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Concept Version 6
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Thyroid Gland

The thyroid gland, the largest endocrine gland, is responsible for the production of the hormones T3, T4, and calcitonin.

Learning Objective

  • Describe the hormones produced by the thyroid and explain how their production is regulated


Key Points

    • The thyroid gland is made up of thyroid follicles, which produce three main hormones.
    • T3 and T4 hormones increase the metabolic activity of the body‘s cells while calcitonin helps regulate calcium concentrations in body fluids.
    • T3 and T4 release is controlled by thyroid stimulating hormone; however, calcitonin release is controlled by calcium ion concentrations.

Terms

  • triiodothyronine

    the most powerful thyroid hormone, affecting almost every process in the body, including body temperature, growth, and heart rate

  • thyrocalcitonin

    a hormone, secreted by parenchymal cells, that regulates calcium and phosphate metabolism

  • thyroxine

    a hormone (an iodine derivative of tyrosine), produced by the thyroid gland, that regulates cell metabolism and growth

  • thyroglobulin

    a globulin, produced by the thyroid gland, that has a role in the production of the thyroid hormones


Full Text

Thyroid Gland

The thyroid gland, one of the largest endocrine glands in the body, is located in the neck, just below the larynx and in front of the trachea . It is a butterfly-shaped gland with two lobes that are connected by the isthmus. It has a dark red color due to its extensive vascular system. When the thyroid swells due to dysfunction, it can be felt under the skin of the neck.

Thyroid gland

The location of the thyroid gland is in the neck below the larynx and in front of the trachea; it is the largest endocrine gland in the body, producing T3, T4, and calcitonin.

The thyroid gland is made up of many spherical thyroid follicles which are lined with a simple cuboidal epithelium. These follicles contain a viscous fluid, called colloid, which stores the glycoprotein thyroglobulin. This glycoprotein is the precursor to the thyroid hormones. The follicles produce hormones that can be stored in the colloid or released into the surrounding capillary network for transport to the rest of the body via the circulatory system.

The thyroid gland produces the hormones T3 (triiodothyronine) and T4 (thyroxine). These hormones increase the metabolic activity of the body‘s cells. Follicle cells are stimulated to release stored T3 and T4 by thyroid-stimulating hormone (TSH), which is produced by the anterior pituitary. These thyroid hormones increase the rates of mitochondrial ATP production.

Another hormone produced by the thyroid gland, thyrocalcitonin, or calcitonin, decreases the concentration of calcium in the blood. Most of the calcium removed from the blood is stored in the bones. Calcitonin is produced by parafollicular cells of the thyroid, either releasing hormones or inhibiting hormones. The hormone's release is not controlled by TSH, but instead is released when calcium ion concentrations in the blood rise. Calcitonin functions to help regulate calcium concentrations in body fluids. It acts in the bones to inhibit osteoclast activity and in the kidneys to stimulate excretion of calcium. The combination of these two events lowers body fluid levels of calcium.

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