17.4 The Thyroid Gland
3 min read•june 18, 2024
The , a butterfly-shaped organ in the neck, plays a crucial role in regulating metabolism. It produces hormones that affect nearly every bodily function, from heart rate to body temperature. Understanding its structure and function is key to grasping how it impacts overall health.
Thyroid hormones and are produced through a complex process involving . These hormones influence metabolism, growth, and development throughout the body. The gland's function is tightly regulated by the , ensuring proper hormone levels are maintained.
Thyroid Gland Anatomy and Function
Structure of thyroid gland
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- Location anterior to the trachea and inferior to the larynx at the level of C5 to T1 vertebrae
- Butterfly-shaped gland consists of two lateral lobes connected by an
- Lobes approximately 5 cm long, 2.5 cm wide, and 20-30 g in weight
- Thin band of tissue called the isthmus connects the lobes
- Blood supply provided by superior thyroid arteries from the external carotid arteries and inferior thyroid arteries from the thyrocervical trunk
- Venous drainage occurs through superior, middle, and inferior thyroid veins
- Innervated by the autonomic nervous system including both sympathetic and parasympathetic fibers
- Histologically composed of follicles filled with colloid surrounded by () with () found between follicles
- can be used to assess gland size, structure, and detect abnormalities
Production of T3 and T4 hormones
- begins with follicular cells trapping iodide (I-) from the bloodstream using the (NIS)
- (TPO) oxidizes iodide and incorporates it into tyrosine residues of (Tg) to form (MIT) and (DIT)
- MIT and DIT combine within the thyroglobulin molecule to form T3 () and T4 ()
- T3 and T4 are stored within the colloid as part of thyroglobulin until from the anterior pituitary stimulates follicular cells to endocytose thyroglobulin
- Lysosomes in follicular cells break down thyroglobulin, releasing T3 and T4 into the bloodstream
- T3 and T4 are transported in the blood primarily bound to
- Regulation occurs through the hypothalamic-pituitary-thyroid axis:
- Hypothalamus secretes (TRH)
- TRH stimulates the anterior pituitary to secrete TSH
- TSH stimulates the thyroid gland to produce and release T3 and T4
- Negative feedback occurs when high levels of T3 and T4 inhibit TRH and TSH secretion
- enzymes in peripheral tissues convert T4 to the more active T3
Thyroid hormones in metabolism regulation
- Increase basal metabolic rate (BMR) by stimulating carbohydrate, protein, and lipid metabolism and increasing oxygen consumption and heat production
- Cardiovascular effects include increased heart rate, cardiac output, blood flow and vasodilation
- Essential for normal growth and development, especially critical for brain development and maturation
- Other effects include:
- Increased intestinal motility and nutrient absorption
- Enhanced muscle contraction and relaxation
- Stimulation of bone remodeling and growth
- Increased sensitivity to catecholamines (epinephrine and norepinephrine)
Calcitonin's role from parafollicular cells
- Parafollicular cells (C cells) are neuroendocrine cells located between thyroid follicles derived from the neural crest during embryonic development
- Secrete in response to elevated blood calcium levels detected by calcium-sensing receptors
- Functions to lower blood calcium levels by:
- Inhibiting osteoclast activity and bone resorption
- Promoting calcium excretion by the kidneys
- Opposing the effects of parathyroid hormone (PTH) on calcium homeostasis
- Not essential for calcium homeostasis in humans but may serve as a marker for (MTC) arising from parafollicular cells
Thyroid Disorders
- : Enlargement of the thyroid gland, often due to iodine deficiency or autoimmune conditions
- : Underactive thyroid gland leading to decreased metabolism and various systemic effects
- : Life-threatening condition characterized by severe thyrotoxicosis, typically occurring in untreated
Key Terms to Review (43)
C Cells: C cells, also known as parafollicular cells, are a type of thyroid gland cell that produce the hormone calcitonin. These cells play a crucial role in the regulation of calcium and phosphate homeostasis within the body.
Calcitonin: Calcitonin is a hormone produced by the thyroid gland that helps regulate calcium levels in the blood by lowering them when they are high. It acts by inhibiting the activity of osteoclasts, cells that break down bone, thereby reducing the release of calcium from bones.
