Hyperparathyroidism in Cats: Causes, High Calcium & Treatment (2026)
Quick Facts: Hyperparathyroidism in Cats
- Three distinct forms: Primary (parathyroid adenoma), renal secondary (driven by CKD), and idiopathic hypercalcemia (high calcium with suppressed PTH — unique to cats).
- Most common cause of high calcium in cats: Idiopathic hypercalcemia — NOT cancer, unlike in dogs.
- Most common form overall: Renal secondary HPT — because CKD is extremely prevalent in senior cats (>30% of cats over 15).
- Key distinction: Idiopathic hypercalcemia has HIGH calcium + LOW PTH. Primary HPT has HIGH calcium + HIGH (or inappropriately normal) PTH.
- Renal secondary HPT bloodwork: HIGH PTH + HIGH phosphorus + HIGH BUN/creatinine + normal or low-normal calcium.
- Treatment: Primary: surgery (curative). Renal secondary: phosphate binders, restricted diet, calcitriol. Idiopathic: dietary change, prednisolone, or alendronate.
- Calcium oxalate stones: A common complication of both primary HPT and idiopathic hypercalcemia in cats.
Trying to understand your cat's high calcium or kidney values?
Upload your cat's blood panel — VetLens explains every value in plain language, flags elevated calcium and PTH patterns, and helps you track results across visits.
Upload My Cat's ResultsWhat Is Hyperparathyroidism in Cats?
Cats have four tiny parathyroid glands adjacent to the thyroid glands in the neck. These glands produce parathyroid hormone (PTH), which is the body's primary calcium-raising signal. PTH pulls calcium from bone, increases calcium reabsorption in the kidneys, and triggers calcitriol production (active vitamin D) to increase calcium absorption from the gut. Normally this system is tightly regulated: when calcium rises, PTH shuts off.
Hyperparathyroidism means PTH production is excessive — either because a tumor is making it autonomously (primary), or because the body is genuinely trying to raise calcium that has fallen too low (secondary). The end result in either case can be harmful: too much PTH pulls calcium from bones and causes soft tissue damage.
In dogs, cancer is the most common cause of high calcium. In cats, idiopathic hypercalcemia — a condition where calcium is high but PTH is actually suppressed — is the most common cause. This matters enormously: the workup, diagnosis, and treatment are all different. Understanding which of the three feline forms is present determines everything about management.
The Three Forms of Feline Hyperparathyroidism
Form 1: Renal Secondary Hyperparathyroidism
The most common form overall. CKD — one of the most frequent diseases in senior cats — sets off a cascade: failing kidneys cannot excrete phosphorus, so phosphorus rises (hyperphosphatemia). High phosphorus suppresses calcitriol (active vitamin D) production. Less calcitriol means less calcium absorbed from the gut. The parathyroid glands detect relative calcium insufficiency and pump out PTH to compensate. PTH raises calcium by pulling it from bone.
This form is medically appropriate — the parathyroid glands are working correctly — but PTH-mediated bone resorption and soft tissue mineralization cause long-term harm. Elevated PTH is itself nephrotoxic, worsening the CKD that started the cascade.
Bloodwork signature: High PTH + high phosphorus + elevated BUN/creatinine/SDMA + normal or low-normal calcium. See our guide to chronic kidney disease in cats for the full CKD bloodwork picture.
Form 2: Primary Hyperparathyroidism
Uncommon but well-documented in cats. A single parathyroid gland develops an adenoma (benign tumor) that secretes PTH autonomously — regardless of how high calcium climbs. Because the feedback loop is broken, calcium rises unchecked. The result is persistent hypercalcemia with all its downstream consequences.
Bloodwork signature: High ionized calcium + low phosphorus + PTH that is inappropriately normal or elevated (any detectable PTH in the context of high calcium is abnormal — it should be near zero). Kidney values are normal unless hypercalcemia has damaged the kidneys.
Form 3: Idiopathic Hypercalcemia (IHC)
The most common cause of persistent high calcium in cats — and technically not hyperparathyroidism at all. In IHC, ionized calcium is elevated but PTH is low or completely suppressed. The parathyroid glands are doing their job correctly (suppressing PTH when calcium is high), but the calcium is rising from a source that doesn't respond to PTH.
