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Thyroid hormones T3 and T4 are primarily composed of serine and require dietary calcium for their synthesis.
Answer: False
Thyroid hormones T3 and T4 are tyrosine-based, not serine-based, and require dietary iodine for their synthesis, not calcium.
Selenium is a critical dietary component necessary for the synthesis of T3 because the enzymes that convert T4 to T3 contain selenium.
Answer: True
Selenium is an essential trace mineral that serves as a cofactor for the deiodinase enzymes responsible for the conversion of T4 to the more active T3.
Selenium is essential for the production of T4, but not T3, as it is a component of the tyrosine molecule used in hormone synthesis.
Answer: False
Selenium is essential for the enzymes that convert T4 to T3, not for the production of T4 itself. Tyrosine is the amino acid backbone, and iodine is incorporated into it.
Thyroid hormones T3 and T4 are chemically based on which amino acid, and what essential dietary element is crucial for their production?
Answer: Tyrosine and Iodine
Thyroid hormones are derived from the amino acid tyrosine and require iodine, obtained from the diet, for their synthesis. These are fundamental building blocks for T3 and T4.
What essential mineral is required for the deiodinase enzymes responsible for converting T4 into the more active T3?
Answer: Selenium
Selenium is a critical component of the deiodinase enzymes that catalyze the conversion of T4 to T3. Its presence is vital for this metabolic activation process.
Which of the following is a key step in the synthesis of thyroid hormones within the thyroid gland?
Answer: Iodination of thyroglobulin tyrosine residues
The iodination of tyrosine residues on the thyroglobulin molecule is a critical early step in the synthesis of thyroid hormones within the thyroid follicular cells.
What is the primary function of the sodium-iodide symporter in thyroid follicular cells?
Answer: Actively transporting iodide ions into the cell
The sodium-iodide symporter is responsible for the active uptake of iodide ions from the bloodstream into thyroid follicular cells, a process essential for hormone synthesis.
Thyroglobulin's role in thyroid hormone production is to:
Answer: Act as a precursor protein where iodination and coupling occur
Thyroglobulin serves as the scaffold upon which iodine atoms are attached (iodination) and subsequently coupled to form T3 and T4 within the thyroid follicle.
How does Thyroid-Stimulating Hormone (TSH) influence thyroid hormone production?
Answer: It stimulates the endocytosis of thyroglobulin, leading to hormone release
TSH binding to receptors on thyroid follicular cells triggers the uptake of thyroglobulin from the colloid, initiating the process that culminates in the release of thyroid hormones into circulation.
Thyroxine (T4) is the most abundant thyroid hormone found in the bloodstream and possesses a half-life of approximately one week.
Answer: True
Thyroxine (T4) represents the predominant thyroid hormone circulating in the peripheral blood and exhibits a relatively long biological half-life, estimated to be approximately one week.
Triiodothyronine (T3) is the most abundant thyroid hormone in the blood, although T4 has a longer half-life.
Answer: False
Thyroxine (T4) is the most abundant thyroid hormone in the blood, while triiodothyronine (T3) is less abundant but more potent. T4 possesses the longer half-life.
The conversion of thyroxine (T4) into the more potent active form, triiodothyronine (T3), occurs within cells via deiodinase enzymes.
Answer: True
The conversion of the prohormone thyroxine (T4) to the biologically active triiodothyronine (T3) is primarily mediated by deiodinase enzymes located within target cells.
T3 is converted into T4 by deiodinases, making T4 the more potent form of the thyroid hormone.
Answer: False
The conversion process involves deiodinases converting T4 into T3. T3 is the more potent form of the thyroid hormone, not T4.
What are the two primary hormones produced and released by the thyroid gland that regulate metabolism?
Answer: Triiodothyronine (T3) and Thyroxine (T4)
The thyroid gland's principal endocrine function involves the synthesis and secretion of triiodothyronine (T3) and thyroxine (T4), which are critical for regulating the body's metabolic processes.
Which thyroid hormone is the major form circulating in the blood, and what is its approximate half-life?
Answer: T4, approximately one week
Thyroxine (T4) is the most abundant thyroid hormone in circulation and has a significantly longer half-life, approximately one week, compared to the more potent T3.
