Serum Magnesium Concentrations in Association with Feline Calcium Metabolic Disorders
ACVIM 2008
P.A. Schenck
Diagnostic Center for Population and Animal Health, Endocrine Diagnostic Section, Michigan State University
Lansing, MI, USA

Serum ionized magnesium (iMg) concentration can influence parathyroid hormone (PTH), and either a deficiency or excess may suppress PTH production. With low serum iMg concentrations, receptors have decreased sensitivity to serum ionized calcium (iCa). In general, low iMg concentrations are associated with significant illness or hypoparathyroidism. However, serum iMg concentrations have not been studied in other calcium metabolic disorders. The objective was to determine the serum iMg status in cats with calcium metabolic disorders. A total of 11,320 feline cases submitted for PTH and iCa measurement were reviewed, and sorted by diagnosis into the following categories: normal, borderline hypercalcemia, secondary hyperparathyroid, primary hyperparathyroid, parathyroid independent hypercalcemia, humoral hypercalcemia of malignancy (with elevated parathyroid hormone related protein; PTHrP), and hypoparathyroid. In addition, those cats with a history of chronic renal failure but without secondary hyperparathyroidism were identified for comparison to those with secondary hyperparathyroidism. The mean iMg concentration in each group (except for hypoparathyroid cats) was within the feline reference range (0.54-0.70 mmol/L); however, there were significant differences in the mean values, and the distribution of iMg concentrations was not the same in each group. Hypoparathyroid cats (n=80) had the lowest mean iMg concentration (0.46 mmol/L), with 85% below the reference range. Cats with secondary hyperparathyroidism (n=1491) or CRF without secondary hyperparathyroidism (n=394) had significantly higher iMg concentrations (mean 0.55 mmol/L in both groups) as compared to hypoparathyroid cats, but these means were still significantly lower than cats with other disorders. In cats with secondary hyperparathyroidism, 49% had iMg values below and 9% had iMg values above the reference range, and in those with CRF but not secondary hyperparathyroidism, 44% of values were below and 6% of values were above the reference range. Cats with normal PTH and iCa (n=2229), or borderline hypercalcemia (n=749) had significantly higher mean iMg concentrations (0.58 and 0.57 mmol/L, respectively) as compared to previous groups. Approximately 33% of cats with normal PTH and iCa and borderline hypercalcemia had iMg concentrations below the reference range. Cats with primary hyperparathyroidism (n=434), parathyroid independent hypercalcemia (n=5847), or humoral hypercalcemia of malignancy with elevated PTHrP (n=184) had the highest mean iMg concentrations (0.62, 0.59, and 0.60 mmol/L respectively). In primary hyperparathyroidism, iMg concentrations showed a wide spread, with 28% of iMg values below, and 27% above the reference range. Approximately 25% of cats with elevated PTHrP and 31% of cats with parathyroid independent hypercalcemia had iMg concentrations below the reference range. These data suggest that difference in serum iMg concentrations do occur in association with calcium metabolic disorders, and that cats are more likely to have serum iMg concentrations below the reference range as compared to dogs.

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Patricia Schenck

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