NAVLE Endocrine

Feline Hyperparathyroidism Study Guide

📅 March 28, 2026 ⏱ 25 min read
Hyperparathyroidism refers to excessive secretion of parathyroid hormone (PTH) from the parathyroid glands, resulting in disturbances in calcium and phosphorus homeostasis.

Overview and Clinical Importance

Hyperparathyroidism refers to excessive secretion of parathyroid hormone (PTH) from the parathyroid glands, resulting in disturbances in calcium and phosphorus homeostasis. In cats, hyperparathyroidism exists in primary and secondary forms, each with distinct etiologies, clinical presentations, and treatment approaches. Understanding these disorders is essential for the NAVLE as they frequently appear in questions involving hypercalcemia, chronic kidney disease (CKD), and metabolic bone disease in felines.

PTH is the principal hormone involved in minute-to-minute regulation of blood calcium concentration through effects on tubular reabsorption of calcium, intestinal calcium absorption (mediated indirectly via calcitriol), and bone resorption of calcium. The parathyroid glands are exquisitely sensitive to fluctuations in ionized calcium (iCa), which is the metabolically active form of calcium.

Fraction Percentage Clinical Significance
Ionized (free) Approximately 50% Biologically active; regulated by PTH
Protein-bound 40-45% Bound to albumin (80%) and globulins (20%)
Complexed 5-10% Bound to citrate, sulfate, phosphate

Calcium and PTH Physiology

The parathyroid glands (PTGs) are small endocrine organs located on or near the thyroid gland. In cats, there are typically four PTGs (two external and two internal) associated with each lobe of the thyroid gland. The chief cells of the PTGs secrete PTH in response to low ionized calcium levels.

PTH Actions

Kidney: Increases calcium reabsorption and phosphorus excretion. Increases 1-alpha-hydroxylase activity, converting 25-hydroxyvitamin D (25OHD) to 1,25-dihydroxyvitamin D3 (calcitriol).

Bone: Stimulates osteoclastic bone resorption, releasing calcium and phosphorus into circulation.

Gastrointestinal Tract (indirect): Calcitriol increases intestinal calcium and phosphorus absorption.

High-YieldOn the NAVLE, remember the inverse sigmoidal relationship between iCa and PTH secretion. When iCa drops, PTH secretion dramatically increases; when iCa rises, PTH secretion decreases. Total serum calcium CANNOT reliably predict ionized calcium in cats, with up to 40% diagnostic discordance reported.

Serum Calcium Fractions

Clinical Sign Pathophysiology
Anorexia and lethargy (most common) Hypercalcemia reduces smooth and skeletal muscle contractility
Vomiting GI smooth muscle dysfunction
Polyuria and polydipsia Calcium interferes with ADH action on renal tubules (nephrogenic DI)
Weight loss Chronic disease, anorexia
Calcium-containing uroliths Hypercalciuria leading to calcium oxalate stones
Palpable cervical mass (some cases) Enlarged PTG or associated cystic structures

Primary Hyperparathyroidism (PHPT)

Primary hyperparathyroidism results from autonomous, inappropriate secretion of PTH by one or more parathyroid glands, leading to hypercalcemia. Although historically considered uncommon in cats, PHPT is more prevalent than previous diagnoses would suggest.

Etiology

  • Parathyroid adenoma: Most common cause (single gland affected)
  • Parathyroid hyperplasia: May affect more than one gland
  • Parathyroid carcinoma: Rare in cats
  • Parathyroid cystadenoma: Cystic variant that may be palpable

Signalment

  • Age: Middle-aged to older cats (8-15 years), average age 13 years
  • Breed: No confirmed breed predisposition; domestic shorthairs most commonly affected; Siamese cats may show some predilection
  • Sex: No sex predisposition documented

Clinical Signs

Clinical signs tend to be non-specific and are primarily due to the effects of hypercalcemia:

NAVLE TipUnlike dogs, cats with PHPT often retain the ability to concentrate urine despite hypercalcemia, with mean urine specific gravity of 1.032 reported in one study. Do not assume hyposthenuria in hypercalcemic cats!

Diagnosis

Laboratory Findings

Imaging

Cervical Ultrasound: High-frequency ultrasonography (7.5-10 MHz) can identify enlarged parathyroid glands. A PTG greater than 4 mm in diameter is highly suspicious for adenoma or carcinoma. Normal PTGs are typically too small to visualize (less than 2 mm).

