Hyperparathyroidism is an endocrine disorder characterized by excessive production and secretion of parathyroid hormone (parathormone, PTH) by the parathyroid chief cells. The disorder may be primary or secondary.
In primary hyperparathyroidism the excessive secretion of parathyroid hormone is due to an adenoma or, more rarely, a carcinoma of the parathyroid chief cells. The primary form of hyperparathryroidism is almost always associated with hypercalcaemia.
In secondary hyperparathyroidism, which may be of nutritional or renal nature, the excessive secretion of hormone occurs as an adaptive response of the body to an imbalance in the homeostasis of calcium and phosphorus. Various conditions can cause excessive secretion of parathyroid hormone, but in both dogs and cats the only two conditions in which the excess hormone can give rise to significant manifestations are chronic renal failure and calcium deficiency during the growth phase. These conditions indirectly affect the parathyroid glands causing diffuse hyperplasia of the glands. Whereas the plasma concentration of calcium is increased in primary hyperparathyroidism, in the secondary forms the plasma calcium concentrations may be normal, increased or decreased.