The Function of the Thyroid and Parathyroid Gland

The thyroid follicles (the structural units of the thyroid gland) are responsible for the production and secretion of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). Iodine is necessary for the synthesis of these hormones because thyroxine contains four atoms of iodine while triiodothyronine contains three. Besides T4 and T3 a third hormone, calcitonin, is produced by the thyroid gland . However, it is secreted by parafollicular cells and its function varies largely from those of thyroxine and triiodothyronine.

Which hormones does the thyroid gland secrete? The functions of the hormones thyroxine (T4) and triiodothyronine (T3) are similar. They control metabolism by regulating energy production and protein synthesis, which contribute to physical growth of the body and normal body functioning, throughout life. Often the term “thyroxine” is used to describe both hormones, which probably results from their similar function. T3 and T4 increase cellular respiration, several reactions and processes that convert nutrients (e.g. carbohydrates, fats, and excess amino acids) into biochemical energy in the form of ATP. Thereby, energy and heat production are also increased. The hormones also increase the rate of protein synthesis within cells. Low levels of T3 and T4 lower your heart rate and you may gain weight. Hence , a balanced production of thyroxine is essential for physical growth, healthy mental development, and a mature reproductive system. Although, thyroid hormones are not crucial to survival (not labelled “vital hormones“), their absence greatly impairs physical and mental growth and abilities.

The secretion of T3 and T4 is stimulated by the thyroid-stimulating hormone (TSH; made in the anterior pituitary gland). This process of secretion by the anterior pituitary gland is in turn controlled by the thyrotropin-releasing hormone (TRH) of the hypothalamus. The hypothalamus secretes TRH when it detects a decrease in the metabolic rate (also including energy production). TRH now stimulates the anterior pituitary gland to release TSH which leads to the secretion of T4 and T3. This increases the metabolic rate by increasing energy production. Afterwards, this negative feedback mechanism shuts off TRH from the hypothalamus until the metabolic rate decreases again.

What is calcitonin? Calcitonin is also produced by the thyroid gland. It regulates calcium and phosphate metabolism, mainly by inhibiting osteoclast activity. This reduces the release of calcium and phosphate ions from the bones to the blood. Calcitonin thus lowers the Ca2+ and PO4- blood levels. This function of calcitonin regulates normal blood levels of calcium and phosphate and also helps in maintaining a stable, strong bone matrix.

The stimulus for calcitonin is hypercalcemia, a high blood calcium level, because then a decrease of calcium in the blood is needed. In this case calcitonin ensures that calcium reabsorption from the bones to the blood is reduced until the calcium in the blood is used up and calcium is needed in the blood again.

What is the function of the parathyroid glands and their hormone? There are 4 parathyroid glands in humans: 2 parathyroid glands on either lobe of the thyroid gland. The hormone they produce is the parathyroid hormone.

Parathyroid hormone (PTH) is an antagonist of calcitonin and maintains normal blood levels of calcium and phosphate. The bones, the small intestine , and the kidneys are the target organs of PTH.

PTH increases the osteoclast activity and, thus, the reabsorption of calcium and phosphate from the bones to the blood and thereby raises their blood levels. Not only reabsorption, but also the uptake of calcium and phosphate from food in the small intestine are increased by PTH. Likewise, this raises their blood levels. In the kidneys, PTH enhances the reabsorption of calcium and the excretion of phosphate (more than is obtained from the bones).

All in all, the effect of PTH is to raise the blood calcium level and lower the blood phosphate level. Secretion of PTH is stimulated by hypocalcemia (low blood calcium level).

Together, the hormones produced by the thyroid and parathyroid glands maintain blood calcium within a physiological range. Calcium in the blood is essential for blood clotting and for the healthy activity of neurons and muscle cells.

BIBLIOGRAPHY 1. Holt EH, Peery H: Basic Medical Endocrinology. 4th edition. Elsevier 2009. 2. http://dict.leo.org. 3. www.dict.cc. 4. www.webster-dictionary.org.

EXERCISE: 1. Where is PTH produced?

a. parathyroid gland b. thyroid gland c. kidneys d. pancreas

2. Which organs are the target organs für PTH?

a. bones b small intestine c. kidneys d. liver e. heart

3. What is the function of calcitonin?

a. hormone regulation b. regulation of calcium and phosphate metabolism c. regulation of adrenaline and noradrenaline

You‘ll find the answer on: http://tinyurl.com/weiterbildung-englisch

Die Autorin: Carmen Lobitz

Carmen Lobitz ist freie Trainerin, Lehrerin und MTA. Sie gründete 1998 in Berlin lobitz seminare, die unter anderem Englischseminare für Health Professionals anbieten.

Kontakt: mail@lobitz-seminare.de, www.lobitz-seminare.de

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Lösung für Exercise:

Answer: 1. a Answer: 2. a, b, c Answer: 3. b

The Function of the Thyroid and Parathyroid Gland

Which hormones does the thyroid gland secrete?

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The Function of the Thyroid and Parathyroid Gland

Which hormones does the thyroid gland secrete?

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