Calcitonin: Calcitonin is a hormone produced by the thyroid gland that plays a crucial role in regulating calcium homeostasis within the body. It functions to lower blood calcium levels by inhibiting the activity of osteoclasts, the cells responsible for bone resorption, and promoting the deposition of calcium into bone tissue.
Deiodinase: Deiodinase is an enzyme that plays a crucial role in the metabolism of thyroid hormones by removing iodine atoms from these molecules. This process is vital for converting thyroxine (T4), which is less active, into the more active form, triiodothyronine (T3). The regulation of deiodinase activity is essential for maintaining appropriate hormone levels that affect metabolism, growth, and development.
Diiodotyrosine: Diiodotyrosine is an iodinated derivative of the amino acid tyrosine, crucial for the synthesis of thyroid hormones. It plays a vital role in the production of thyroxine (T4) and triiodothyronine (T3), as it is formed when two iodine atoms are attached to the tyrosine molecule, which occurs in the thyroid gland during hormone synthesis. Understanding diiodotyrosine is essential to grasp how thyroid hormones are produced and their functions in regulating metabolism and growth.
Follicular Cells: Follicular cells, also known as thyroid follicular cells or thyroid epithelial cells, are the predominant cell type found in the thyroid gland. These cells form the functional units of the thyroid, known as thyroid follicles, and are responsible for the production and secretion of thyroid hormones.
Goiter: A goiter is an abnormal enlargement of the thyroid gland, which can lead to a noticeable swelling in the neck. This condition can result from iodine deficiency, autoimmune diseases, or thyroid tumors.
Goiter: A goiter is an abnormal enlargement of the thyroid gland, which is located at the base of the neck and plays a critical role in regulating metabolism through hormone production. This condition can result from various factors, including iodine deficiency, autoimmune disorders like Hashimoto's thyroiditis, or the presence of thyroid nodules. Goiters may vary in size and can cause symptoms such as difficulty swallowing or breathing if they become large enough to compress surrounding structures.
Hyperthyroidism: Hyperthyroidism is a condition where the thyroid gland produces an excessive amount of thyroid hormones, leading to a speeding up of the body's metabolism. Symptoms can include rapid heartbeat, weight loss, and increased anxiety.
Hyperthyroidism: Hyperthyroidism is a condition in which the thyroid gland produces an excessive amount of thyroid hormones, leading to an overactive metabolism. This condition is closely related to the topics covered in Chapter 17.4, which discusses the structure and function of the thyroid gland.
Hypothalamic-pituitary-thyroid axis: The hypothalamic-pituitary-thyroid axis is a complex set of interactions between the hypothalamus, pituitary gland, and thyroid gland that regulates metabolism and energy balance in the body. This axis starts with the hypothalamus releasing thyrotropin-releasing hormone (TRH), which stimulates the pituitary gland to secrete thyroid-stimulating hormone (TSH), prompting the thyroid gland to produce thyroid hormones T3 (triiodothyronine) and T4 (thyroxine). These hormones play a crucial role in controlling metabolic processes and influencing growth, development, and overall homeostasis.
Hypothyroidism: Hypothyroidism is a condition where the thyroid gland does not produce enough thyroid hormones to meet the body's needs. This deficiency can slow down metabolic processes, leading to various health issues.
Hypothyroidism: Hypothyroidism is a condition in which the thyroid gland does not produce enough thyroid hormones, leading to a slowing of the body\'s metabolic processes. This hormonal imbalance can have significant impacts on an individual\'s overall health and development, particularly in the context of the thyroid gland\'s function and the aging of the endocrine system.
Iodine: Iodine is a essential trace mineral that plays a crucial role in the proper functioning of the thyroid gland. It is a key component of the thyroid hormones thyroxine (T4) and triiodothyronine (T3), which regulate metabolism, growth, and development throughout the body.
Iodine uptake: Iodine uptake refers to the process by which the thyroid gland absorbs iodine from the bloodstream to synthesize thyroid hormones, mainly thyroxine (T4) and triiodothyronine (T3). This process is crucial for regulating metabolism, growth, and development in the body, as thyroid hormones play a key role in energy expenditure and the overall functioning of various physiological systems.
