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For an explanation of causes of most abnormal blood and urine tests, go here
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The parathyroid glands do so by constantly monitoring the blood calcium levels in your dog or cat and secreting a hormone, PTH when those levels become too low. PTH causes your pet’s kidneys to conserve calcium, by lowering the amount lost in its urine while, at the same time, increasing the amount of phosphate purged from the body in urine.
PTH also encourages its kidneys to produce the active form of vitamin D (calcitriol) to enhance calcium absorption from your pet’s food. At the same time PTH causes certain bone cells (osteoclasts) to release bone calcium into the pet's blood stream.
It is not possible to make any decisions regarding PTH levels and the status of your pet’s parathyroid glands without simultaneously knowing the pet's blood calcium and phosphorus levels.
Renal secondary hyperparathyroidism. This is the most common form of parathyroid gland disease or over-activity.
It occurs when your pet’s kidneys are failing and can no longer conserve the calcium that passes through them. In that situation, too much calcium spills out in the pet’s urine. In response to sensing low blood calcium levels, your pet’s parathyroid glands increase their production of PTH. It is to no avail however, because the kidneys no longer have the capacity to react to the PTH. What unfortunately does occur, however is a weakening of bone throughout the pet's body (osteomalacia). The condition is more common in dogs than in cats.
Renal secondary hyperparathyroidism can also occur on calcium or vitamin D3 deficient diets and in cases of starvation and general malnutrition.
Over active parathyroid glands (primary hyperparathyroidism) will also cause high PTH levels. The usual cause for over-active parathyroid glands are one or more benign tumors within them (adenomas). Blood calcium levels also increase in this problem.
A normal PTH level in a pet whose blood calcium levels are high (and therefore should not be producing much PTH) is also suggestive of hyperparathyroidism.
This condition is quite rare in dogs or cats. When it does occur, it is almost always due to neck surgery in which the parathyroid glands were accidentally removed. Generally that surgery involved the thyroid gland but it could have involved the larynx or, perhaps removal of cancerous lymph nodes or other neck tumors.
In humans, low magnesium and autoimmune parathyroid gland destruction have been reported. I know of no cases reported in dogs or cats.
CBC/WBC and blood chemistry Panel including blood calcium and phosphate levels, urinalysis, evaluation of the calcium content and vitamin D3 availability in the pet’s diet, perhaps a PTHrP assay for evidence of lymphoma if Ca levels are high