Feline Hyperparathyroidism Study Guide

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.

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.