Isthmus: An isthmus is a narrow strip of land that connects two larger landmasses and separates two bodies of water. It plays a crucial role in the anatomy and physiology of the thyroid gland, which is the focus of topic 17.4 in the course.
Medullary Thyroid Cancer: Medullary thyroid cancer is a rare type of thyroid cancer that originates from the C cells of the thyroid gland, which are responsible for producing the hormone calcitonin. This form of thyroid cancer is distinct from the more common types, such as papillary and follicular thyroid cancers, which develop from the thyroid follicular cells.
Monoiodotyrosine: Monoiodotyrosine is an amino acid derivative formed by the iodination of the amino acid tyrosine, specifically incorporating one iodine atom. It plays a crucial role in the synthesis of thyroid hormones, particularly in the production of thyroxine (T4) and triiodothyronine (T3), which are essential for regulating metabolism and various physiological processes in the body.
Neonatal hypothyroidism: Neonatal hypothyroidism is a condition in newborns where the thyroid gland is underactive and does not produce enough thyroid hormones, crucial for growth and brain development. Early diagnosis and treatment are essential to prevent intellectual disability and growth failure.
Parafollicular Cells: Parafollicular cells, also known as C-cells, are a type of specialized endocrine cells located within the thyroid gland. They are responsible for the production and secretion of the hormone calcitonin, which plays a crucial role in the regulation of calcium homeostasis in the body.
Radioactive Iodine Uptake: Radioactive iodine uptake (RAIU) is a diagnostic test that measures the amount of radioactive iodine absorbed by the thyroid gland. It is used to evaluate thyroid function and diagnose various thyroid disorders, such as hyperthyroidism and thyroid cancer.
Sodium-Iodide Symporter: The sodium-iodide symporter (NIS) is a transmembrane protein that actively transports iodide (I-) into thyroid follicular cells, a crucial step in the production of thyroid hormones. It is a key component in the regulation of thyroid function and iodine homeostasis within the body.
T3: T3, or triiodothyronine, is a thyroid hormone that plays a crucial role in regulating metabolism, growth, and development in the body. It is one of the primary hormones produced and released by the thyroid gland and works in conjunction with other hormones to maintain homeostasis and support various physiological processes.
T4: T4, or thyroxine, is a hormone produced by the thyroid gland that plays a crucial role in regulating the body's metabolism, growth, and development. It is one of the primary hormones secreted by the thyroid gland and is essential for maintaining overall physiological homeostasis.
Thyrocytes: Thyrocytes, also known as thyroid follicular cells, are the primary functional cells of the thyroid gland. They are responsible for the production and secretion of thyroid hormones, which play a crucial role in regulating the body's metabolism, growth, and development.
Thyroglobulin: Thyroglobulin is a large glycoprotein produced by the follicular cells of the thyroid gland. It serves as a precursor for the synthesis of the thyroid hormones, triiodothyronine (T3) and thyroxine (T4), and is essential for the proper functioning of the thyroid gland.
Thyroid gland: The thyroid gland is a butterfly-shaped endocrine gland located in the front part of the neck below the Adam's apple, responsible for producing hormones that regulate metabolism, energy generation, and body temperature. It releases triiodothyronine (T3) and thyroxine (T4), which play key roles in the body's metabolic processes.
Thyroid Hormone Receptor: The thyroid hormone receptor is a type of nuclear receptor that binds to thyroid hormones, such as triiodothyronine (T3) and thyroxine (T4), and regulates the expression of genes involved in various physiological processes. These receptors play a crucial role in the regulation of the thyroid gland and its hormones.
Thyroid hormone synthesis: Thyroid hormone synthesis is the biochemical process through which the thyroid gland produces hormones, primarily thyroxine (T4) and triiodothyronine (T3), from iodide and the amino acid tyrosine. This process involves a series of steps including the uptake of iodide, its oxidation and incorporation into thyroglobulin, and the enzymatic conversion to the active hormones. The production of these hormones plays a critical role in regulating metabolism, growth, and development in the body.
Thyroid Lobes: The thyroid gland is composed of two lobes, the left and right thyroid lobes, connected by a narrow isthmus. These lobes are the primary functional units of the thyroid gland, responsible for the production and secretion of essential hormones that regulate various physiological processes in the body.