How is the more potent active form of thyroid hormone, T3, primarily produced from T4?
Answer: Through conversion by deiodinase enzymes within cells
The conversion of thyroxine (T4) into the more biologically active triiodothyronine (T3) is predominantly achieved through the action of deiodinase enzymes located within peripheral tissues.
Compare the biological activity and blood concentration of T3 and T4.
Answer: T3 is more potent but less abundant than T4
Triiodothyronine (T3) exhibits significantly higher biological potency than thyroxine (T4), although T4 circulates in greater quantities in the bloodstream.
How are thyroid hormones primarily transported in the blood?
Answer: Bound to transport proteins like TBG, TTR, and albumin
The vast majority of thyroid hormones circulate in the blood bound to specific transport proteins, primarily thyroxine-binding globulin (TBG), transthyretin (TTR), and albumin.
Why is measuring *free* thyroid hormones (like free T4 and free T3) important for diagnosis?
Answer: Because only the unbound hormones are biologically active
Only the unbound, free fractions of thyroid hormones (free T4 and free T3) are biologically active and capable of interacting with cellular receptors; therefore, their measurement is crucial for accurate clinical assessment.
In blood plasma, what is the approximate percentage of thyroid hormone bound to Thyroxine-Binding Globulin (TBG)?
Answer: Approximately 70%
Thyroxine-Binding Globulin (TBG) is the primary carrier protein for thyroid hormones in the blood, binding approximately 70% of the total circulating hormone.
How do thyroid hormones (T3 and T4) primarily cross cell membranes to exert their effects?
Answer: Through carrier-mediated transport systems
Despite their lipophilic nature, thyroid hormones utilize specific carrier proteins for active transport across cell membranes, rather than relying solely on passive diffusion.
What is the primary mechanism through which thyroid hormones exert their cellular effects?
Answer: Binding to nuclear receptors to influence gene transcription
Thyroid hormones primarily exert their effects by binding to nuclear receptors, which then modulate gene expression, thereby altering cellular function and metabolism.
The thyroid gland is the primary endocrine organ responsible for the synthesis and secretion of triiodothyronine (T3) and thyroxine (T4), hormones indispensable for the regulation of systemic metabolic rate.
Answer: True
The thyroid gland primarily produces and releases triiodothyronine (T3) and thyroxine (T4), which are crucial for regulating the body's metabolism.
Thyroid hormones influence energy expenditure by encouraging the creation of new mitochondria and stimulating processes that generate heat (thermogenesis).
Answer: True
Thyroid hormones are key regulators of basal metabolic rate, promoting mitochondrial biogenesis and increasing oxygen consumption, which contributes to thermogenesis.
Thyroid hormones decrease energy expenditure by inhibiting mitochondrial activity and reducing heat production in the body.
Answer: False
Thyroid hormones increase energy expenditure by stimulating mitochondrial activity and enhancing thermogenesis, not decreasing it.
Thyroid hormones play a role in neural maturation and increase the body's sensitivity to catecholamines, in addition to regulating metabolic rate.
Answer: True
Thyroid hormones are essential for normal neural development and function, and they potentiate the effects of catecholamines, thereby influencing cardiovascular and metabolic responses.
Thyroid hormones primarily function to decrease protein synthesis and inhibit long bone growth.
Answer: False
Thyroid hormones generally promote protein synthesis and are necessary for normal long bone growth, particularly in conjunction with growth hormone.
At the cellular level, thyroid hormones regulate the metabolic processes involving proteins, fats, and carbohydrates, and also stimulate vitamin metabolism.
Answer: True
Thyroid hormones exert widespread effects on cellular metabolism, influencing the breakdown and synthesis of proteins, carbohydrates, and lipids, as well as impacting vitamin utilization.
Thyroid hormones exclusively affect carbohydrate metabolism and have no influence on the metabolism of fats or proteins.
Answer: False
Thyroid hormones influence the metabolism of carbohydrates, fats, and proteins, playing a central role in regulating the body's overall energy balance.
How do thyroid hormones primarily modulate energy expenditure?
Answer: By promoting mitochondrial biogenesis and stimulating thermogenesis
Thyroid hormones enhance energy expenditure by stimulating the production of mitochondria and increasing metabolic processes that generate heat (thermogenesis).
