Finally, do you have a particular protocol for addressing renal secondary hyperparathyroidism?
Pressler: The first step in treating renal secondary hyperparathyroidism is normalizing the serum phosphorus concentration, as hyperphosphatemia
increases serum parathyroid hormone (PTH) concentrations. This is best done first by transitioning to a renal-formulated diet.
If the serum phosphorus concentration is still increased or in the upper end of the reference range, then an oral phosphate
binder should be administered; I most commonly use nonflavored, powdered aluminum hydroxide. Only once phosphorus has reached
this target do I measure serum PTH concentrations in patients with azotemic chronic kidney disease. If the concentration is
increased, then it is the most likely cause of decreased active vitamin D production by the diseased kidneys, and calcitriol
should be considered.
Specifically, how and when do you use calcitriol?
Pressler: I personally consider calcitriol only in those patients with chronic kidney disease in which the PTH concentration is increased
despite the serum phosphorus concentration being in the lower half of the reference range. However, I always balance prescribing
calcitriol with a number of other factors, including whether the owner has demonstrated good compliance with previous medications
since calcitriol has a low safety margin and errors in administration can have severe consequences. I also consider the owner's
financial situation because visits for measurement of serum PTH and ionized calcium concentrations to determine appropriate
dosing can be costly. The last consideration is the rate of disease progression in the patient.
Unfortunately, the true benefit of calcitriol administration has not been determined as of yet through prospective studies,
so the veterinary nephrology community is still not unified as to when and how best to incorporate calcitriol in the management
of patients with chronic kidney disease. For example, azotemic patients with normal serum PTH concentrations still have intracellular
biochemical alterations consistent with renal secondary hyperparathyroidism. However, although this does suggest that earlier
use of calcitriol than what I suggest could eventually prove to be beneficial, in most nephrologists' opinions, these are
outweighed at this time by the safety and cost considerations I outlined above.
Thank you, Dr. Pressler, for your time and detailed answers regarding several controversial areas of renal disease treatment.
There are always new avenues of interest coming out of human medicine, and only time and research will tell if they prove
to be effective treatments for small animals.
Barrak Pressler, DVM, PhD, Dipl. ACVIM, is an assistant professor of small-animal internal medicine at Purdue University
in West Lafayette, Ind., where he is the principal investigator in the Comparative Nephrology Laboratory. His main clinical
and basic science interests are veterinary nephrology and urology, particularly the pathogenesis and immune dysfunction of
glomerular and autoantibody diseases. He has published multiple research articles in these fields and is a Veterinary Medicine editorial advisory board member.
Dr. Blake is a freelance technical editor and writer in Eudora, Kan.