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Introduction to the Thyroid Gland


The thyroid gland is one of the glands of the endocrine system which lies outside of a body cavity. It is a small gland weighing about 1 ounce (28 grams) and is located in the neck, just below the larynx. This gland secretes the hormones thyroxine (T4), triiodothyronine(T3), and calcitonin (thyrocalcitonin). Thyroxine affects the growth rate and metabolism of all of the body's cells. It controls reflexes and regulates the rate at which the body produces energy and transforms food into body components. This hormone causes cells to speed up the release of energy from foods as needed by stimulating catabolism (increasing the basal metabolism rate - BMR). All body functions depend upon the normal supply of energy, normal thyroid secretion is, therefore, vital to the body.

One of the main components of thoroxyne is iodine, an important body building agent. Babies cannot grow properly without iodine. In older people, iodine deficiency causes hair loss, slowed speech, and drying and thickening of the skin. An average adults iodine requirement is only about a millionth of an ounce (0.00003 grams) per day but it is vital to the delicate balance between health and sickness. Thyroxine also controls temperature. People with overactive thyroid glands tend to feel uncomfortably hot in cool conditions, while those whose thyroid gland is under-active tend to feel cool even on hot days. The other hormone produced by the thyroid gland, calcitonin, tends to decrease the amoumt of calcium in the blood, the opposite effect of parathormone from the parathyroid glands. Calcitonin functions to help maintain homeostasis of blood calcium. It prevents a harmful excess of calcium in the blood, called hypercemia, from developing.

The thyroid gland is somewhat heavier in females and becomes enlarged during menstration and pregnancy. The gland is divided into two lobes, a right lobe and a left lobe, each of which is about two inches in length. These lobes are connected by a bridge called the isthmus, which is about an inch in length and width. The isthmus connects the lower two-thirds of the lobes and covers the second and third rings of the trachea.

The pyramidal lobe, when it is present, has a conical shape and extends above the isthmus. The pyramidal lobe may also extend from the left lobe, and on a rare occaision from the right lobe. The pyramidal lobe varies in size from person to person and can ascend as high as the hyoid bone.

The inferior thyroid veins are a network of small branches of veins surrounding the thyroid gland. These veins drain blood from the thyroid and surrounding structures in front of the neck and return the blood to the internal jugular vein, also called the superior thyroid vein, which returns the blood back to the heart. Two to four small veins that drain the thyroid gland also unite to empty into the inominate vein.


This page is maintained by Alkam (alkam@fs1.scg.man.ac.uk)
Created: 11 November 1996, 23:42:35
Last Updated: 13 November 1996 - 10:13:13