Module 12: The Endocrine System

Lesson 4: The Thyroid Gland

Tuyến Giáp

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Mỗi bài học (lesson) bao gồm 4 phần chính: Thuật ngữ, Luyện Đọc, Luyện Nghe, và Bàn Luận.
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Dưới đây là danh sách những thuật ngữ Y khoa của module The Endocrine System.
Khái quát được số lượng thuật ngữ sẽ xuất hiện trong bài đọc và nghe sẽ giúp bạn thoải mái tiêu thụ nội dung hơn. Sau khi hoàn thành nội dung đọc và nghe, bạn hãy quay lại đây và luyện tập (practice) để quen dần các thuật ngữ này. Đừng ép bản thân phải nhớ các thuật ngữ này vội vì bạn sẽ gặp và ôn lại danh sách này trong những bài học (lesson) khác của cùng một module.

Medical Terminology: The Endocrine System

acromegaly
disorder in adults caused when abnormally high levels of GH trigger growth of bones in the face, hands, and feet
adenylyl cyclase
membrane-bound enzyme that converts ATP to cyclic AMP, creating cAMP, as a result of G-protein activation
adrenal cortex
outer region of the adrenal glands consisting of multiple layers of epithelial cells and capillary networks that produces mineralocorticoids and glucocorticoids
adrenal glands
endocrine glands located at the top of each kidney that are important for the regulation of the stress response, blood pressure and blood volume, water homeostasis, and electrolyte levels
adrenal medulla
inner layer of the adrenal glands that plays an important role in the stress response by producing epinephrine and norepinephrine
adrenocorticotropic hormone (ACTH)
anterior pituitary hormone that stimulates the adrenal cortex to secrete corticosteroid hormones (also called corticotropin)
alarm reaction
the short-term stress, or the fight-or-flight response, of stage one of the general adaptation syndrome mediated by the hormones epinephrine and norepinephrine
aldosterone
hormone produced and secreted by the adrenal cortex that stimulates sodium and fluid retention and increases blood volume and blood pressure
alpha cell
pancreatic islet cell type that produces the hormone glucagon
angiotensin-converting enzyme
the enzyme that converts angiotensin I to angiotensin II
antidiuretic hormone (ADH)
hypothalamic hormone that is stored by the posterior pituitary and that signals the kidneys to reabsorb water
atrial natriuretic peptide (ANP)
peptide hormone produced by the walls of the atria in response to high blood pressure, blood volume, or blood sodium that reduces the reabsorption of sodium and water in the kidneys and promotes vasodilation
autocrine
chemical signal that elicits a response in the same cell that secreted it
beta cell
pancreatic islet cell type that produces the hormone insulin
calcitonin
peptide hormone produced and secreted by the parafollicular cells (C cells) of the thyroid gland that functions to decrease blood calcium levels
chromaffin
neuroendocrine cells of the adrenal medulla
colloid
viscous fluid in the central cavity of thyroid follicles, containing the glycoprotein thyroglobulin
cortisol
glucocorticoid important in gluconeogenesis, the catabolism of glycogen, and downregulation of the immune system
cyclic adenosine monophosphate (cAMP)
second messenger that, in response to adenylyl cyclase activation, triggers a phosphorylation cascade
delta cell
minor cell type in the pancreas that secretes the hormone somatostatin
diabetes mellitus
condition caused by destruction or dysfunction of the beta cells of the pancreas or cellular resistance to insulin that results in abnormally high blood glucose levels
diacylglycerol (DAG)
molecule that, like cAMP, activates protein kinases, thereby initiating a phosphorylation cascade
downregulation
decrease in the number of hormone receptors, typically in response to chronically excessive levels of a hormone
endocrine gland
tissue or organ that secretes hormones into the blood and lymph without ducts such that they may be transported to organs distant from the site of secretion
endocrine system
cells, tissues, and organs that secrete hormones as a primary or secondary function and play an integral role in normal bodily processes
epinephrine
primary and most potent catecholamine hormone secreted by the adrenal medulla in response to short-term stress; also called adrenaline
erythropoietin (EPO)
protein hormone secreted in response to low oxygen levels that triggers the bone marrow to produce red blood cells
estrogens
class of predominantly female sex hormones important for the development and growth of the female reproductive tract, secondary sex characteristics, the female reproductive cycle, and the maintenance of pregnancy
