The Thyroid and Parathyroids

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The thyroid gland is a double-lobed structure located in the neck. Embedded in its rear surface are the four parathyroid glands.
Link to graphic showing the location of the thyroid, parathyroids, and other endocrine glands (92K).

The Thyroid Gland

The thyroid gland synthesizes and secretes:

T4 and T3

Both hormones are derivatives of the amino acid tyrosine with four atoms of iodine in T4 , three in T3. The thyroid secretes mainly (80%) T4 , but when T4 enters target cells, one atom of iodine is removed from it converting it into T3. T3 is the more potent of the two hormones. It has many effects. Among the most prominent of these are: The thyroid cells responsible for the synthesis of T4 and T3 take up circulating iodine from the blood and attach them to tyrosine residues in the protein thyroglobulin. This action, as well as the synthesis of the hormones, is stimulated by the binding of thyroid stimulating hormone (TSH; also known as thyrotropin) to transmembrane receptors at the cell surface.

Diseases of the thyroid

1. hypothyroid diseases; caused by inadequate production of T3

2. hyperthyroid diseases; caused by excessive secretion of thyroid hormones

Graves´ disease. Autoantibodies against the TSH receptor bind to the receptor mimicking the effect of TSH binding. Result: excessive production of thyroid hormones. Graves´ disease is an example of an autoimmune disease.

Osteoporosis. High levels of thyroid hormones suppress the production of TSH through the negative-feedback mechanism mentioned above. The resulting low level of TSH causes an increase in the numbers of bone-reabsorbing osteoclasts resulting in osteoporosis.


Calcitonin

Calcitonin is a polypeptide of 32 amino acids. The thyroid cells in which it is synthesized have receptors that bind calcium ions (Ca2+) circulating in the blood. A rise in its level, such as would occur with the absorption of calcium from a meal, stimulates the cells to release calcitonin. Calcitonin prevents a sharp rise in blood calcium by

Because it slows the loss of Ca2+ from bones, calcitonin has been examined as a possible treatment for osteoporosis, a weakening of the bones that is a leading cause of hip and other bone fractures in the elderly. Being a polypeptide, calcitonin cannot be given by mouth (it would be digested), and giving by injection is not appealing. However, inhaling calcitonin appears to be an effective way to get therapeutic levels of the hormone into the blood. A synthetic version of calcitonin (trade name = Miacalcin) is now available as a nasal spray.


The Parathyroid Glands

The parathyroid glands are 4 tiny structures embedded in the rear surface of the thyroid gland. They secrete parathyroid hormone (PTH) a polypeptide of 84 amino acids. PTH increases the concentration of Ca2+ in the blood in three ways. PTH promotes PTH also regulates the level of phosphate in the blood. Secretion of PTH reduces the efficiency with which phosphate is reclaimed in the proximal tubules of the kidney causing a drop in the phosphate concentration of the blood.

Control of the Parathyroids: the calcium receptor

The cells of the parathyroid glands have surface G-protein-coupled receptors that bind Ca2+ (the same type of receptor is found on the calcitonin-secreting cells of the thyroid and on the calcium absorbing cells of the kidneys). Binding of Ca2+ to this receptor depresses the secretion of PTH and thus leads to a lowering of the concentration of Ca2+ in the blood. Two classes of inherited disorders involving mutant genes encoding the Ca2+ receptor occur: Rare autoimmune disorders can mimic one or the other of these inherited disorders. In each case, autoantibodies bind to the receptors.

Hyperparathyroidism

Tumors in the parathyroids elevate the level of PTH causing a rise in the level of blood Ca2+ at the expense of calcium stores in the bones. So much calcium may be withdrawn from the bones that they become brittle and break.

Until recently, treatment has been the removal of most — but not all — of the parathyroid tissue (i.e. the goal is the removal of 3 1/2 glands). Now clinical trials have begun on a drug (designated R-568) that mimics the action of calcium on the parathyroids, resulting in a drop in PTH and blood Ca2+ and sparing the calcium stores in the bone.

Hypoparathyroidism

Causes: Treatment:
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12 May 2011