THYROID GLAND Structure and Function - PowerPoint Presentation

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THYROID GLAND

Structure and Function

Location and Structure The thyroid gland is the largest of all the endocrine glands, weighing from 20 to 30 grams in adult human being.

The

two lobes of the gland,

and the

isthmus

of tissue joining them, lie just below the larynx or voice box, just anterior to the upper portion of the \trachea or windpipe.

The gland is well supplied with blood, has connective tissue framework, and contains many microscopic cavities or vesicles.

The vesicles are lined with a single layer of cuboidal epithelial cells which are the actual secretory cells responsible for the production of the thyroid hormones.

The

hollow centre of each vesicle is filled with a gelatinous colloidal material made up mostly of a large complex glycoprotein called

thyroglobulin.

It is now known that the thyroid hormone consists of atleast four physiologically active iodinated organic compounds collectively called thyroid

hormones.

The most important and abundant of the four is called

thyroxine

or tetraiodothyronine (T4) which until recently was believed to be the only physiologically active component or hormone secreted by the gland.

In human beings, thyroid glands also secrete calcitonin, which is a large polypeptide with a molecular weight of about 3400 and a chain of 32 amino acids

.

The so-called

parafollicular cells, or C-cells, in the interstitial tissue between the follicles of the human thyroid gland which constitute only 0.1 percent of the thyroid gland, secrete

calcitonin

.

Thyroid secretion is controlled primarily by thyroid-stimulating hormone (TSH) secreted by the adenohypophysis.

The structure of

thyroxine

(C

15H11O4I4N) is the iodinated derivative of the essential amino acid

L-tyrosine

.

Being an amino acid, thyroxine is not digested in the gut, and it can, therefore, be taken by mouth.

Functions of the Thyroid Hormones1. Increased Metabolic Activity

:

The thyroid hormones stimulate the rate of oxygen consumption or

oxidative metabolism

and resulting heat production (calorigenesis) of the various cell and tissues of the body. Thyroid hormone, in the homeotherms, stimulates many phases of carbohydrate metabolism, from the absorption of sugars to their utilization in the tissues.

When thyroxine is given to an animal, the mitochondria in most cells of the body increase in size and number.

Therefore

, one of the principal functions of thyroxine might be simply to increase the number and activity of mitochondria, and they in turn increase the ATP formation to energize cellular function.

Thyroid hormone also increases active transport of joins through the cell memberanes. One of the enzymes that is increases in response to thyroid hormone is Na, K-ATPase which in turn increases the rate of transport of both Na and K through the cell membrances of some tissues.

Because this process uses energy and increases the metabolic rate of the body, increased metabolism in the tissues affects blood flow and cardiac output and also affects respiration function of muscles and sleep

.

2. Regulatory Role:

Thyroid gland also has a second important function, namely a

regulatory role (which it performs by its effects on the cellular metabolism) in general process of growth and development of all cells and tissues.

These include the sexual development, maturation of bones and teeth, mental development, and energy metabolism, etc.

thyroid deprivation results in reduced skeletal and somatic growth, retarded sexual development, mental deficiency, low basal metabolic rate, and increased risk of infection.

3. Effect on Other Endocrine Glands:Increased thyroid secretion increases the rate of secretion of most other endocrine glands, but it also increases the need of the tissues for hormones.

For

instance, increased thyroxine secretion increases the rate of glucose metabolism in the body and therefore a corresponding need for increased insulin secretion by the pancreas.

Thyroid hormone also increases many metabolic activities related to bone formation and, as a consequence, increases the need for parathyroid hormone.

Thyroid hormone increases the rate at which adrenal glucocorticoid is inactivated by the liver. This leads to feedback increase in adrenocorticotropic hormone production by the adenohypophysis, and therefore, increases rate of secretion of glucocorticoid by the adrenal gland.

Effect of Calcitonin on Blood Calcium:Calcitonin, secreted by thyroid gland in human beings, tends to reduce the blood calcium ion concentration (an effect which is opposite to that of parathyroid hormone).

However, it should be noted that calcitonin mechanism acts only weakly and only as a short-term regulator of

Ca

ion concentration because it is rapidly dominated by the much more powerful parathyroid control mechanism.

Effect on development:In these vertebrates the hormone is specifically required in the

metamorphosis.

If thyroid function in an embryo or a young tadpole is in some way inhibited (e.g., by excising the thyroid gland), then the animal remains a tadpole permanently.

I

f a young tadpole is given an excess of thyroxine, then the larva metamorphoses prematurely into a tiny froglet.

Hypothyroidism. :The phenomenon of

under-secretion by the thyroid gland

is called

hypothyroidism.

The occurrence of hypothyroidism (which may by due to a variety of factors including atrophy of the gland, or deficient dietary supply of iodine, etc.) in Infancy it is called as Cretinism characerised by lack of skeletal development resulting in deformation of bones, abnormal development of Nervous system, large protruding tongue, slow pulse rate, Sex organs and secondary sexual characters becomes reduced

.

In

adults

produces the condition called

myxedema (Gull’s disease),

a disease characterized by a low metabolic rate, increased plasma cholesterol, thickness and puffiness of the skin, coarseness and brittleness of the hair and finger nails, and a general physical and mental lethargy.

There is usually a weight-gain, loss of hair, slower pulse rate, reduced blood pressure, and a decreased body temperature. The administration of adequate dose of the thyroid extracts quickly restores normal function to individuals with myxedema.

Cretinism:When hypothyroidism is present from the birth,

the condition is called

cretinism

.

It results from congenital lack of a thyroid gland (congenital cretinism), from failure of the thyroid gland to produce thyroid hormone, or from iodine deficiency in the diet (endemic cretinism).

Cretinism

is characterized by retarded physical, sexual and mental development and low metabolic rate. Such children are stunted and of low intelligence

.

An early administration of the thyroid hormones by mouth or injection is necessary to prevent permanent effects of cretinism.

Goitre:Thyroxine deficiency may be accompanied by thyroid

enlargement

or goiter. Here the number of thyroid cells increases and more hormones may then be secreted by the enlarged gland.

One of the causes of simple goiter is dietary deficiency of iodine and so the addition of iodine to certain foods (e.g. table salt) and drinking water, or the administration of adequate amounts of thyroxine from an external source reduces its incidence.

Hyperthyroidism:The phenomenon of over secretion by the thyroid gland is called hyperthyroidism or

thyrotoxicosis

. It is accompanied by physiological effects and symptoms which are the opposite of those observed in hypothyroidism.

One of the most severe conditions of hyperthyroidism is called

Graves’ disease or Exophthalmic goiter. It is characterized by an enlargement of the entire gland, an elevated basal metabolic rate, weight-loss, profuse perspiration, higher pulse rate, and a typical protrusion of the eyeballs.

The condition may be treated by surgical removal of a portion of the thyroid gland.

A

dministration of

radioactive iodine which accumulates in the thyroid destroying some of its tissues. use of certain thyroid-inhibiting drugs which interfere with the synthesis of thyroid hormones.