The cause is incompletely understood. Current theories include: abnormal intestinal calcium absorption (possibly diet-related), altered receptor sensitivity in bone cells, and possible links to high-oxalate diets or high-protein commercial diets. IHC is associated with calcium oxalate urinary stones and sometimes chronic GI signs (vomiting, constipation, weight loss). Many cats are diagnosed incidentally when a wellness panel shows an unexpected calcium elevation.
Bloodwork signature: High ionized calcium + LOW PTH (suppressed) + normal phosphorus + normal kidney values. This pattern is the key distinguishing feature from primary HPT (which has normal-to-high PTH with the same high calcium).
Symptoms
When Calcium Is High (Primary HPT & Idiopathic Hypercalcemia)
Excessive thirst & urination
High calcium impairs ADH signaling in kidney tubules — water pours out even though the cat is thirsty
Lethargy & weakness
Elevated calcium impairs neuromuscular transmission
Decreased appetite
Hypercalcemia reduces GI motility and can cause nausea
Vomiting
GI smooth muscle dysfunction and direct mucosal irritation
Constipation
Reduced colonic motility; also worsened by dehydration from PU/PD
Weight loss
Combination of poor appetite and increased metabolic demand
Calcium oxalate urinary stones
Excess calcium spills into urine → crystals form → straining, hematuria, recurrent UTIs
Depression / social withdrawal
Neurological effects of sustained hypercalcemia
Renal Secondary HPT (Calcium Usually Normal)
Because calcium is not elevated in most cats with renal secondary HPT, the hypercalcemic symptoms above are typically absent. Instead, cats show signs of the underlying CKD: progressive weight loss, poor coat quality, reduced appetite, intermittent vomiting, increased thirst and urination, and general weakness. In advanced, prolonged cases, excess PTH causes bone resorption leading to jaw bone softening (fibrous osteodystrophy / “rubber jaw”) — a rare late-stage finding.
Both renal secondary HPT and idiopathic hypercalcemia can be completely asymptomatic when first discovered on routine bloodwork. A wellness panel in a senior cat showing elevated phosphorus or elevated calcium may be the first hint — one reason routine annual or biannual bloodwork is recommended for cats over 7–8 years.
What Bloodwork Shows
Measuring ionized calcium and PTH together is the key that unlocks the diagnosis. Total calcium alone is insufficient — it can be falsely lowered by low albumin and misses the distinction between idiopathic hypercalcemia and primary HPT.
| Test | Primary HPT | Renal Secondary HPT | Idiopathic Hypercalcemia |
|---|---|---|---|
| Ionized calcium | HIGH | Normal or low | HIGH |
| PTH | Normal or HIGH (inappropriate) | HIGH (appropriate) | LOW (suppressed) |
| Phosphorus | LOW (PTH increases renal excretion) | HIGH (poor renal excretion) | Normal |
| BUN / Creatinine / SDMA | Normal (unless hypercalcemia damaged kidneys) | HIGH — reflects CKD | Normal (or mildly elevated if stones caused damage) |
| Kidney values | May worsen over time | Abnormal — CKD present | Usually normal |
Ionized calcium is the accurate test
How Vets Diagnose It
Confirm true hypercalcemia (if calcium is elevated)
Measure ionized calcium. If ionized calcium is consistently elevated on two samples, true hypercalcemia is confirmed. Rule out dehydration (which can falsely raise total calcium) and low albumin (which can falsely lower it).
Measure PTH alongside ionized calcium
This is the most important diagnostic step. High ionized calcium + high PTH = primary HPT. High ionized calcium + low PTH = idiopathic hypercalcemia (or vitamin D toxicity). Normal ionized calcium + high PTH + high phosphorus + kidney disease = renal secondary HPT. Most cats fit clearly into one of these three patterns.
Evaluate kidney function
BUN, creatinine, SDMA, urine specific gravity, and urine protein-to-creatinine ratio assess CKD severity. Even cats with primary HPT or idiopathic hypercalcemia may have concurrent CKD — hypercalcemia causes kidney damage over time.
Cervical ultrasound (if primary HPT is suspected)
A parathyroid adenoma appears as a small, hypoechoic nodule near the thyroid gland — usually identifiable by an experienced ultrasonographer. Confirming the adenoma location before surgery is important because only the abnormal gland is removed.
