Calcium is a vital mineral essential for numerous bodily functions, extending far beyond bone health. It plays a critical role in muscle contraction, including voluntary skeletal muscles, involuntary smooth muscles, and the indispensable heart muscle. Furthermore, calcium acts as a messenger to activate enzymes and regulate various physiological processes. Consequently, blood calcium levels are meticulously maintained within a narrow, healthy range.
How Calcium is Organized and Regulated in the Body
In the bloodstream, calcium exists in two primary forms: ionized calcium and bound calcium. Ionized calcium is freely circulating and readily available for use in bodily functions. Bound calcium, on the other hand, is attached to albumin, a blood protein, or complexed with other ions. Total calcium, as reported on most blood chemistry panels, is the sum of ionized and bound calcium. A significant portion of the body’s calcium is also stored in bone tissue, which acts as a reservoir that can be mobilized when circulating calcium levels drop.
The body employs a sophisticated hormonal system to adjust blood calcium levels. Parathyroid hormone (PTH) and calcitriol (a form of vitamin D) are the key players. Calcitriol promotes calcium absorption from the intestines, facilitates calcium release from bone, and reduces calcium excretion by the kidneys, all leading to increased blood ionized calcium. PTH also stimulates the release of calcium from bone and decreases renal calcium excretion. These hormones work in a feedback loop: as blood calcium levels rise, PTH production is suppressed, which in turn reduces calcitriol activation and allows calcium levels to fall again.
Symptoms of Hypocalcemia (Low Blood Calcium)
When blood calcium levels become abnormally low, it can lead to muscle dysfunction and nervous system hyperexcitability. Symptoms can include:
- Seizures (Hypocalcemic Tetany): Particularly in dogs, seizures can occur when calcium levels drop below 6 mg/dL (1.50 mmol/L), sometimes exacerbated by exercise.
- Neurological Signs: Nervousness, disorientation, uncoordinated gait (“drunken walk”).
- Physiological Changes: Fever, weak pulses, excessive panting, muscle tension, twitches, and tremors.
- Behavioral Changes: Cats may exhibit increased listlessness and third eyelid elevation. Painful muscle cramping can lead to aggression.
If blood calcium levels fall to 4 mg/dL (1.00 mmol/L) or below, it can be life-threatening.
Causes of Hypocalcemia in Pets
Several conditions can disrupt the body’s calcium regulation system:
Nursing a Large Litter
Small mother dogs attempting to nurse large litters can become severely depleted of calcium. This is typically diagnosed by the patient’s history and confirmed with blood tests. Treatment involves calcium injections followed by oral supplements. It’s important to note that preemptive oral calcium supplementation can sometimes suppress the mother’s natural ability to mobilize calcium, potentially worsening the situation if a deficiency arises.
Chronic Kidney Disease
Damaged kidneys may not respond appropriately to hormonal signals, leading to imbalances in both calcium and phosphorus. Management focuses on controlling phosphorus levels and addressing the underlying kidney issue.
Acute Pancreatitis
Inflammation of the pancreas can lead to the digestion of body fats, releasing triglycerides that bind to calcium. This process can significantly lower ionized calcium levels, resulting in hypocalcemia.
Toxin Exposure
- Phosphate-Based Enemas: Using human phosphate enemas in pets introduces a large amount of phosphate, which binds to circulating calcium, potentially causing a crisis, especially in smaller animals.
- Antifreeze (Ethylene Glycol) Poisoning: This severe poisoning can lead to acute kidney failure and may include low blood calcium as part of the syndrome.
Parathyroid Hormone (PTH) Deficiency (Hypoparathyroidism)
This condition occurs when the parathyroid glands do not produce sufficient PTH, failing to adequately raise low blood calcium levels. This also leads to an inability to regulate phosphorus and a reduced activation of vitamin D. The average age of onset is around 5 years, with certain breeds like toy poodles, miniature schnauzers, and Labrador retrievers being more commonly affected.
Diagnosis of Hypocalcemia
Diagnosis involves a thorough history, physical examination, and laboratory tests, including a basic blood panel and urinalysis. Key indicators include low calcium and, in some cases, high phosphorus levels. If kidney failure is suspected, other blood and urine tests will help assess kidney function. If the cause remains unclear, a PTH blood level test can help distinguish between hypoparathyroidism and other conditions. Low magnesium levels can also cause secondary hypoparathyroidism and should be ruled out.
Treatment and Long-Term Management
Treatment strategies depend on the severity of the hypocalcemia:
- Acute Crisis: For patients experiencing seizures or critically low calcium levels, hospitalization and intravenous calcium administration are necessary.
- Long-Term Management: This typically involves oral calcium and vitamin D supplementation. It can take several days for oral medications to take full effect, so intravenous or subcutaneous calcium may be administered initially.
Vitamin D Forms for Treatment
Three forms of vitamin D are used:
- Vitamin D2 (Ergocalciferol): Not ideal for treating hypoparathyroidism due to its slow onset of action and storage in body fat, making fluctuations in calcium levels difficult to manage.
- DHT (Dihydrotachysterol): Offers a faster onset of action (1-7 days), but calcium levels can take 4-21 days to normalize if problems arise. Liquid forms are often preferred.
- Calcitriol: The preferred medication for managing hypoparathyroidism, offering a rapid onset of effect (1-4 days) and a relatively quick decrease in calcium levels after discontinuation if levels become too high. Compounding pharmacies are often needed to prepare appropriate pet dosages.
Oral Calcium Supplements
Calcium carbonate is generally the supplement of choice due to its efficacy and lower risk of stomach irritation compared to calcium chloride. It is crucial to consult with a veterinarian before changing calcium supplement types, as dosing calculations will need adjustment.
Monitoring
Stable patients with hypoparathyroidism require regular monitoring, typically quarterly, with blood calcium level tests to ensure levels remain within the target range (8-9 mg/dL or 2.0-2.25 mmol/L) and to make any necessary dose adjustments. Signs of excessively high blood calcium, such as vomiting, diarrhea, increased thirst, and listlessness, should be reported to a veterinarian immediately.
