Assignment 1: Description of PTH

Parathyroid Hormone

Parathyroid hormone, in association with calcitonin and vitamin D, is responsible for regulating calcium metabolism in the body (Griffin & Ojeda, 2004). In adults, the primary action of PTH is to raise serum calcium levels when the body senses a low concentration of ionized calcium in the blood. To induce a response that raises extracellular calcium levels, PTH acts on bone, kidneys and intestine (Many, 2005) in the following ways:

(i) Stimulating specific bone cells that release stored calcium
(ii) Increasing calcium absorption from the gut in a pathway involving vitamin D (1,25-(OH)2 D)
(iii) Acting on the kidney to enhance the calcium being reabsorbed from the urine (Hadley & Levine, 2006).

The parathyroid glands, which secrete parathyroid hormone,

are small pea-sized glands found posterior to the thyroid gland (Griffin & Ojeda, 2004). Four of these glands are normally found, but a small percentage of the population can have as few as two or as many as eight. The parathyroid gland produces PTH by first synthesizing pre-pro-PTH which is cleaved to pro-PTH and then to PTH, an 84-amino acid polypeptide. The hormone is secreted to the bloodstream directly after it is produced and has a half-life of less than 5 minutes in the serum.

The amount of PTH produced depends on the ionized calcium concentration of the blood and is regulated by a negative feedback mechanism (Griffin & Ojeda, 2004). In another feedback loop, the amount of vitamin D directly inhibits PTH secretion. These feedback loops are important to protect the individual from hypercalcemia. Due to these processes, the calcium concentration of blood is kept within a narrow range optimal for proper functioning (Hadley & Levine, 2006).

References:

Griffin, J. E., Ojeda, S. R. 2004. Textbook of Endocrine Physiology. Oxford University Press, New York.

Hadley, M. E., Levine, J. E. 2006. Endocrinology, 6th Ed. Pearson Prentice Hall, New Jersey.

Many, T. N. 2005. Molecular Biology of the Parathyroid. Kluwer Academic/Plenum Publishers, New York.