Thyroid Peroxidase: Thyroid peroxidase is an enzyme found in the thyroid gland that plays a crucial role in the production of thyroid hormones. It catalyzes the iodination and coupling of tyrosine residues to form the thyroid hormones triiodothyronine (T3) and thyroxine (T4), which are essential for regulating metabolism, growth, and development in the body.
Thyroid Stimulating Hormone (TSH): Thyroid Stimulating Hormone (TSH) is a crucial hormone produced by the pituitary gland that regulates the function of the thyroid gland. It plays a central role in maintaining proper thyroid hormone levels and overall endocrine system balance.
Thyroid storm: Thyroid storm is a life-threatening condition characterized by an extreme and acute exacerbation of hyperthyroidism, often triggered by stress, infection, or surgery. It is marked by a rapid increase in metabolism and can lead to severe symptoms like fever, tachycardia, and confusion, reflecting the overwhelming effects of excessive thyroid hormones on the body. Understanding this condition is crucial for recognizing the importance of the thyroid gland's role in regulating metabolism and maintaining overall homeostasis.
Thyroid Ultrasound: A thyroid ultrasound is a diagnostic imaging procedure that uses high-frequency sound waves to create real-time images of the thyroid gland, allowing healthcare providers to evaluate its size, structure, and any abnormalities that may be present. This non-invasive technique is commonly used to assess thyroid function and detect potential issues within the thyroid gland.
Thyroid-binding globulin: Thyroid-binding globulin (TBG) is a protein produced by the liver that plays a crucial role in the transport of thyroid hormones, specifically thyroxine (T4) and triiodothyronine (T3), in the bloodstream. By binding to these hormones, TBG helps regulate their availability to tissues and maintains a stable supply of thyroid hormones, which are essential for metabolic processes throughout the body.
Thyroid-stimulating hormone: Thyroid-stimulating hormone (TSH) is a peptide hormone produced by the anterior pituitary gland that regulates the production of thyroid hormones by the thyroid gland. TSH plays a crucial role in maintaining metabolic homeostasis by stimulating the synthesis and release of thyroxine (T4) and triiodothyronine (T3), which are vital for various physiological processes, including growth, development, and metabolism.
Thyroid-stimulating hormone (TSH): Thyroid-Stimulating Hormone is a pituitary hormone that stimulates the thyroid gland to produce and release thyroid hormones, which regulate metabolism, energy generation, and overall growth and development. It acts as part of a feedback system that maintains hormonal balance within the body.
Thyroid-stimulating hormone test: The thyroid-stimulating hormone test measures the level of thyroid-stimulating hormone (TSH) in the blood, which is produced by the pituitary gland and regulates the production of hormones by the thyroid gland. This test is crucial for diagnosing thyroid disorders such as hypothyroidism and hyperthyroidism, as it provides insights into how well the thyroid is functioning and how the body is regulating its hormone levels.
Thyrotropin-Releasing Hormone: Thyrotropin-releasing hormone (TRH) is a peptide hormone produced in the hypothalamus that stimulates the release of thyroid-stimulating hormone (TSH) from the anterior pituitary gland. TRH plays a crucial role in regulating the activity of the thyroid gland, which is responsible for producing hormones that control metabolism, growth, and development.
Thyroxine: Thyroxine, also known as T4, is a hormone produced by the thyroid gland that plays a crucial role in regulating metabolism, growth, and development. It influences almost every cell in the body by controlling the rate at which they use energy.
Thyroxine: Thyroxine, also known as T4, is a hormone produced by the thyroid gland that plays a crucial role in regulating the body's metabolism and energy balance. It is the primary thyroid hormone released into the bloodstream and is responsible for a wide range of physiological processes, including growth, development, and temperature regulation.
Triiodothyronine: Triiodothyronine, also known as T3, is a thyroid hormone that plays a crucial role in the body's metabolic rate, heart and digestive functions, muscle control, brain development, and maintenance of bones. It is produced by the thyroid gland after being stimulated by the thyroid-stimulating hormone (TSH).
Triiodothyronine: Triiodothyronine, also known as T3, is a thyroid hormone that plays a crucial role in regulating the body's metabolism and energy balance. It is produced by the thyroid gland and acts on target cells throughout the body to influence a wide range of physiological processes.