Endocrine System: Parathyroid Glands

What is the action of calcitonin?

1. Inhibits osteoclast activity, thus decreasing blood calcium levels. 2. Increases calcium deposition in bone from blood.

Parathyriod Hormone is the major regulator of what 3 things?

Blood Levels of: 1. Calcium. 2. Magnesium. 3. Phosphate.

Parathyroid hormone and calcitonin secretion are both directly controlled by what?

Blood calcium levels via negative feed-back loops.

What role does calcitriol play?

It increases absorption of calcium, phosphate, and magnesium in the GI tract.

Lower than normal blood calcium levels stimulates which cells?

Stimulates chief/principle cells of the parathyroid to secrete PTH, which promotes increased calcium blood levels.

Higher than normal blood calcium levels stimulates which cells?

Stimulates parafollicular cells of the thyroid to release calcitonin.

What is calcitriol?

The active form of Vitamin D [1,25-dihydroxyvitamin D3].

How many parathyroid glands are there and where are they located?

*4 glands. *2 located on the posterior aspect of each lateral lobe of the thyroid.

Histology of the parathyroid glands.

*Cells arranged in thick branching cords. *2 cells types: 1. Chief/Principle Cells [more numerous]. 2. Oxyphil Cells.

What is the function of the 2 cells that make of the parathyroid glands?

*Chief/Principle Cells --> Produce and secrete parathyroid hormone [PTH]. *Oxyphil Cells - Function unknown in normal physiology.

Which of the 3 ions regulated by PTH are increased when PTH is secreted? Which are decreased?

*Increased --> Calcium and Magnesium. *Decreased --> Phosphate.

2 target organs of PTH to regulate calcium, magnesium, and phosphate levels.

1. Bone. 2. Kidneys.

What effect does PTH have on bone?

1. Increases the number and activity of osteoclasts. 2. This increases bone resorption, thus releasing calcium and phosphate levels into the blood.

Calcium is tightly regulated within what range?

9-11 mg/100 mL.

What 3 effects does PTH have on the kidney?

1. Slows the rate calcium and magnesium are lost in urine. 2. Increases the rate that phosphate is lost in urine. 3. Promotes formation of calcitriol [active form of Vitamin D] to increase calcium, phosphate, and magnesium absorption in the GI tract.

PTH causes resorption of phosphate into the blood from bone. So why do phosphate levels decrease in response to PTH?

Because PTH also stimulates the kidneys to excrete more phosphate, and this loss of phosphate exceeds the amount that enters the blood from bone resorption.

Which hormone works antagonistically to PTH and where is it secreted from?

Calcitonin, secreted by parafollicular cells of the hypothalamus.

Is PTH secretion regulated by the hypothalamus or pituitary gland at all?

No, it is strictly regulated by blood calcium levels via negative feed-back.

What secretes calcitonin?

Parafollicular cells of the thyroid gland.