Calcitonin

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Calcitonin

http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/thyroid/calcitonin

html

Calcitonin is a hormone known to participate in calcium and phosphorus

metabolism. In mammals, the major source of calcitonin is from the

parafollicular or C cells in the thyroid gland, but it is also synthesized

in a wide variety of other tissues, including the lung and intestinal tract.

In birds, fish and amphibians, calcitonin is secreted from the

ultimobrachial glands.

Calcitonin is a 32 amino acid peptide cleaved from a larger prohormone. It

contains a single disulfide bond, which causes the amino terminus to assume

the shape of a ring. Alternative splicing of the calcitonin pre-mRNA can

yield a mRNA encoding calcitonin gene-related peptide; that peptide appears

to function in the nervous and vascular systems. The calcitonin receptor has

been cloned and shown to be a member of the seven-transmembrane, G

protein-coupled receptor family.

Physiologic Effects of Calcitonin

A large and diverse set of effects has been attributed to calcitonin, but in

many cases, these were seen in response to pharmacologic doses of the

hormone, and their physiologic relevance is suspect. It seems clear however,

that calcitonin plays a role in calcium and phosphorus metabolism. In

particular, calcitonin has the ability to decrease blood calcium levels at

least in part by effects on two well-studied target organs:

Bone: Calcitonin suppresses resorption of bone by inhibiting the activity of

osteoclasts, a cell type that " digests " bone matrix, releasing calcium and

phosphorus into blood.

Kidney: Calcium and phosphorus are prevented from being lost in urine by

reabsorption in the kidney tubules. Calcitonin inhibits tubular reabsorption

of these two ions, leading to increased rates of their loss in urine.

It seems clear that there are species differences in the importance of

calcitonin as a factor affecting calcium homeostasis. In fish, rodents and

some domestic animals, calcitonin appears to play a significant role in

calcium homeostais. In humans, calcitonin has at best a minor role in

regulating blood concentrations of calcium. One interesting piece of

evidence to support this statement is that humans with chronically increased

(medullary thyroid cancer) or decreased (surgical removal of the thyroid

gland) levels of calcitonin in blood do not show alterations from normal in

serum calcium concentration.

Addition information on calcitonin and calcium balance can be found in the

section Endocrine Control of Calcium and Phosphate Homeostasis.

Control of Calcitonin Secretion

The most prominent factor controlling calcitonin secretion is the

extracellular concentration of ionized calcium. Elevated blood calcium

levels strongly stimulate calcitonin secretion, and secretion is suppressed

when calcium concentration falls below normal. A number of other hormones

have been shown to stimulate calcitonin release in certain situations, and

nervous controls also have been demonstrated.

Disease States

A large number of diseases are associated with abnormally increased or

decreased levels of calcitonin, but pathologic effects of abnormal

calcitonin secretion per se are not generally recognized.

There are several therapeutic uses for calcitonin. It is used to treat

hypercalcemia resulting from a number of causes, and has been a valuable

therapy for Paget disease, which is a disorder in bone remodeling.

Calcitonin also appears to be a valuable aid in the management of certain

types of osteoporosis.

Advanced and Supplemental Topics

Endocrine Control of Calcium and Phosphate Homeostasis

Index of: Thyroid and Parathyroid Glands

Control of Thyroid Hormone Synthesis and SecretionParathyroid Hormone

http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/thyroid/calcitonin.htm\

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