thyroid gland
DEFINITION:
An important organ of the endocrine system. The thyroid gland is situated in the front of the neck, just below the larynx (voice box). It consists of two lobes, one on each side of the trachea (windpipe), joined by a narrower portion of tissue called the isthmus.
STRUCTURE:
Thyroid tissue is composed of two types of secretory cells---follicular cells and parafollicular cells (or C cells). Follicular cells make up most of the gland. They are arranged in the form of hollow, spherical follicles, and secrete the iodine-containing hormones thyroxine (T4) and triiodothyronine (T3). The space inside the follicles is filled with a semifluid, colloid material that is essential for the production of T4 and T3.
Parafollicular cells occur singly or in small groups in the spaces between the follicles. They secrete the hormone calcitonin. Also between the follicles are numerous blood capillaries, small lymphatic vessels, and connective tissue. The entire thyroid gland is encased in a thin outer layer of connective tissue.
FUNCTION:
T4 and T3 play an important role in controlling body metabolism. Calcitonin acts in conjunction with parathyroid hormone to regulate calcium balance in the body.
DISORDERS:
The function of the thyroid gland is controlled by both the pituitary gland and the hypothalamus, so thyroid disorders may be due not only to defects in the gland itself, but also to disruption of the hypothalamic-pituitary hormonal control system. Thyroid disorders may cause symptoms due to overproduction of thyroid hormones (hyperthyroidism), underproduction of these hormones (hypothyroidism), or enlargement or distortion of the gland. Idiopathic (of unknown cause) goiter (enlargement of the gland), Graves' disease, and Hashimoto's thyroiditis are the common disorders of thyroid function. Goiter sometimes occurs without any accompanying abnormality of the thyroid function.
Other disorders include congenital defects (either a missing, underdeveloped, or maldeveloped thyroid gland), genetic disorders, infection (thyroiditis), tumors, autoimmune disorders, myxedema, hormonal disorders, nutritional disorders, and post-irradiation induced tumors.
thyroid gland
DEFINITION:
An important organ of the endocrine system. The thyroid gland is situated in the front of the neck, just below the larynx (voice box). It consists of two lobes, one on each side of the trachea (windpipe), joined by a narrower portion of tissue called the isthmus.
STRUCTURE:
Thyroid tissue is composed of two types of secretory cells---follicular cells and parafollicular cells (or C cells). Follicular cells make up most of the gland. They are arranged in the form of hollow, spherical follicles, and secrete the iodine-containing hormones thyroxine (T4) and triiodothyronine (T3). The space inside the follicles is filled with a semifluid, colloid material that is essential for the production of T4 and T3.
Parafollicular cells occur singly or in small groups in the spaces between the follicles. They secrete the hormone calcitonin. Also between the follicles are numerous blood capillaries, small lymphatic vessels, and connective tissue. The entire thyroid gland is encased in a thin outer layer of connective tissue.
FUNCTION:
T4 and T3 play an important role in controlling body metabolism. Calcitonin acts in conjunction with parathyroid hormone to regulate calcium balance in the body.
DISORDERS:
The function of the thyroid gland is controlled by both the pituitary gland and the hypothalamus, so thyroid disorders may be due not only to defects in the gland itself, but also to disruption of the hypothalamic-pituitary hormonal control system. Thyroid disorders may cause symptoms due to overproduction of thyroid hormones (hyperthyroidism), underproduction of these hormones (hypothyroidism), or enlargement or distortion of the gland. Idiopathic (of unknown cause) goiter (enlargement of the gland), Graves' disease, and Hashimoto's thyroiditis are the common disorders of thyroid function. Goiter sometimes occurs without any accompanying abnormality of the thyroid function.
Other disorders include congenital defects (either a missing, underdeveloped, or maldeveloped thyroid gland), genetic disorders, infection (thyroiditis), tumors, autoimmune disorders, myxedema, hormonal disorders, nutritional disorders, and post-irradiation induced tumors.
iodine
DEFINITION:
1. A halogen element of a peculiar odor and acrid taste; symbol, I; atomic number, 53; atomic weight, 126.904. It is a nonmetallic element, occurring in heavy, grayish black plates or granules. Iodine is essential in nutrition, being especially necessary for the synthesis of thyroid hormones (thyroxine and triiodothyronine), which regulate the metabolic rate in all cells.
2. [USP] A preparation of iodine used as a topical anti-infective. Iodine, usually in the form of iodides, is used in the treatment of hyperthyroidism.
iodine 123 - a radioactive isotope of iodine, atomic mass 123, having a half-life of 13.2 hours; it decays by electron capture, emitting gamma rays (0.159 MeV) and x-rays. It is used as a tracer in diagnostic imaging and as a radiation source in radiation therapy.
iodine 125 - A radioisotope of iodine, atomic mass 125, having a half-life of 60.14 days and emitting gamma rays (0.035 MeV); used as a radioactive tracer, particularly as a label in radioimmunoassays and other in vitro tests, and also for thyroid imaging.
iodine 131 - A radioactive isotope of iodine, atomic mass 131, having a half-life of 8.04 days; it emits beta particles (0.607, 0.81, 0.336 MeV) and gamma rays (0.080, 0.284, 0.364, 0.637, 0.723 MeV) and is used as a tracer in diagnostic imaging and as a radiation source in radiation therapy.