The Parathyroid Glands
The parathyroid glands are typically four small, ovoid endocrine glands located on the posterior surface of the thyroid lobes. They are the primary regulators of serum calcium and phosphate balance.
Microscopic Cell Types
| Cell Type | Function |
|---|---|
| Chief Cells | The primary functional cells secrete Parathyroid Hormone (PTH). |
| Oxyphil Cells | Larger, eosinophilic cells with unknown function; their number increases with age. |
PTH Mechanism of Action
PTH increases serum calcium via three mechanisms:
- Bone: Stimulates osteoclast activity (indirectly via RANKL) to release calcium into the blood.
- Kidney: Increases calcium reabsorption in distal tubules and promotes phosphate excretion.
- Small Intestine: Stimulates the synthesis of 1,25-dihydroxyvitamin D, which increases intestinal calcium absorption.
High-Yield Exam Pearl:
Primary Hyperparathyroidism is classically identified by the mnemonic “stones, bones, groans, and psychiatric overtones” (kidney stones, osteitis fibrosa cystica, abdominal pain, and depression/lethargy). It is often caused by a benign parathyroid adenoma.