Treatment

High-YieldPost-operative hypocalcemia occurs because chronic hypercalcemia suppresses and atrophies the remaining normal PTGs. Signs of hypocalcemia include facial rubbing, muscle tremors, tetany, and seizures. The remaining PTGs typically regain function within days to weeks.
Parameter Expected Finding Notes
Total Calcium Increased Often the first finding prompting workup
Ionized Calcium Increased Must be measured for accurate diagnosis
Phosphorus Low to low-normal Due to PTH-induced phosphaturia; hyperphosphatemia argues against PHPT
PTH Increased or inappropriately normal Even high-normal PTH is inappropriate with hypercalcemia
PTHrP Normal or undetectable Elevated in hypercalcemia of malignancy

Nutritional Secondary Hyperparathyroidism (NSHP)

Nutritional secondary hyperparathyroidism occurs when dietary imbalances lead to hypocalcemia, stimulating compensatory PTH secretion. This is most commonly seen in young, growing animals fed inappropriate diets.

Etiology

  • All-meat diets: Very low calcium:phosphorus (Ca:P) ratio (meat has low calcium, high phosphorus)
  • Calcium deficiency: Absolute or relative deficiency in diet
  • Excess dietary phosphorus: Alters Ca:P ratio even with normal calcium
  • Vitamin D deficiency: Less common contributor

Recommended Ca:P ratio for growing kittens: Approximately 1:1 to 1.2:1

Signalment

Age: Young, growing kittens (typically 2-6 months old) are most commonly affected due to increased calcium demands for bone growth and minimal calcium stores.

Clinical Signs

Clinical signs are related to hypocalcemia and osteopenia:

Diagnosis

Laboratory Findings

  • Calcium: Normal or decreased (may be normal if PTH is compensating)
  • Phosphorus: Normal or decreased (due to PTH-induced phosphaturia)
  • PTH: Elevated
  • Alkaline phosphatase: Often elevated due to bone isoenzyme associated with growth
  • Calcitriol: May be increased (PTH stimulates activation of vitamin D)

Radiographic Findings

  • Diffuse osteopenia (generalized decrease in bone opacity)
  • Thin cortices
  • Folding fractures (bones bend rather than break cleanly)
  • Pathological fractures of long bones, pelvis, spine
  • Bony deformities

Treatment

Prognosis

Good for uncomplicated cases. Calcium, phosphorus, and PTH should normalize within 2 weeks of dietary correction. Appropriate bone mineralization may occur within 4-8 weeks. Guarded to poor for animals with spinal fractures or severe bony changes causing permanent neurological deficits.

NAVLE TipWhen you see a young kitten on an all-meat diet presenting with reluctance to move, hindlimb weakness, and radiographic evidence of osteopenia with folding fractures, think NSHP immediately. Treatment is simple: balanced diet + cage rest = good prognosis.
Treatment Details
Parathyroidectomy Treatment of choice. Surgical removal of affected PTG(s). Good long-term prognosis for adenoma/cystadenoma.
Pre-surgical stabilization IV 0.9% NaCl diuresis. May add furosemide (2-4 mg/kg IV). Consider glucocorticoids if needed.
Post-operative monitoring Monitor serum calcium 1-2 times daily for at least 1 week. Risk of hypocalcemia especially if pre-surgical Ca greater than 14 mg/dL.
Hypocalcemia management Calcium gluconate 10% (0.5-1.5 mL/kg IV slowly) for acute signs. Oral calcium and calcitriol supplementation until remaining PTGs recover.

Renal Secondary Hyperparathyroidism (RSHP)

Renal secondary hyperparathyroidism is a complex metabolic disorder that develops as a consequence of chronic kidney disease (CKD). It is characterized by PTG hyperplasia and elevated PTH levels in response to phosphorus retention, decreased calcitriol production, and subsequent hypocalcemia.

Pathophysiology

The development of RSHP involves a cascade of events:

  • Decreased GFR: Progressive loss of functional nephrons reduces glomerular filtration rate
  • Phosphorus retention: Decreased renal phosphorus excretion leads to hyperphosphatemia
  • Decreased calcitriol: Reduced 1-alpha-hydroxylase activity in failing kidneys decreases activation of vitamin D
  • Ionized hypocalcemia: Low calcitriol reduces intestinal calcium absorption; phosphorus complexes with calcium
  • PTH secretion: Low calcium, high phosphorus, and low calcitriol all stimulate PTH release
  • PTG hyperplasia: Chronic stimulation leads to increased PTG mass and non-suppressible basal PTH secretion

Role of FGF-23

Fibroblast Growth Factor-23 (FGF-23) is a phosphatonin secreted by osteoblasts and osteocytes in response to hyperphosphatemia. It promotes renal phosphorus excretion and downregulates 1-alpha-hydroxylase. FGF-23 concentrations are elevated early in CKD and are associated with disease progression and mortality. FGF-23 requires Klotho as a co-receptor for activity; Klotho expression is decreased in CKD, leading to end-organ resistance to FGF-23.

