Related Subjects:
|PTH Physiology
|Calcium Physiology
|Magnesium Physiology
|Calcitonin
|Hypocalcaemia
|Hypercalcaemia
|Hypomagnesaemia
|Hypermagnesaemia
|Primary Hyperparathyroidism
|Familial hypocalciuric hypercalcaemia (FHH)
|Sarcoidosis
Overview of Parathyroid Physiology
The parathyroid glands are small endocrine glands located on the posterior surface of the thyroid gland. They play a critical role in regulating calcium and phosphate balance in the body through the secretion of parathyroid hormone (PTH).
Anatomy of the Parathyroid Glands
- Typically, there are four parathyroid glands, two superior and two inferior, located on the posterior aspect of the thyroid gland.
- Each gland is about the size of a grain of rice (3-5 mm in diameter).
Synthesis and Secretion of Parathyroid Hormone (PTH)
- Synthesis:
- PTH is synthesized and secreted by the chief cells of the parathyroid glands.
- PTH is produced as a precursor (preproPTH), which is then processed to proPTH and finally to active PTH.
- Secretion:
- PTH secretion is primarily regulated by the serum calcium level via a negative feedback mechanism.
- Low blood calcium levels stimulate PTH secretion, while high blood calcium levels inhibit it.
Actions of Parathyroid Hormone (PTH)
- Bone:
- PTH stimulates osteoclast activity, leading to bone resorption and release of calcium and phosphate into the bloodstream.
- Increases the number and activity of osteoclasts indirectly by stimulating osteoblasts to produce RANKL (Receptor Activator of Nuclear factor Kappa-B Ligand), which activates osteoclasts.
- Kidneys:
- Increases renal reabsorption of calcium in the distal convoluted tubules, reducing calcium excretion.
- Decreases reabsorption of phosphate in the proximal convoluted tubules, increasing phosphate excretion.
- Stimulates the conversion of 25-hydroxyvitamin D to its active form, 1,25-dihydroxyvitamin D (calcitriol), which enhances intestinal absorption of calcium.
- Intestines:
- Indirectly increases intestinal absorption of calcium and phosphate by stimulating the production of active vitamin D in the kidneys.
Regulation of Parathyroid Hormone (PTH) Secretion
- Calcium-Sensing Receptors (CaSR):
- Located on the surface of parathyroid cells, these receptors sense extracellular calcium levels.
- Activation of CaSR by high calcium levels inhibits PTH secretion.
- Low calcium levels reduce CaSR activation, stimulating PTH secretion.
- Vitamin D:
- Active vitamin D (calcitriol) inhibits PTH gene expression and secretion, providing a negative feedback mechanism.
- Phosphate:
- High serum phosphate levels indirectly stimulate PTH secretion by lowering serum calcium levels (due to calcium-phosphate binding).
Clinical Relevance
- Hyperparathyroidism:
- Characterized by excessive production of PTH, leading to hypercalcemia.
- Primary hyperparathyroidism is usually caused by a parathyroid adenoma.
- Secondary hyperparathyroidism is often due to chronic kidney disease, leading to hypocalcemia and compensatory PTH secretion.
- Symptoms: Bone pain, kidney stones, gastrointestinal disturbances, and neuropsychiatric symptoms.
- Hypoparathyroidism:
- Characterized by insufficient production of PTH, leading to hypocalcemia.
- Causes include surgical removal of the parathyroid glands, autoimmune destruction, or genetic disorders.
- Symptoms: Muscle cramps, tetany, seizures, and cardiac arrhythmias.
Diagnosis and Management
- Laboratory Tests:
- Serum calcium, phosphate, and PTH levels are measured to assess parathyroid function.
- Vitamin D levels may also be evaluated.
- Imaging Studies:
- Ultrasound and sestamibi scans are used to identify parathyroid adenomas or hyperplasia.
- Treatment:
- Hyperparathyroidism may require surgical removal of the overactive gland(s).
- Hypoparathyroidism is managed with calcium and vitamin D supplements.
- Medications such as calcimimetics may be used to control hypercalcemia in certain cases.
Summary
The parathyroid glands are essential for maintaining calcium and phosphate homeostasis through the secretion of parathyroid hormone (PTH). PTH acts on bones, kidneys, and intestines to regulate these minerals. Dysregulation of PTH secretion can lead to conditions such as hyperparathyroidism and hypoparathyroidism, with significant clinical implications. Diagnosis involves laboratory tests and imaging, while treatment focuses on correcting the underlying hormonal imbalance.