Journal Scan: Parathyroid hormone and renal function in geriatric cats

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Why they did it

Researchers sought to determine whether cats with early (nonazotemic) chronic kidney disease (CKD) have elevations in their parathyroid hormone (PTH) concentrations compared with healthy geriatric cats.

What they did

The prospective longitudinal study enrolled clinically normal geriatric (> 9 years) cats with creatinine concentrations < 2 mg/dl.

The cats were evaluated over a 12-month period and categorized based on changes in serum creatinine concentration:

• Group 1 (n = 35): Creatinine concentration ≤ 1.6 mg/dl

• Group 2 (n = 52): Creatinine concentration > 1.6 mg/dl and < 2 mg/dl or creatinine concentration ≥ 2 mg/dl and a urine specific gravity > 1.035

• Group 3 (n = 31): Creatinine concentration ≥ 2 mg/dl and a urine specific gravity < 1.035 or persistent azotemia on at least two consecutive visits a minimum of 14 days apart.

Cats in groups 1 and 2 were considered nonazotemic. Calcitriol concentrations were measured at enrollment for a subset of 30 cats.

What they found

About 35% (11/31) of cats that became azotemic during the study period (group 3) had elevations in their PTH concentrations at the 12-month follow-up. There were no significant differences in calcium and phosphorus concentrations between the groups. The authors hypothesize that, as in people, the increase in PTH concentrations is a compensatory mechanism to aid in phosphorus homeostasis in CKD as 81% of the cats in group 3 had plasma phosphate concentrations within the normal reference range.

The authors found only a weak inverse correlation between total calcium and PTH concentrations, suggesting that "variables other than plasma total calcium concentration and phosphate concentration may be involved in the regulation of plasma PTH concentration." The authors note that, while there was an association between low calcitriol concentrations and PTH concentration, the role of calcitriol in the development of hyperparathyroidism in this population remains unclear and requires further research.

Take-home message

Renal secondary hyperparathyroidism may develop before the onset of azotemia and before changes in serum phosphorus and calcium concentrations. Early identification of this condition may aid in the management of cats with CKD.

The authors note that optimal management of patients with this syndrome, however, is still a source of debate. "Randomized controlled clinical trials need to be conducted with cats to determine whether dietary protein restriction and the administration of calcitriol, calcimimetics, and phosphate binders during the early stages of CKD are beneficial in preventing the development of renal secondary hyperparathyroidism," conclude the authors.

Finch NC, Syme HM, Elliott J. Parathyroid hormone concentration in geriatric cats with various degrees of renal function. J Am Vet Med Assoc  2012;241(10):1326-1335.

Link to abstract: http://avmajournals.avma.org/doi/abs/10.2460/javma.241.10.1326