Thyroid Gland Overview
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Questions and Answers

Which metabolic enzyme is NOT induced by thyroid hormones?

  • Cytochrome oxidase
  • Lactase (correct)
  • α-glycerophosphate dehydrogenase
  • NADPH cytochrome C reductase

    • How do thyroid hormones affect protein metabolism?

  • Increase protein synthesis and degradation, resulting in a net decrease in muscle mass. (correct)
  • Increase protein synthesis and degradation, resulting in a net increase in muscle mass.
  • Decrease protein synthesis and degradation, resulting in a net decrease in muscle mass.
  • Decrease protein synthesis and degradation, resulting in a net increase in muscle mass.

    • What is the primary effect of thyroid hormone on the cardiovascular system?

  • Decreases heart rate and stroke volume.
  • Decreases heart rate and increases stroke volume.
  • Increases heart rate and stroke volume. (correct)
  • Increases heart rate and decreases stroke volume.

    • Which of these statements is TRUE about the effect of thyroid hormone on cholesterol levels?

    <p>Thyroid hormone decreases cholesterol levels by inducing increased numbers of LDL receptors on liver cells. (D)</p> Signup and view all the answers

    What is the effect of thyroid hormone on the sympathetic nervous system?

    <p>Thyroid hormone enhances the effect of the sympathetic nervous system. (C)</p> Signup and view all the answers

    How does thyroid hormone affect the growth of children?

    <p>Thyroid hormone promotes the growth of children. (D)</p> Signup and view all the answers

    Which of the following is NOT a symptom of hypothyroidism in adults?

    <p>Increased mental capacity (A)</p> Signup and view all the answers

    How are the effects of thyroid hormones and catecholamines on the body related?

    <p>The effects are synergistic, meaning they enhance each other. (A)</p> Signup and view all the answers

    Which of the following statements correctly describes the primary function of thyroxine (T4) and triiodothyronine (T3)?

    <p>They control metabolic processes in the body (D)</p> Signup and view all the answers

    What is the primary role of thyroid-stimulating hormone (TSH) in the regulation of thyroid hormone secretion?

    <p>TSH stimulates the release of stored thyroid hormones from the thyroid gland (B)</p> Signup and view all the answers

    The thyroid gland contains C cells that secrete calcitonin. What is the primary function of calcitonin?

    <p>Calcitonin lowers calcium levels in the blood. (B)</p> Signup and view all the answers

    Which of the following is NOT a critical step in the synthesis of thyroid hormones?

    <p>Production of parathyroid hormone. (D)</p> Signup and view all the answers

    What is the role of thyroglobulin in the synthesis of thyroid hormones?

    <p>Thyroglobulin serves as a storage protein for thyroid hormones in the follicular lumen. (D)</p> Signup and view all the answers

    What is the mechanism by which thyroid hormones are released from the thyroid gland?

    <p>Thyroid hormones are released into the follicular lumen and then absorbed by cells through endocytosis. (C)</p> Signup and view all the answers

    Which of the following substances inhibits the Na+–I– cotransport system, thereby reducing iodide uptake into the thyroid gland?

    <p>Perchlorate anions (B)</p> Signup and view all the answers

    What is the therapeutic use of propylthiouracil in the treatment of hyperthyroidism?

    <p>Propylthiouracil inhibits the production of thyroid hormones, reducing hyperthyroidism. (D)</p> Signup and view all the answers

    Which of the following is NOT a direct effect of TSH on thyroid gland function?

    <p>Increased production of thyroid-stimulating immunoglobulins (B)</p> Signup and view all the answers

    What is the role of the paraventricular nucleus (PVN) in thyroid hormone regulation?

    <p>It synthesizes and secretes Thyrotropin-releasing hormone (TRH) (B)</p> Signup and view all the answers

    How does TSH exert its trophic effect on the thyroid gland?

    <p>By promoting hypertrophy and hyperplasia of thyroid follicular cells (D)</p> Signup and view all the answers

    Which of the following is CORRECT regarding the role of cAMP in thyroid hormone regulation?

    <p>cAMP acts as a second messenger, mediating the stimulatory effects of TSH (D)</p> Signup and view all the answers

    Which of the following statements is TRUE about thyroid hormone action?

    <p>T3 binds to nuclear receptors, affecting gene expression and protein synthesis (B)</p> Signup and view all the answers

    How does the brain protect its levels of T3 during starvation?

    <p>By activating a different type of 5′-iodinase that is not inhibited by starvation (A)</p> Signup and view all the answers

    Which of the following is a hallmark of Graves' disease?

    <p>Increased circulating levels of thyroid-stimulating immunoglobulins (C)</p> Signup and view all the answers

    Which of the following mechanisms does thyroid hormone use to increase basal metabolic rate (BMR)?

    <p>By inducing the synthesis of Na+-K+ ATPase, leading to increased oxygen consumption and heat production (B)</p> Signup and view all the answers

    Flashcards

    5′-iodinase

    An enzyme responsible for converting T4 into T3 in the brain

    Regulation of Thyroid Hormone Secretion

    Controlled by TRH from the hypothalamus that stimulates TSH production

    TSH

    Thyroid-stimulating hormone increases thyroid secretion and activity

    cAMP in TSH action

    cAMP mediates TSH's stimulatory effect on thyroid hormone production

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    Trophic effect of TSH

    Long-term TSH elevation causes hypertrophy and hyperplasia of thyroid cells

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    Graves Disease

    Autoimmune disorder causing hyperthyroidism via TSH receptor antibodies

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    Negative feedback in Graves Disease

    High thyroid hormone levels inhibit TSH secretion due to feedback

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    Action of T3

    T3 binds to nuclear receptors, stimulating gene transcription and protein synthesis

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    Thyroid Gland Location

    Located below the larynx, anterior to the trachea.

