and to determine what clinical consequences of renal secondary hyperparathyroidism have been reported in domestic cats.