Hyperparathyroidism
Basic Information 
Epidemiology
Species, Age, Sex
• Hyperparathyroidism has been reported in horses of all ages.
• Clinical signs of hyperparathyroidism secondary to nutritional imbalances are more evident in young, growing animals.
• In nutritional hyperparathyroidism, many horses can be affected in one farm, but only one horse is affected in primary hyperparathyroidism.
Clinical Presentation
Disease Forms/Subtypes
• Hyperparathyroidism may be primary or secondary.
Primary hyperparathyroidism: An autonomous and excessive secretion of PTH from the parathyroid glands. The parathyroid glands in these horses are not responsive to calcium concentrations (refractory).
• Another condition associated with abnormal calcium and phosphorus concentrations is humoral hypercalcemia of malignancy (HHM; pseudohyperparathyroidism).
Physical Exam Findings
• Horses with nutritional secondary hyperparathyroidism (also termed big head, Miller’s disease) may have generalized swelling of the skull and narrowing of the nasal meatus, which may lead to upper airway stridor.
• Whereas facial changes or deformations are evident in young animals, lameness and decreased bone mass are the main findings in old horses.
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