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    Main Thyroid Hormones

    The thyroid secretes T4 (thyroxine) and T3 (triiodothyronine).

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    Thyroid Hormone Secretion Control

    Thyroid secretion is controlled by TSH from the anterior pituitary gland.

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    Hormone Potency

    T3 is four times more potent than T4 but lasts shorter.

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    Colloid in Thyroid Follicles

    Colloid is composed of thyroid hormones attached to thyroglobulin.

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    Thyroglobulin Production

    Thyroglobulin synthesized from tyrosine in follicular cells.

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    Iodide Pump Function

    Na+–I– cotransport pump absorbs iodide in follicular cells.

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    Organification Process

    Tyrosine combines with reactive iodine to form MIT and DIT.

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    Cardiac Output

    The volume of blood the heart pumps effectively per minute, influenced by heart rate and stroke volume.

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    Thyroid Hormones

    Hormones that regulate metabolism, influencing heart rate, and enzymatic synthesis.

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    β1-Adrenergic Receptors

    Receptors in the heart that increase the heart's contractility and rate when stimulated.

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    Metabolic Enzymes

    Enzymes that facilitate biochemical reactions in metabolism, regulated by thyroid hormones.

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    Lipolysis

    The breakdown of fats into fatty acids and glycerol, increased by thyroid hormones.

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    Growth Effects

    Thyroid hormones enhance growth and development, particularly in children and during fetal life.

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    CNS Effects

    Thyroid hormones influence brain function, with hypothyroidism causing cognitive impairments.

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    Sympathetic Nervous System Interaction

    Thyroid hormones interact with this system, influencing heart rate and metabolism similarly to catecholamines.

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    Study Notes

    The Thyroid Gland

    • Located immediately below the larynx, on each side of and anterior to the trachea
    • Secretes thyroxine (T4) and triiodothyronine (T3), major metabolic hormones
    • Thyroid secretion primarily controlled by thyroid-stimulating hormone (TSH) from the anterior pituitary gland
    • Contains C cells that secrete calcitonin

    Synthesis and Secretion of Thyroid Hormones

    • 93% of the metabolically active hormones secreted by the thyroid gland is thyroxine (T4), 7% is triiodothyronine (T3)
    • Almost all T4 is converted to T3 in the tissues, both are functionally important
    • T3 is about four times more potent than T4 but persists for a shorter time

    Thyroid Hormone Synthesis

    • Synthesized by follicular epithelial cells
    • Cells have a basal membrane facing the blood and an apical membrane facing the follicular lumen
    • Follicular lumen contains colloid, composed of newly synthesized thyroid hormones attached to thyroglobulin (TG)
    • When stimulated, colloidal hormone absorbed into follicular cells by endocytosis

    Thyroid Hormone Biosynthesis (detailed steps)

    • Iodide (I-) is taken up via sodium/iodide symporter (NIS)
    • Iodide secreted via pendrin channel
    • Iodide converted to iodide (I2) via oxidation
    • I2 combines with thyroglobulin (Tg) to form MIT and DIT
    • MIT and DIT combine to form T3 and T4
    • Vesicles fuse with lysosomes, cleaving T3 and T4 from Tg
    • Free T3 and T4 secreted into the blood

    Thyroid Hormone Action

    • T4 and T3 bind to nuclear receptors and activate gene transcription of various proteins
    • Increased basal metabolic rate (BMR)
    • Increases oxygen consumption and heat production
    • Increases heart rate and contractility
    • Affects growth, development of CNS, and other functions

    Thyroid Hormone Regulation

    • Hypothalamus releases thyrotropin-releasing hormone (TRH)
    • TRH stimulates the anterior pituitary to release TSH
    • TSH stimulates the thyroid gland to produce and release T3 and T4
    • Negative feedback: elevated T3 and T4 levels inhibit TRH and TSH release

    Graves Disease

    • Autoimmune disorder
    • Increased circulating levels of thyroid-stimulating immunoglobulins (TSI)
    • TSI cause similar response as TSH
    • Thyroid hormones increase
    • TSH levels are lower than normal due to negative feedback

    Hypothyroidism

    • Decreased thyroid hormone production
    • Deficiencies in iodine, autoimmune diseases, or pituitary/hypothalamic problems

    Symptoms of Hyperthyroidism

    • Increased excitability, intolerance to heat, sweating, weight loss, muscle weakness, diarrhea.
    • Nervousness, psychic disorders, extreme fatigue, tremor in hands, exophthalmos (bulging eyes).

    Symptoms of Hypothyroidism

    • Decreased metabolic rate, cold intolerance, decreased sweating, weight gain, slowness of movement/speech.
    • Lethargy and sleepiness, constipation, mental sluggishness, myxedema (edema).

    Goiter

    • Enlarged thyroid gland
    • Caused by increased TSH (high TSH, not usually due to a tumor, but usually an autoimmune condition), iodine deficiency, or certain tumors

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    The Thyroid Gland PDF

    Description

    Explore the essential functions and characteristics of the thyroid gland in this quiz. From hormone secretion to metabolic impact, test your knowledge on thyroid hormones like thyroxine (T4) and triiodothyronine (T3). Understand how these hormones affect overall metabolism.

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