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Excessive Levels of Parathyroid Hormone in the Blood in Cats

Hyperparathyroidism is medical condition in which abnormally high levels of parathyroid hormone (also known as parathormone or PTH) are circulating in the blood as the result of an overactive parathyroid gland. The parathyroid hormone is responsible for regulating calcium and phosphorus levels in the blood, increasing blood calcium levels by causing calcium to be reabsorbed from bone. The parathyroid glands are small, hormone-secreting glands that are located on or near the thyroid glands. The term para- refers to adjacent or alongside, and thyroid refers to the actual thyroid gland; the thyroid and parathyroid glands are located side by side in the neck, near the windpipe or trachea

Primary hyperparathyroidism refers to a condition in which a tumor in the parathyroid gland produces excessive levels of parathyroid hormone, leading to increased blood calcium levels (hypercalcemia).

Secondary hyperparathyroidism can be caused by a deficiency of calcium and vitamin D, and is associated with malnutrition or long-term (chronic) kidney disease.

There is no known genetic cause for primary hyperparathyroidism, but its association with certain breeds suggests a possible hereditary basis in some cases. Secondary hyperparathyroidism can develop in association with hereditary kidney disease (known as hereditary nephropathy), but is not inherited per se. Siamese cats seem to show some predilection for this disease. In cats, the average age is 13 years, with a range of 8 to 15 years of age.

Symptoms and Types

• Most cats with primary hyperparathyroidism do not appear ill
• Signs are usually mild and are due solely to the effects of high levels of calcium in the blood
• Increased urination
• Increased thirst
• Lack of appetite
• Sluggishness
• Vomiting
• Weakness
• Presence of stones in the urinary tract
• Stupor and coma
• Veterinarian may be able to feel enlarged parathyroid glands in the neck
• Nutritional secondary hyperparathyroidism is caused by diets that have too little calcium and vitamin D or too much phosphorus – it is a type of malnutrition
• Nutritional secondary hyperparathyroidism sometimes is associated with bone fractures and general poor body condition

Causes

• Primary hyperparathyroidism – PTH-secreting tumor of the parathyroid gland; in most cases only one gland has a tumor; malignant tumors of the parathyroid glands are uncommon
• Secondary hyperparathyroidism is related to malnutrition – nutritional deficiency of calcium and vitamin D or nutritional excess of phosphorus
• Secondary hyperparathyroidism is also related to long-term (chronic) kidney disease. Calcium is lost through the kidneys and absorption of calcium is reduced through the intestinal tract due to deficiency of a hormone known as calcitriol (which regulates levels and absorption of calcium in the intestines), which is produced by the kidneys; may also be due to the retention of phosphorus in the body
• Primary hyperparathyroidism – unknown
• Secondary hyperparathyroidism – related to calcium/vitamin D malnutrition or to long-term (chronic) kidney disease

Diagnosis

Your veterinarian will be looking for cancer first and foremost for the cause for this disease. However, several other possibilities will also be considered, such as renal failure and vitamin D intoxication, which have been known to be found in some rodenticides. Other possibilities are too much calcium in the blood in cats. A urinalysis will reveal calcium and phosphate levels.

Serum ionized calcium determination is often normal in patients with chronic renal failure and high in patients with primary hyperparathyroidism or hypercalcemia that is associated with a malignancy. If kidney stones are suspected, your veterinarian may use x-ray and ultrasound imaging of the parathyroid gland to discover whether there is tumor there. If nothing can be found using these diagnostic techniques, your veterinarian may need to use surgery to explore the area of the thyroid and parathyroid.