exocrine system
cells, tissues, and organs that secrete substances directly to target tissues via glandular ducts
first messenger
hormone that binds to a cell membrane hormone receptor and triggers activation of a second messenger system
follicle-stimulating hormone (FSH)
anterior pituitary hormone that stimulates the production and maturation of sex cells
G protein
protein associated with a cell membrane hormone receptor that initiates the next step in a second messenger system upon activation by hormone–receptor binding
general adaptation syndrome (GAS)
the human body’s three-stage response pattern to short- and long-term stress
gigantism
disorder in children caused when abnormally high levels of GH prompt excessive growth
glucagon
pancreatic hormone that stimulates the catabolism of glycogen to glucose, thereby increasing blood glucose levels
glucocorticoids
hormones produced by the zona fasciculata of the adrenal cortex that influence glucose metabolism
goiter
enlargement of the thyroid gland either as a result of iodine deficiency or hyperthyroidism
gonadotropins
hormones that regulate the function of the gonads
growth hormone (GH)
anterior pituitary hormone that promotes tissue building and influences nutrient metabolism (also called somatotropin)
hormone
secretion of an endocrine organ that travels via the bloodstream or lymphatics to induce a response in target cells or tissues in another part of the body
hormone receptor
protein within a cell or on the cell membrane that binds a hormone, initiating the target cell response
hyperglycemia
abnormally high blood glucose levels
hyperparathyroidism
disorder caused by overproduction of PTH that results in abnormally elevated blood calcium
hyperthyroidism
clinically abnormal, elevated level of thyroid hormone in the blood; characterized by an increased metabolic rate, excess body heat, sweating, diarrhea, weight loss, and increased heart rate
hypoparathyroidism
disorder caused by underproduction of PTH that results in abnormally low blood calcium
hypophyseal portal system
network of blood vessels that enables hypothalamic hormones to travel into the anterior lobe of the pituitary without entering the systemic circulation
hypothalamus
region of the diencephalon inferior to the thalamus that functions in neural and endocrine signaling
hypothyroidism
clinically abnormal, low level of thyroid hormone in the blood; characterized by low metabolic rate, weight gain, cold extremities, constipation, and reduced mental activity
infundibulum
stalk containing vasculature and neural tissue that connects the pituitary gland to the hypothalamus (also called the pituitary stalk)
inhibin
hormone secreted by the male and female gonads that inhibits FSH production by the anterior pituitary
inositol triphosphate (IP3)
molecule that initiates the release of calcium ions from intracellular stores
insulin
pancreatic hormone that enhances the cellular uptake and utilization of glucose, thereby decreasing blood glucose levels
insulin-like growth factors (IGF)
protein that enhances cellular proliferation, inhibits apoptosis, and stimulates the cellular uptake of amino acids for protein synthesis
leptin
protein hormone secreted by adipose tissues in response to food consumption that promotes satiety
luteinizing hormone (LH)
anterior pituitary hormone that triggers ovulation and the production of ovarian hormones, and the production of testosterone
melatonin
amino acid–derived hormone that is secreted in response to low light and causes drowsiness
mineralocorticoids
hormones produced by the zona glomerulosa cells of the adrenal cortex that influence fluid and electrolyte balance
neonatal hypothyroidism
condition characterized by cognitive deficits, short stature, and other signs and symptoms in people born to people who were iodine-deficient during pregnancy
norepinephrine
secondary catecholamine hormone secreted by the adrenal medulla in response to short-term stress; also called noradrenaline
osmoreceptor
hypothalamic sensory receptor that is stimulated by changes in solute concentration (osmotic pressure) in the blood
oxytocin
hypothalamic hormone stored in the posterior pituitary gland and important in stimulating uterine contractions in labor, milk ejection during breastfeeding, and feelings of attachment (produced by males and females)
pancreas
organ with both exocrine and endocrine functions located posterior to the stomach that is important for digestion and the regulation of blood glucose
pancreatic islets
specialized clusters of pancreatic cells that have endocrine functions; also called islets of Langerhans
paracrine