Rule out other causes of hypercalcemia
In cats, malignancy-associated hypercalcemia (PTHrP-secreting tumors) is less common than in dogs but still occurs — lymphoma and squamous cell carcinoma are the most frequent. PTHrP measurement helps exclude this. Vitamin D toxicity (certain plants, over-supplementation, rodenticides) is also possible and requires a careful history.
Treatment by Form
Primary Hyperparathyroidism: Surgery
Parathyroidectomy — removal of the abnormal parathyroid gland — is curative in most cats. Only the adenoma-containing gland is removed; the remaining three glands stay in place.
Post-operative hypocalcemia is the primary risk. When the adenoma is removed, calcium drops — sometimes abruptly. The remaining parathyroid glands have been functionally suppressed by chronic hypercalcemia and need 2–8 weeks to resume normal PTH production. During this period, cats require:
- Frequent calcium monitoring in the first 24–72 hours post-surgery
- Oral calcium supplementation (calcium carbonate or calcium gluconate)
- Calcitriol (active vitamin D) to support calcium absorption
Signs of hypocalcemia to watch at home: muscle twitching, facial rubbing or pawing, stiff gait, hypersensitivity to touch, or seizures. These require prompt veterinary attention but are manageable when caught early.
Renal Secondary Hyperparathyroidism: Medical Management
The parathyroid glands are not treated. Management targets the CKD-phosphorus cascade:
Phosphorus-restricted diet
Renal diets (Hill's k/d, Royal Canin Renal, Purina NF) are formulated with reduced phosphorus. Reducing dietary phosphorus is the first and most important step. Both wet and dry formulations are available; wet food also supports hydration.
Phosphate binders
Given with every meal to bind dietary phosphorus in the gut before it enters the bloodstream. Options include aluminum hydroxide (effective but long-term use risks aluminum accumulation), calcium carbonate, lanthanum carbonate (Lantharenol), and sevelamer. Binders are added when diet alone does not achieve target phosphorus levels.
Calcitriol supplementation
Supplementing active vitamin D directly suppresses PTH secretion. Studies suggest calcitriol slows CKD progression in some cats by reducing PTH-mediated nephrotoxicity. Must be used cautiously — excess calcitriol causes hypercalcemia, which worsens kidney damage. Ionized calcium must be monitored regularly when calcitriol is used.
Idiopathic Hypercalcemia: Dietary & Medical
Dietary change (first line)
Switching to a low-oxalate diet, a hydrolyzed protein diet, or a novel protein diet resolves hypercalcemia in some cats — suggesting a dietary trigger. High-fiber diets reduce intestinal calcium absorption and are sometimes used. A 4–8 week dietary trial is typically the first step.
Prednisolone
Corticosteroids reduce intestinal calcium absorption and increase renal calcium excretion. Effective in many cats with IHC. Long-term prednisolone carries risks of diabetes mellitus and immunosuppression, so the minimum effective dose is used and cats are monitored for glucose elevation.
Alendronate (bisphosphonate)
A bisphosphonate that inhibits bone resorption. Used weekly (typically 10 mg orally once weekly) for cats that do not respond to diet or prednisolone. Effective at normalizing calcium in refractory IHC. Must be given carefully — esophageal irritation is a risk if the cat does not swallow the pill promptly with water.
Urinary stones are a complication to monitor
Managing your cat's kidney disease or high calcium long-term?
VetLens reads your cat's lab results, tracks phosphorus, calcium, PTH, and kidney values over time, and explains what each trend means — so you can have more informed conversations with your vet.