Which of the following is NOT a primary function of thyroid hormones within the body's cells?
Answer: Decreasing sensitivity to catecholamines
Thyroid hormones actually increase sensitivity to catecholamines. They are crucial for increasing basal metabolic rate, supporting growth, and aiding neural maturation.
At the cellular level, thyroid hormones regulate the metabolism of which macronutrients?
Answer: Proteins, fats, and carbohydrates
Thyroid hormones exert a comprehensive influence on cellular metabolism, regulating the catabolism and anabolism of proteins, carbohydrates, and fats.
Why is fetal thyroid hormone production significant during gestation?
Answer: It is crucial for proper brain development, becoming significant around 18-20 weeks
Fetal thyroid hormone production, particularly becoming significant around the second trimester, is vital for the proper development of the fetal brain.
A deficiency in dietary iodine can cause the thyroid gland to enlarge, resulting in a condition known as simple goiter, due to insufficient hormone production.
Answer: True
Insufficient dietary iodine impairs thyroid hormone synthesis, leading to increased TSH stimulation and thyroid gland enlargement, a condition termed simple goiter.
Simple goiter is a condition caused by an excess of dietary iodine, leading to overproduction of thyroid hormones.
Answer: False
Simple goiter is caused by a deficiency, not an excess, of dietary iodine, which leads to insufficient hormone production and subsequent thyroid enlargement.
Deficiency in which essential dietary element leads to decreased thyroid hormone production and can cause the thyroid gland to enlarge, resulting in simple goiter?
Answer: Iodine
Iodine is indispensable for the synthesis of thyroid hormones. Its deficiency impairs hormone production, leading to compensatory enlargement of the thyroid gland, known as simple goiter.
Levothyroxine, a synthetic form of T4, was among the most frequently prescribed medications in the US in 2020, exceeding 98 million prescriptions.
Answer: True
Levothyroxine is a widely prescribed medication for hypothyroidism, and data from 2020 indicates it was the second most prescribed drug in the US, with over 98 million prescriptions.
Levothyroxine, despite being a common thyroid medication, was prescribed less than 10 million times in the US in 2020.
Answer: False
Levothyroxine was prescribed significantly more than 10 million times in the US in 2020; it exceeded 98 million prescriptions.
T3 and T4 are effective oral treatments for hypothyroidism, which is a condition characterized by thyroid hormone deficiency.
Answer: True
Hypothyroidism, resulting from insufficient thyroid hormone production, is effectively managed with oral administration of T3 and T4 preparations due to good gastrointestinal absorption.
T3 and T4 are primarily used to treat hyperthyroidism, a condition of excess thyroid hormone production.
Answer: False
T3 and T4 are used to treat hypothyroidism (deficiency), not hyperthyroidism (excess). Treatments for hyperthyroidism aim to reduce hormone levels.
Levothyroxine, a synthetic T4 preparation, is frequently chosen for thyroid hormone replacement because its slower metabolism allows for convenient once-daily dosing.
Answer: True
Levothyroxine's pharmacokinetic profile, characterized by a longer half-life and slower metabolism compared to T3, facilitates convenient once-daily dosing regimens for patients with hypothyroidism.
Levothyroxine is preferred because it is the active T3 form and requires multiple daily doses due to its rapid metabolism.
Answer: False
Levothyroxine is a synthetic T4 preparation, not T3. Its preference stems from its slower metabolism allowing for once-daily dosing, unlike the more rapidly metabolized T3.
Natural desiccated thyroid hormone treatments, derived from animal thyroids, contain a mix of hormones including T3, T4, and smaller amounts of T2, T1, and calcitonin.
Answer: True
Natural desiccated thyroid preparations are sourced from the thyroid glands of animals and contain a spectrum of thyroid-related compounds, including T3, T4, and trace amounts of other iodothyronines and calcitonin.
Natural desiccated thyroid hormones are purely synthetic preparations containing only T4 and are derived from bovine sources.
Answer: False
Natural desiccated thyroid hormones are derived from animal thyroids (commonly porcine), are not purely synthetic, and contain a mixture of T3 and T4, not just T4.
What is the medical significance of levothyroxine in the United States, based on 2020 data?