chemical signal that elicits a response in neighboring cells; also called paracrine factor
parathyroid glands
small, round glands embedded in the posterior thyroid gland that produce parathyroid hormone (PTH)
parathyroid hormone (PTH)
peptide hormone produced and secreted by the parathyroid glands in response to low blood calcium levels
phosphodiesterase (PDE)
cytosolic enzyme that deactivates and degrades cAMP
phosphorylation cascade
signaling event in which multiple protein kinases phosphorylate the next protein substrate by transferring a phosphate group from ATP to the protein
pineal gland
endocrine gland that secretes melatonin, which is important in regulating the sleep-wake cycle
pinealocyte
cell of the pineal gland that produces and secretes the hormone melatonin
pituitary dwarfism
disorder in children caused when abnormally low levels of GH result in growth retardation
pituitary gland
bean-sized organ suspended from the hypothalamus that produces, stores, and secretes hormones in response to hypothalamic stimulation (also called hypophysis)
PP cell
minor cell type in the pancreas that secretes the hormone pancreatic polypeptide
progesterone
predominantly female sex hormone important in regulating the female reproductive cycle and the maintenance of pregnancy
prolactin (PRL)
anterior pituitary hormone that promotes development of the mammary glands and the production of breast milk
protein kinase
enzyme that initiates a phosphorylation cascade upon activation
second messenger
molecule that initiates a signaling cascade in response to hormone binding on a cell membrane receptor and activation of a G protein
stage of exhaustion
stage three of the general adaptation syndrome; the body’s long-term response to stress mediated by the hormones of the adrenal cortex
stage of resistance
stage two of the general adaptation syndrome; the body’s continued response to stress after stage one diminishes
testosterone
steroid hormone secreted by the testes and important in the maturation of sperm cells, growth and development of the reproductive system, and the development of secondary sex characteristics
thymosins
hormones produced and secreted by the thymus that play an important role in the development and differentiation of T cells
thymus
organ that is involved in the development and maturation of T-cells and is particularly active during infancy and childhood
thyroid gland
large endocrine gland responsible for the synthesis of thyroid hormones
thyroid-stimulating hormone (TSH)
anterior pituitary hormone that triggers secretion of thyroid hormones by the thyroid gland (also called thyrotropin)
thyroxine
(also, tetraiodothyronine, T4) amino acid–derived thyroid hormone that is more abundant but less potent than T3 and often converted to T3 by target cells
triiodothyronine
(also, T3) amino acid–derived thyroid hormone that is less abundant but more potent than T4
upregulation
increase in the number of hormone receptors, typically in response to chronically reduced levels of a hormone
zona fasciculata
intermediate region of the adrenal cortex that produce hormones called glucocorticoids
zona glomerulosa
most superficial region of the adrenal cortex, which produces the hormones collectively referred to as mineralocorticoids
zona reticularis
deepest region of the adrenal cortex, which produces the steroid sex hormones called androgens
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Dưới đây là các bài văn nằm ở bên trái. Ở bên phải là các bài luyện tập (practice) để đánh giá khả năng đọc hiểu của bạn. Sẽ khó khăn trong thời gian đầu nếu vốn từ vựng của bạn còn hạn chế, đặc biệt là từ vựng Y khoa. Hãy kiên nhẫn và đọc nhiều nhất có kể, lượng kiến thức tích tụ dần sẽ giúp bạn đọc thoải mái hơn.
A butterfly-shaped organ, the thyroid gland is located anterior to the trachea, just inferior to the larynx (Figure 1). The medial region, called the isthmus, is flanked by wing-shaped left and right lobes. Each of the thyroid lobes are embedded with parathyroid glands, primarily on their posterior surfaces. The tissue of the thyroid gland is composed mostly of thyroid follicles. The follicles are made up of a central cavity filled with a sticky fluid called colloid. Surrounded by a wall of epithelial follicle cells, the colloid is the center of thyroid hormone production, and that production is dependent on the hormones’ essential and unique component: iodine.
Hormones are produced in the colloid when atoms of the mineral iodine attach to a glycoprotein, called thyroglobulin, that is secreted into the colloid by the follicle cells. The following steps outline the hormones’ assembly:

  1. As a result, the concentration of iodide ions “trapped” in the follicular cells is many times higher than the concentration in the bloodstream.
  2. Iodide ions then move to the lumen of the follicle cells that border the colloid. There, the ions undergo oxidation (their negatively charged electrons are removed). The oxidation of two iodide ions (2 I–) results in iodine (I2), which passes through the follicle cell membrane into the colloid.
  3. In the colloid, peroxidase enzymes link the iodine to the tyrosine amino acids in thyroglobulin to produce two intermediaries: a tyrosine attached to one iodine and a tyrosine attached to two iodines. When one of each of these intermediaries is linked by covalent bonds, the resulting compound is triiodothyronine (T3), a thyroid hormone with three iodines. Much more commonly, two copies of the second intermediary bond, forming tetraiodothyronine, also known as thyroxine (T4), a thyroid hormone with four iodines.

These hormones remain in the colloid center of the thyroid follicles until TSH stimulates endocytosis of colloid back into the follicle cells. There, lysosomal enzymes break apart the thyroglobulin colloid, releasing free T3 and T4, which diffuse across the follicle cell membrane and enter the bloodstream.

In the bloodstream, less than one percent of the circulating T3 and T4 remains unbound. This free T3 and T4 can cross the lipid bilayer of cell membranes and be taken up by cells. The remaining 99 percent of circulating T3 and T4 is bound to specialized transport proteins called thyroxine-binding globulins (TBGs), to albumin, or to other plasma proteins. This “packaging” prevents their free diffusion into body cells. When blood levels of T3 and T4 begin to decline, bound T3 and T4 are released from these plasma proteins and readily cross the membrane of target cells. T3 is more potent than T4, and many cells convert T4 to T3 through the removal of an iodine atom.
The release of T3 and T4 from the thyroid gland is regulated by thyroid-stimulating hormone (TSH). As shown in Figure 2, low blood levels of T3 and T4 stimulate the release of thyrotropin-releasing hormone (TRH) from the hypothalamus, which triggers secretion of TSH from the anterior pituitary. In turn, TSH stimulates the thyroid gland to secrete T3 and T4. The levels of TRH, TSH, T3, and T4 are regulated by a negative feedback system in which increasing levels of T3 and T4 decrease the production and secretion of TSH.
The thyroid hormones, T3 and T4, are often referred to as metabolic hormones because their levels influence the body’s basal metabolic rate, the amount of energy used by the body at rest. When T3 and T4 bind to intracellular receptors located on the mitochondria, they cause an increase in nutrient breakdown and the use of oxygen to produce ATP. In addition, T3 and T4 initiate the transcription of genes involved in glucose oxidation. Although these mechanisms prompt cells to produce more ATP, the process is inefficient, and an abnormally increased level of heat is released as a byproduct of these reactions. This so-called calorigenic effect (calor- = “heat”) raises body temperature.