Prevalence

The prevalence of RSHP in cats with CKD has been reported as high as 84%. PTH concentrations may increase even at IRIS Stage 1-2 CKD, before azotemia is detected. PTH is considered a uremic toxin and may contribute to ongoing renal damage, anemia, and bone marrow suppression.

Clinical Signs

Clinical signs are primarily related to the underlying CKD:

  • Polyuria and polydipsia
  • Weight loss
  • Decreased appetite
  • Vomiting
  • Dehydration

Severe or chronic RSHP may cause:

  • "Rubber jaw" (fibrous osteodystrophy) - loosening of teeth, soft mandible
  • Soft tissue mineralization (metastatic calcification)
  • Bone pain and pathological fractures (less common than in NSHP)

Diagnosis

IRIS Phosphorus Targets

Treatment

High-YieldSerum phosphorus less than 4.5 mg/dL has been reported as a surrogate for PTH that is not increased in dogs with CKD. Phosphorus control through diet and binders is the cornerstone of RSHP management. NEVER start calcitriol until phosphorus is controlled - risk of soft tissue mineralization if Ca x P product exceeds 6 (SI units).
Clinical Sign Description
Reluctance to move Due to bone pain and weakness
Hindlimb lameness Weak, painful bones
Bowing of legs Bone deformities from osteopenia
Folding or pathological fractures Long bones, pelvis, spine commonly affected; bones slowly fold rather than snap
Muscle tremors and seizures Hypocalcemia-induced neuromuscular excitability
Paresis or paralysis Vertebral body fractures causing neurological deficits
"Rubber jaw" (fibrous osteodystrophy) Severe demineralization causing jaw to become pliable

Differential Diagnosis Comparison

"PHPT = Primary Problem in the PTG" - The gland itself is abnormal (tumor/hyperplasia) causing autonomous PTH secretion and hypercalcemia.

"NSHP = Nutritional Need Not Met" - Young kitten + all-meat diet + folding fractures = calcium deficiency causing compensatory PTH rise.

"RSHP = Renal Retention of Phosphorus" - CKD leads to phosphorus buildup, calcitriol deficiency, and compensatory PTH elevation.

"Phosphorus Predicts PTH in RSHP" - Hyperphosphatemia is a reliable indicator of hyperparathyroidism in cats with CKD. Control phosphorus first!

Treatment Details
Dietary correction Transition to complete and balanced commercial diet formulated for growth. This is the primary treatment.
Acute hypocalcemia Parenteral calcium gluconate or short-term oral calcium supplements for animals with tremors/seizures.
Cage rest Reduce risk of fractures during recovery.
Analgesia For animals with fractures or bony deformities.
Parameter Expected Finding Notes
Creatinine/BUN Elevated (azotemia) Indicates CKD; IRIS staging guides therapy
Phosphorus Usually elevated Hyperphosphatemia indicates RSHP is likely
Calcium Variable (low, normal, or high) Ionized calcium usually normal; total Ca may be elevated
PTH Elevated May increase before azotemia is detected
FGF-23 Elevated Early marker; increases with IRIS stage
IRIS Stage Target Serum Phosphorus
Stage 2 Less than 4.5 mg/dL (1.45 mmol/L)
Stage 3 Less than 5.0 mg/dL (1.61 mmol/L)
Stage 4 Less than 6.0 mg/dL (1.94 mmol/L)
Treatment Details
Renal diet Phosphorus-restricted, protein-restricted diet. Most proven treatment; cats on renal diets survived 20.8 months vs 8.7 months on maintenance diet.
Phosphate binders Aluminum hydroxide, calcium carbonate/acetate, lanthanum carbonate, sevelamer. Given with meals to bind dietary phosphorus in GI tract.
Calcitriol therapy Low-dose calcitriol (2.5-3.5 ng/kg/day) to suppress PTH and prevent PTG hyperplasia. Use only after phosphorus is controlled. Monitor for hypercalcemia.
Treat underlying CKD Fluid therapy, anti-emetics, appetite stimulants, manage hypertension, address proteinuria.
Condition Total Ca iCa Phosphorus PTH Key Feature
PHPT High High Low-normal High/inappro-priately normal PTG tumor on ultrasound
NSHP Normal/Low Normal/Low Normal/Low High Young animal, all-meat diet, folding fractures
RSHP Variable Normal/Low High High Azotemia, CKD history
Idiopathic Hypercalcemia High High Normal Low-normal Most common cause in cats; mild-moderate hypercalcemia
Malignancy High High Normal/Low Low PTHrP may be elevated; tumor identified

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