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THYROID GLAND

 
The thyroid is a dual lobed gland located at the front of the neck, just beneath the larynx (voice box). One lobe of the thyroid is located on each side of the trachea (windpipe). The two lobes of the thyroid gland are connected by a narrow strip of tissue known as the isthmus. As a component of the endocrine system, the thyroid secretes hormones that control important functions including metabolism, growth, heart rate, and body temperature. Found within thyroid tissue are structures known as parathyroid glands. These tiny glands secrete parathyroid hormone, which regulates calcium levels in the blood.

Thyroid Structure

The thyroid is highly vascular, meaning that it has a wealth of blood vessels.javascript:void(0); It is composed of follicles that absorb iodine, which is needed to produce thyroid hormones. These follicles store iodine and other substances necessary for hormone production. Surrounding the follicles are folliclar cells. These cells produce and secrete thyroid hormones into circulation via blood vessels. The thyroid also contains cells known as parafollicular cells. These cells are responsible for the production and secretion of the hormone calcitonin.

Thyroid Hormones

The thyroid gland produces the hormones thyroxine, triiodothyronine, and calcitonin. Thyroxine and triiodothyronine are produced by thyroid folliclar cells. Thyroid cells absorb iodine from certain foods and combine the iodine with tyrosine, an amino acid, to make thyroxine (T4) and triiodothyronine (T3). The hormone T4 has four atoms of iodine, while T3 has three atoms of iodine. T4 and T3 regulate metabolism, growth, heart rate, body temperature, and affect protein synthesis. The hormone calcitonin is produced by thyroid parafollicular cells. Calcitonin helps to regulate calcium concentrations by lowering blood calcium levels when the levels are high.

Thyroid Hormone Regulation

Thyroid hormones T4 and T3 are regulated by the pituitary gland. This small endocrine gland is located in the middle of the base of the brain. It controls a multitude of important functions in the body. The pituitary gland is termed the "Master Gland" because it directs other organs and endocrine glands to suppress or induce hormone production. One of the many hormones produced by the pituitary gland is thyroid stimulating hormone (TSH). When levels of T4 and T3 are too low, TSH is secreted to stimulate the thyroid to produce more thyroid hormones. As levels of T4 and T3 rise and enter the blood stream, the pituitary senses the increase and reduces its production of TSH. This type of regulation is an example of a negative feedback mechanism. The pituitary gland is itself regulated by the hypothalamus. Blood vessel connections between the hypothalamus and pituitary gland allow hypothalamic hormones to control pituitary hormone secretion. The hypothalamus produces thyrotropin-releasing hormone (TRH). This hormone stimulates the pituitary to release TSH.

Thyroid Problems

When the thyroid gland is not functioning properly, several thyroid disorders may develop. These disorders can range from a slightly enlarged gland to thyroid cancer. An iodine deficiency may cause the thyroid to become enlarged. An enlarged thyroid gland is referred to as a goiter. When the thyroid produces hormones in excess of the normal amount, it causes a condition called hyperthyroidism. When the thyroid does not produce enough thyroid hormone, hypothyroidism occurs. Many instances of hyperthyroidism and hypothyroidism are caused by autoimmune thyroid diseases. In autoimmune disease, the immune system attacks the body's own normal tissues and cells. Autoimmune thyroid diseases can cause the thyroid to become overactive or to stop producing hormones entirely.

Parathyroid Glands

Parathyroid glands are small tissue masses located on the posterior side of the thyroid. These glands vary in number, but typically two or more may be found in the thyroid. Parathyroid glands contain many cells that secrete hormones and have access to extensive blood capillary systems. Parathyroid glands produce and secrete parathyroid hormone. This hormone helps to regulate calcium concentrations by increasing blood calcium levels when these levels dip below normal. Parathyroid hormone counteracts calcitonin, which decreases blood calcium levels. Parathyroid hormone increases calcium levels by promoting the break down of bone to release calcium, by increasing calcium absorption in the digestive system, and by increasing calcium absorption by the kidneys. Calcium ion regulation is vital to the proper functioning of organ systems such as the nervous system and muscular system.
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THYROID GLAND





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