Adequate levels of thyroid hormones are also required for protein synthesis and for fetal and childhood tissue development and growth. They are especially critical for normal development of the nervous system both in utero and in early childhood, and they continue to support neurological function in adults. As noted earlier, these thyroid hormones have a complex interrelationship with reproductive hormones, and deficiencies can influence libido, fertility, and other aspects of reproductive function. Finally, thyroid hormones increase the body’s sensitivity to catecholamines (epinephrine and norepinephrine) from the adrenal medulla by upregulation of receptors in the blood vessels. When levels of T3 and T4 hormones are excessive, this effect accelerates the heart rate, strengthens the heartbeat, and increases blood pressure. Because thyroid hormones regulate metabolism, heat production, protein synthesis, and many other body functions, thyroid disorders can have severe and widespread consequences.
The thyroid gland also secretes a hormone called calcitonin that is produced by the parafollicular cells (also called C cells) that stud the tissue between distinct follicles. Calcitonin is released in response to a rise in blood calcium levels. It appears to have a function in decreasing blood calcium concentrations by:

  • Inhibiting the activity of osteoclasts, bone cells that release calcium into the circulation by degrading bone matrix.
  • Increasing osteoblastic activity.
  • Decreasing calcium absorption in the intestines Increasing calcium loss in the urine.

However, these functions are usually not significant in maintaining calcium homeostasis, so the importance of calcitonin is not entirely understood. Pharmaceutical preparations of calcitonin are sometimes prescribed to reduce osteoclast activity in people with osteoporosis and to reduce the degradation of cartilage in people with osteoarthritis. The hormones secreted by thyroid are summarized in Table 1.

Of course, calcium is critical for many other biological processes. It is a second messenger in many signaling pathways, and is essential for muscle contraction, nerve impulse transmission, and blood clotting. Given these roles, it is not surprising that blood calcium levels are tightly regulated by the endocrine system. The organs involved in the regulation are the parathyroid glands.

OpenStax. (2022). Anatomy and Physiology 2e. Rice University. Retrieved June 15, 2023. ISBN-13: 978-1-711494-06-7 (Hardcover) ISBN-13: 978-1-711494-05-0 (Paperback) ISBN-13: 978-1-951693-42-8 (Digital). License: Attribution 4.0 International (CC BY 4.0). Access for free at openstax.org.

The thyroid gland is located in the neck where it wraps around the trachea. (a) Anterior view of the thyroid gland. (b) Posterior view of the thyroid gland. (c) The glandular tissue is composed primarily of thyroid follicles. The larger parafollicular cells often appear within the matrix of follicle cells. LM × 1332. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

A classic negative feedback loop controls the regulation of thyroid hormone levels.

Associated hormonesChemical classEffect
Thyroxine (T4), triiodothyronine (T3)AmineStimulate basal metabolic rate
CalcitoninPeptideReduces blood Ca2+ levels
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Script:
  1. The thyroid gland is a butterfly-shaped organ located in the neck anterior to the trachea.
  2. Its hormones regulate basal metabolism, oxygen use, nutrient metabolism, the production of ATP, and calcium homeostasis.
  3. They also contribute to protein synthesis and the normal growth and development of body tissues, including maturation of the nervous system, and they increase the body’s sensitivity to catecholamines.
  4. The thyroid hormones triiodothyronine (or T3) and thyroxine (or T4) are produced and secreted by the thyroid gland in response to thyroid-stimulating hormone (or TSH) from the anterior pituitary.
  5. Synthesis of the T3 and T4 hormones requires iodine.
  6. Insufficient amounts of iodine in the diet can lead to goiter, cretinism, and many other disorders.
  7. When blood levels of T3 and T4 are low, the hypothalamus releases thyrotropin-releasing hormone (or TRH), prompting the anterior pituitary to secrete TSH.
  8. Subsequently, TSH stimulates the thyroid gland to release T3 and T4.
  9. However, as T3 and T4 levels rise, they inhibit the production and secretion of TSH, thus maintaining balance within the system.
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