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The Thyroid

Table of Contents
The Lungs
The Trachea
The Testes
The Submaxillary Gland
The Liver
The Stomach and Duodenum
The Blood
The Arteries and Veins
The Adipose Tissue
The Skin
The Pituitary Gland
The Pancreas
The Thyroid
The Kidney
The Spinal Cord
The Cerebellum
The Elastic Cartilage
The Bone
The Smooth Muscle
The Striated Muscle


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The thyroid is the gland found at the base of the neck, which releases hormones that regulate the metabolic processes of cells.


The thyroid gland secretes two main hormones that control the rate of bodily processes, thyroxine (T4) and triiodothyronine (T3). Both are iodine containing hormone that controls the rate at which glucose is used up to provide energy for the cells, which is also plays an important role in the growth and development of body systems. The only difference between the two is the extra iodine atom found in thyroxine, which is also is secreted more regularly than T3.


The release of thyroid hormones is regulated by interactions between the hypothalamus in the brain, the pituitary gland, the thyroid gland, and thyroid hormone levels in the body. When thyroid hormone levels are low, the hypothalamus is activated and sends signals to the pituitary gland to release thyroid stimulating hormone (TSH). In turn, the thyroid gland releases thyroid hormones until their levels in the body are normal. Once normal levels are reached, the hypothalamus’ detection is shut off, and the cycle is shut down.


People who live in areas where they can not receive sufficient iodine develop goiters. A goiter is an enlargement of the thyroid gland due to the overproduction if incomplete thyroid hormones. When thyroid hormone levels are low, the pituitary gland keeps sending TSH to the thyroid to produce T4 and T3, but the thyroid can not produce complete thyroid hormones because it lacks the iodine required to create them. Therefore, the hypothalamus never gets the feedback signal to stop the production of TSH, and the thyroid becomes enlarged in efforts to produce adequate amounts of thyroid hormone.


Overproduction of thyroid hormones is described as hyperthyroidism, characterized by high metabolic rates, rapid heart and respiratory rates, agitated behavior, and in some cases, protruding eyes.


Insufficient thyroid hormone production is called hypothyroidism, characterized by sluggishness, obesity, low body temperature and poor muscle tone.


The thyroid gland also produces calcitonin, which decreases calcium levels in the body by depositing it in the bones.

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