Calcitonin and Parathyroid Hormone

Calcitonin

1.) polypeptide hormone, secretion from parafollicular cells of thyroid

2.) released in response to a rise in blood calcium levels

3.) lower calcium (and phosphate) in plasma

4.) uses adenylyl cyclase mechanism

Calcitonin lower calcium (and phosphate) in plasma by

1.) inhibits the activity of osteoclasts

2.) increasing osteoblastic activity

3.) decreasing calcium absorption in the intestines

4.) increasing calcium loss in the urine

Calcitonin clinical uses

1.) osteoporosis treatment if patient has adverse reactions to bisphosphonates (drugs that prevent loss of bone density)

2.) prevent fractures from osteoporosis

3.) paget’s disease

Paget’s

1.) dx: history, exam, X-rays, alkaline phosphatase blood test (high), bone scan;

2.) pain in bone and spine, headaches, increased head size, fractures, bowed legs, etc

3.) abnormal bone remodeling and bone growth; bone is broken down and absorbed faster than in normal bone remodeling

4.) body speeds up the bone remodeling process and builds very weak and brittle bones

Parathyroid hormone

The four parathyroid glands lie immediately behind the thyroid gland. Almost all of PTH is synthesized and secreted by the chief cells. Function of oxyphil cells is uncertain, but they may be modified or depleted chief cells that no longer secrete PTH

Parathyroid gland

1.) arterial supply like that of thyroid gland: inferior thyroid artery (since it supplies the posterior aspect of the thyroid gland)

2.) venous drainage: superior, middle and inferior thyroid veins

3.) innervation: sympathetic nerves derived from the thyroid branches of the superior and middle cervical ganglia

PTH function

1.) increase calcium levels when too low in blood; release of calcium from the bones into the blood (increases bone destruction (OC) and decreases the formation of new bone (OB)

2.) decrease calcium loss of the urine (kidney); inhibits phosphate reabsorption, also stimulates the production of active vitamin D from kidneys

3.) increase calcium absorption from food in intestines— due to vitamin D

4.) uses adenylyl cyclase mechanism

Bones act as calcium pools

1.) release calcium into bloodstream when levels are low

2.) high PTH levels signal bones to release calcium by increasing OC activity and decreasing OB activity = resorption

3.) inhibit phosphate reabsorption (PCT) — phosphate lost in urine; stimulate calcium resorption

PTH: intestines

1.) PTH triggers kidney to convert vitamin D to active form and increases intestinal calcium absorption (1 hydroxylase enzyme-from the kidney)

2.) vitamin D receptors in SI cells —> calcium channel proteins —> increase absorption of calcium

Primary hyperparathyroidism

1.) leads too much calcium in the blood (hypercalcemia); high PTH, high Ca2+, due to enlargement of 1 or more of parathyroid glands

2.) treatment: surgery

Secondary hyperthyroidism

1.) another disease first causes low calcium levels, and over time an increase in PTH

2.) high PTH, low Ca2+ (kidney failure)

3.) tx: paricalcitol “activated vitamin D”

Hypoparathyroidism

1.) too little calcium in blood (hypocalcemia); low PTH, low Ca2+

2.) Tx: oral calcium and vitamin D