Answer: It was the second most commonly prescribed medication with over 98 million prescriptions
In 2020, levothyroxine ranked as the second most prescribed medication in the U.S., indicating its widespread clinical importance for managing conditions like hypothyroidism.
T3 and T4 are commonly used orally to treat which medical condition?
Answer: Hypothyroidism
Hypothyroidism, characterized by a deficiency in thyroid hormones, is effectively treated with oral administration of T3 and T4 preparations.
Why is levothyroxine (synthetic T4) often preferred for thyroid hormone replacement therapy?
Answer: It is metabolized more slowly, allowing for once-daily administration
Levothyroxine's slower metabolic clearance rate compared to T3 makes it suitable for once-daily dosing, simplifying treatment regimens for patients with hypothyroidism.
Natural desiccated thyroid hormones, used for hypothyroid treatment, are derived from:
Answer: The thyroid glands of animals, primarily pigs
Natural desiccated thyroid preparations are sourced from the thyroid glands of animals, most commonly pigs, and contain a mixture of thyroid hormones.
Which of the following represents a formulation of thyroid hormone other than levothyroxine or natural desiccated thyroid?
Answer: Liothyronine (pure T3)
Liothyronine is the pharmaceutical name for synthetic triiodothyronine (T3), representing a distinct formulation from levothyroxine (T4) or desiccated thyroid extracts.
Edward Calvin Kendall successfully isolated thyroxine in the year 1915.
Answer: True
American chemist Edward Calvin Kendall is credited with the isolation of thyroxine in 1915.
Thyroxine was first isolated by Charles Robert Harington in the year 1915.
Answer: False
Edward Calvin Kendall isolated thyroxine in 1915. Charles Robert Harington, along with George Barger, later achieved the first synthesis of thyroxine in 1926.
Who is credited with isolating thyroxine, and in which year?
Answer: Edward Calvin Kendall, 1915
Edward Calvin Kendall successfully isolated thyroxine in 1915, a significant milestone in understanding thyroid physiology.
Who were the scientists credited with the first successful synthesis of thyroxine, and in what year?
Answer: Charles Robert Harington and George Barger, 1926
The first successful chemical synthesis of thyroxine was achieved by Charles Robert Harington and George Barger in 1926.
While calcitonin is secreted by the thyroid gland, it is not typically classified under the definition of 'thyroid hormones,' which refers specifically to T3 and T4.
Answer: True
Calcitonin, produced by the parafollicular cells (C cells) of the thyroid, plays a role in calcium homeostasis but is distinct from the primary metabolic regulators, T3 and T4.
Calcitonin is considered one of the two primary thyroid hormones, alongside thyroxine (T4).
Answer: False
The primary thyroid hormones are T3 and T4. Calcitonin, while produced by the thyroid, has a different function (calcium regulation) and is not classified as a 'thyroid hormone' in this context.
Thyronamines are known to inhibit neuronal activity and are implicated in physiological processes such as mammalian hibernation and avian molting.
Answer: True
Thyronamines exhibit inhibitory effects on neuronal activity and are associated with specific physiological states like hibernation in mammals and molting in birds.
Thyronamines stimulate neuronal activity and are primarily involved in regulating sleep cycles in humans.
Answer: False
Thyronamines are known to inhibit, rather than stimulate, neuronal activity and are linked to processes like hibernation and molting, not primarily human sleep cycles.
Calcitonin, a hormone produced by the thyroid gland, is generally not considered a 'thyroid hormone' because:
Answer: It is a peptide hormone, unlike T3 and T4
While calcitonin originates from the thyroid's C cells, its classification differs from the primary thyroid hormones (T3 and T4) due to its peptide nature and distinct physiological role in calcium homeostasis.
Thyronamines are involved in physiological processes such as:
Answer: Mammalian hibernation and avian molting
Thyronamines have been implicated in the regulation of physiological states such as hibernation in mammals and molting in birds, suggesting roles in metabolic adaptation.
What potential medical application has been proposed for thyronamines?
Answer: Controlled induction of hypothermia to prevent brain damage
Thyronamines have been investigated for their potential therapeutic use in inducing controlled hypothermia, a state that may offer neuroprotection following ischemic events.