Thyroid Gland and Hormone Structure Function

The thyroid gland, is a very vascular structure that consists of two large lobes connected by a broad isthmus. It is located just below the larynx on either side and in front of the trachea. It has a special ability to remove iodine from the blood.

Structure of the Gland

The thyroid gland is covered by a capsule of connective tissue and is made up of many secretory parts called follicles. The cavities of the follicles are lined with a single layer of Cuboidal epithelial cells and are filled with a clear, viscous Glycoprotein called colloid. The follicle cells produce and secrete hormones that may be stored in the colloid or released into the blood of nearby capillaries.

Structure of the Gland

Thyroid Hormones and Their Functions

The thyroid gland produces several hormones that have marked effects on the metabolic rates of most body cells and one hormone that influences the level of blood calcium. Of the hormones that affect metabolic rates, the most important are thyroxine and triiodothyronine. 

Thyroxine and Triodothyronine

They act to increase the rate of energy release from carbohydrates and the rate of protein synthesis. They also accelerate growth in young persons and stimulate activities of the nervous system. As was explained earlier, the release of these hormones is controlled by the hypothalamus and pituitary gland.

Thyroid Gland

Before follicle cells can produce thyroxine and triiodothyronine, they must be supplied with iodine salts (iodides). Such salts are normally obtained from foods, and after they have been absorbed from the intestine, they are carried by the blood to the thyroid gland. 

An efficient active transport mechanism called the iodine pump moves the iodides into the follicle cells, where they are used together with an amino acid (tyrosine) in the synthesis of the hormones Follicle cells also secrete the substance called thyroglobulin, which is the main ingredient of thyroid colloid. 

Thvroglobulin is used to store thyroid hormones whenever they are produced in excess. The stored hormones are bonded to the thyroglobulin until the hormone concentration of the body fluids drops below a certain level; then enzymes cause the hormones to be released from the colloid, and they diffuse into the blood. 

Once they are in the blood, thyroid hormones combine with blood proteins (alpha globulins) and are transported to body cells in this form.

Although triiodothyronine is nearly five times more potent, thyroxine accounts for at least 95% of the circulating thyroid hormone.

The thyroid hormone that influences blood calcium levels is a polypeptide called calcitonin.

Calcitonin

This substance helps regulate the calcium level by inhibiting the rate at which calcium leaves the bones and enters the extracellular fluids.

This is accomplished by decreasing the bone resorbing activity of osteoclasts. At the same time, calcitonin causes an increase in the rate of calcium deposit in bone matrix by stimulating the activity of osteoblasts.

Thus, calcitonin acts to lower the concentration of blood calcium—an effect exactly opposite that promoted by parathyroid hormone. 

The secretion of calcitonin is thought to be controlled directly by the blood calcium level. As this level increases, so does the secretion of calcitonin.

Following chart reviews the actions and controls of the thyroid hormones -

Actions and controls of the thyroid hormones

Growth Hormone

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Growth hormone Notes Summary

Growth hormone (GH or somatotropin) is a protein that generally stimulates body cells to increase in size and undergo more rapid cell division than usual. How GH accomplishes this action is not completely understood. It is known, however, that the hormone enhances the movement of amino acids through cell membranes and causes an increase in the rate at which cells convert these molecules into proteins.

GH also causes cells to decrease the rate at which they utilize carbohydrates and to increase the rate at which they use fats. The hormone's effect on amino acids seems to be the more important one.

Although the exact mechanism for controlling growth hormone secretion is unknown, it appears to involve two substances from the hypothalamus called growth hormone-releasingfactor (GRF) and growth hormone release-inhibiting factor (GIH or somatostatin).

A person's nutritional state also seems to play a role in the control of GH, for more of it is released during periods of protein deficiency and of abnormally low blood glucose concentration. Conversely, when blood protein and glucose levels are increased, there is a resulting decrease in growth hormone secretion. 

Apparently the hypothalamus is able to sense changes in the concentrations of certain blood nutrients, and it releases GRF in response to some of them.

If growth hormone is not secreted in sufficient amounts during childhood, body growth is limited, and a type of dwarfism (hypopituitary dwarfism) results. In this condition, body parts are usually correctly proportioned and mental development is normal. However, an abnormally low secretion of growth hormone is usually accompanied by lessened secretions from other anterior lobe hormones, leading to additional hormone deficiency symptoms. For example, a hypopituitary dwarf often fails to develop adult sexual features unless hormone therapy is provided.

Hypopituitary dwarfism is sometimes treated by administering growth hormone, and this treatment may stimulate a rapid increase in height. The procedure, however, must be started before the epiphyseal disks of the person's long bones have become ossified. Otherwise growth in height is not possible.

An over secretion of growth hormone during childhood may result in gigantism— a condition in which the person's height may exceed 8 feet. Gigantism, which is relatively rare, is usually accompanied by a tumor of the pituitary gland. In such cases, various pituitary hormones in addition to GH are likely to be secreted excessively, so that a giant often suffers from a variety of metabolic disturbances and has a shortened life expectancy. 

Gigantism

If growth hormone is secreted excessively in an adult, after the epiphyses of the long bones have ossified, the person does not grow taller. The soft tissues, however, may continue to enlarge and the bones may become thicker. As a consequence, an affected individual may develop greatly enlarged hands and feet, a protruding jaw, and a large tongue and nose. This condition is called acromegaly, and like gigantism, it is often associated with a pituitary tumor.

Acromegaly patient 

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