Analyze My Cat's BloodworkMonitoring After Diagnosis
After Parathyroidectomy (Primary HPT)
- Calcium every 12–24 hours while hospitalized post-surgery
- Ionized calcium recheck at 1–2 weeks after discharge
- Taper calcium and calcitriol supplementation as the remaining glands recover (weeks to months)
- Long-term: annual calcium and kidney value checks — hypercalcemia may have caused residual kidney damage that manifests later
Ongoing Monitoring for Renal Secondary HPT
- Phosphorus every 3–6 months (target <4.5–5 mg/dL for most CKD stages)
- BUN, creatinine, and SDMA to track kidney function trajectory — see our SDMA guide
- Ionized calcium when using calcitriol (must not overshoot into hypercalcemia)
- Blood pressure — hypertension is common with CKD and must be managed separately
Ongoing Monitoring for Idiopathic Hypercalcemia
- Ionized calcium every 3–6 months to assess response to treatment
- Urine analysis and abdominal imaging (X-ray or ultrasound) to screen for stone formation
- Blood glucose monitoring if on prednisolone — corticosteroid-induced diabetes is a real risk in cats
- Kidney values annually — IHC can cause calcium nephropathy over time
Managing feline hyperparathyroidism means repeated bloodwork — pet insurance helps
Between ionized calcium and PTH testing, cervical ultrasound, parathyroidectomy, and long-term monitoring for kidney values and urinary stones, hyperparathyroidism in cats can be an expensive condition to manage. Pet insurance can cover a significant portion of these costs.
Get a Free Quote →We may earn a commission if you purchase through this link, at no extra cost to you.
Frequently Asked Questions
What is hyperparathyroidism in cats?
Overproduction of parathyroid hormone (PTH), which raises blood calcium. In cats, three distinct forms occur: primary (parathyroid adenoma, autonomous PTH secretion), renal secondary (CKD drives compensatory PTH rise with normal-to-low calcium), and idiopathic hypercalcemia (high ionized calcium with suppressed PTH — technically not true HPT, but the most common cause of persistent hypercalcemia in cats).
What is idiopathic hypercalcemia in cats?
A feline-specific condition where ionized calcium is persistently elevated but PTH is low — the opposite of hyperparathyroidism. Cause is unknown but may involve increased intestinal calcium absorption or altered bone turnover. Associated with calcium oxalate stones and chronic GI signs. Treated with dietary change (low-oxalate, novel protein), prednisolone, or alendronate for refractory cases.
What are the symptoms of hyperparathyroidism in cats?
Primary HPT and idiopathic hypercalcemia (high calcium): excessive thirst and urination, lethargy, decreased appetite, weight loss, vomiting, constipation, and calcium oxalate urinary stones (causing straining or blood in urine). Renal secondary HPT (calcium usually normal): signs from CKD — weight loss, poor appetite, nausea, PU/PD. Advanced secondary HPT can cause jaw softening from prolonged bone resorption.
What does bloodwork show with hyperparathyroidism in cats?
Primary: high ionized calcium, low phosphorus, inappropriately normal or elevated PTH. Renal secondary: elevated PTH, high phosphorus, high BUN/creatinine/SDMA, normal or low-normal calcium. Idiopathic hypercalcemia: high ionized calcium, LOW PTH (suppressed), normal phosphorus, normal kidney values. Measuring ionized calcium and PTH together is essential — total calcium alone is not enough.
How is primary hyperparathyroidism treated in cats?
Surgery (parathyroidectomy) to remove the adenoma is curative in most cats. Post-operative hypocalcemia is the main risk — the remaining parathyroid glands recover over 2–8 weeks, during which calcium and calcitriol supplementation is needed. Ultrasound-guided ethanol ablation is an alternative for poor surgical candidates.
How is renal secondary hyperparathyroidism managed in cats?
Phosphorus-restricted diet (renal diet), phosphate binders given with meals, and calcitriol supplementation to suppress PTH directly. The parathyroid glands are not removed. Calcitriol must be used carefully — excess causes hypercalcemia — and requires regular calcium monitoring.
Get clear answers from your cat's bloodwork
Upload your cat's lab results to VetLens — we'll explain every value, flag high calcium and phosphorus patterns, and help you track changes across visits.
Upload My Cat's ResultsRelated Reading
Chronic Kidney Disease in Cats
CKD is the most common driver of renal secondary hyperparathyroidism — learn the full picture of feline kidney disease.
Cat Kidney Values Explained
BUN, creatinine, SDMA, and phosphorus — what each kidney value means and what the trends tell you.
SDMA in Cats
SDMA detects kidney disease earlier than creatinine — learn how it fits into CKD and HPT monitoring.
Why Is My Cat Drinking Excessive Water?
Hypercalcemia and CKD are both causes of polyuria and polydipsia — learn how vets work through the differential.
Hyperparathyroidism in Dogs
How canine hyperparathyroidism differs from the feline forms — including why cancer is the top differential in dogs.