Endocrine Physiology: Parathyroid Function

Endocrine Physiology: Parathyroid and Calcium Regulation

• Parathyroid glands are small, bean-shaped glands located on the posterior surface of the thyroid gland. They exist in pairs.
• Parathyroid hormone (PTH) is released from chief cells within the parathyroid glands.
• PTH regulates calcium levels in the blood, a vital function.
• Its main role is to increase plasma calcium levels and decrease plasma phosphate levels.
• PTH works by acting on three target organs: bone, kidney, and intestine.
• On bone, PTH increases bone resorption, releasing calcium from the bone matrix into the blood. Normal levels of PTH affect both osteoblasts and osteoclasts, but high levels only affect osteoclasts.
• On the kidney, PTH increases calcium reabsorption and phosphate excretion.
• On the intestine, PTH indirectly increases calcium absorption through its interaction with vitamin D3.
• Calcium homeostasis is controlled by serum calcium levels.
• There is no higher control of the parathyroid gland from the hypothalamus and pituitary.
• Abnormalities in parathyroid hormone (PTH) secretion can lead to either hyperparathyroidism or hypoparathyroidism.
• Hyperparathyroidism is characterized by excessive PTH secretion, resulting in increased blood calcium levels.
• Causes of primary hyperparathyroidism include parathyroid gland abnormalities (adenomas).
• Causes of secondary hyperparathyroidism include hypocalcemia (e.g., vitamin D3 deficiency or chronic renal failure).
• Hypercalcemia (excess calcium in the blood) can lead to symptoms like stones (kidney stones), bones (bone pain and deformities), and abdominal groans (GI issues).
• Hypoparathyroidism results in low PTH levels, leading to low blood calcium levels.
• Causes include accidental removal of the parathyroid during thyroid surgery.
• Hypocalcemia (low blood calcium) can manifest as tetany, a state of spastic muscle contractions due to increased neuromuscular excitability.
• Manifest tetany usually appears during rest when calcium levels are below 7 mg%.
• It has symptoms like carpopedal spasm, generalized convulsions, laryngeal and respiratory spasm, and suffocation.
• Latent tetany may appear during stress when calcium levels are between 7-9mg%. It can be identified by tests like Trousseau's sign and Chvosteck's test.
• Causes of tetany include decreased calcium intake, hypoparathyroidism, vitamin D3 deficiency, and renal failure.
• Normal blood calcium levels are typically 9-11 mg/dL.
• Vitamin D3 (1,25(OH)2D3) is crucial for maintaining calcium balance; it works on the intestine (increasing calcium absorption), kidneys (increasing calcium reabsorption), and bones (supporting osteoblast activity and bone matrix calcification).
• Vitamin D3 production is initiated by sunlight on the skin.
• Calcitonin is a hormone that decreases calcium levels in the blood. It is produced by parathyroid cells in the thyroid and counteracts the effects of PTH. The effects involve reducing bone resorption by inhibiting osteoclasts, increasing the excretion of calcium by the kidneys, and reducing absorption of calcium in the gut.

Calcium Homeostasis

Calcium (Ca) makes up 1.5% of total body mass, 99% stored in bones.

Plasma calcium levels are 9-11 mg/dL (average 10 mg/dL).

Calcium is in two forms, free/ionized (active) and bound (inactive).

• Calcium metabolism involves intake from the diet, interchange and exchange with bones, and excretion.
• There is a balance in the exchange between the intestine, blood, and kidneys in calcium metabolism to regulate normal levels in the body.

Functions of Calcium

• Calcium is essential for:
  • Bone and tooth formation
  • Blood clotting
  • Neuromuscular function and coupling
  • Endocrine function and hormone actions

Hormones Involved in Calcium Homeostasis

• Parathyroid Hormone (PTH): increases blood calcium levels
• Vitamin D3: Increases blood calcium levels
• Calcitonin decreased blood calcium levels

Vitamin D3 (1,25(OH)2D3) Function

Acts on the intestine: promotes calcium absorption from the digestive tract

Acts on the kidneys: increases calcium reabsorption

Acts on bones: promotes osteoblast activity and bone calcification

Calcitonin Function

• Acts on bone to decrease bone resorption
• Acts on kidneys to increase calcium excretion
• Acts on the intestine to decrease calcium absorption.

Calcium Homeostasis Regulation

• The body maintains a balance between PTH, vitamin D3, and calcitonin to monitor calcium in the blood, bones, and gut.
• Blood calcium levels trigger the production or reduction of these hormones.

Assignment: Comparing Hormone Effects

• Students need to compare the ways PTH, Calcitonin, and vitamin D3 impact calcium homeostasis.