21.10
The Thyroid Gland
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Q1: Where is the thyroid gland located and what is its basic structure?
The thyroid gland is a small, butterfly-shaped gland located in the neck, covering the anterior surface of the trachea. It consists of two lateral lobes connected by a thin tissue mass called the isthmus. Internally, each lobe contains many small spherical structures called thyroid follicles, which are surrounded by a network of blood vessels that supply the gland.
Q2: What are thyroid follicles and what cells line them?
Thyroid follicles are small spherical structures within each lobe of the thyroid gland. Each follicle has a central cavity lined by simple cuboidal to squamous epithelial cells called follicular cells. These follicular cells produce the glycoprotein thyroglobulin and store it in the central colloid cavity, which serves as a precursor for thyroid hormone production.
Q3: What are T3 and T4 hormones and what role do they play?
T3 (triiodothyronine) and T4 (thyroxine) are thyroid hormones produced from thyroglobulin, a glycoprotein stored in thyroid follicles. These hormones are essential for maintaining the body's metabolic rate and energy production. Understanding their synthesis and regulation is critical to comprehending thyroid function and metabolic control.
Q4: What is calcitonin and which cells produce it?
Calcitonin is a hormone produced by parafollicular cells, also called C cells, which are scattered between thyroid follicles. This hormone regulates calcium metabolism in the body, helping maintain proper calcium levels in the blood and supporting bone health through its effects on calcium homeostasis.
Q5: How does the pituitary gland control thyroid hormone secretion?
The thyroid gland is controlled by thyroid-stimulating hormone (TSH) produced by the pituitary gland. When thyroid hormone levels rise, they send an inhibitory signal back to the pituitary gland to stop secreting TSH. This negative feedback mechanism maintains hormonal balance and prevents excessive thyroid hormone production.
Q6: What is the isthmus and how does it connect the thyroid lobes?
The isthmus is a thin tissue mass that connects the two lateral lobes of the thyroid gland. This structure allows the butterfly-shaped gland to function as a unified organ while maintaining distinct left and right lobes. The isthmus is located anterior to the trachea in the neck region.
Q7: How do thyroid follicles and blood vessels work together?
Thyroid follicles are surrounded by an extensive network of blood vessels that supply oxygen and nutrients while removing hormones and metabolic waste. This rich vascularization enables efficient hormone synthesis, storage, and secretion into the bloodstream. The close relationship between follicles and blood vessels ensures rapid hormone distribution throughout the body.