Thyroid Hormones Overview
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What is the primary function of the thyroid receptor (TR) when thyroid hormone (TH) is absent?

  • Repress gene transcription (correct)
  • Increase gene transcription
  • Promote carbohydrate metabolism
  • Enhance protein synthesis

    • Thyroid hormones have significant effects only during adulthood and have no impact on development and growth.

    False

    What role do thyroid hormones play in the metabolism of carbohydrates, fats, and proteins?

    They increase the turnover of carbohydrates, fats, and proteins to ensure adequate cellular energy.

    The thyroid receptor functions as a _______ with Retinoic Acid X receptor (RXR).

    <p>heterodimer</p> Signup and view all the answers

    Match the following physiological effects of thyroid hormones to their corresponding descriptions:

    <p>Increased metabolic rate = Promotes energy availability Improved responsiveness to catecholamines = Increases adrenoceptor expression Enhanced oxygen consumption = Adjusts heat loss through sweating Stimulated growth of neurones = Important for CNS maturation</p> Signup and view all the answers

    What is one of the physiological effects of thyroid hormones on the cardiovascular system?

    <p>Enhanced sensitivity of beta-1 adrenoceptors</p> Signup and view all the answers

    Iodide stored in the thyroid gland has a low turnover rate, which leads to a deficiency when not absorbed daily.

    <p>False</p> Signup and view all the answers

    What are two primary categories of biological responses influenced by thyroid hormones?

    <p>Effects on cellular differentiation and development, and effects on metabolic pathways.</p> Signup and view all the answers

    What is the primary action of thioureylenes in the treatment of hyperthyroidism?

    <p>Inhibit thyroid hormone synthesis</p> Signup and view all the answers

    Radioiodine therapy is safe for use in pregnant mothers.

    <p>False</p> Signup and view all the answers

    What major effect does potassium iodide (KI) have on thyroid hormone biosynthesis when intake exceeds 2 mg/day?

    <p>Suppresses NADPH oxidase activity and thyroid peroxidase gene expression.</p> Signup and view all the answers

    The major method of treating hyperthyroidism is __________.

    <p>radioiodine (I131)</p> Signup and view all the answers

    Match the following drugs to their primary mechanism of action in hyperthyroidism treatment:

    <p>Propylthiouracil = Inhibits deiodinase enzyme Carbimazole = Inhibits iodination of tyrosine Radioiodine = Causes localized cell damage Potassium Iodide = Suppresses hormone biosynthesis</p> Signup and view all the answers

    What hormone is responsible for stimulating the secretion of thyroid hormones?

    <p>TRH</p> Signup and view all the answers

    Hyperthyroidism can be caused by autoimmune diseases like Hashimoto's Disease.

    <p>False</p> Signup and view all the answers

    What role does iodide play in the synthesis of thyroid hormones?

    <p>Iodide is essential for the synthesis of thyroid hormones and aids in their production.</p> Signup and view all the answers

    The metabolic effects of thyroid hormones include an increase in _____ and response to starvation.

    <p>basal metabolic rate (BMR)</p> Signup and view all the answers

    Match the thyroid conditions with their causes:

    <p>Hyperthyroidism = Graves Disease Hypothyroidism = Hashimoto Disease Goitre = Enlargement due to hormone imbalance Secondary Hyperthyroidism = Excess TSH from a tumor</p> Signup and view all the answers

    Which of the following is NOT a characteristic of catecholamine action related to thyroid hormones?

    <p>Decreased intracellular cAMP</p> Signup and view all the answers

    Thyroid hormones enhance wakefulness and alertness.

    <p>True</p> Signup and view all the answers

    What is the primary function of T3 and T4 hormones?

    <p>They increase metabolism in peripheral tissues.</p> Signup and view all the answers

    Excessive secretion of _____ from a tumor can lead to secondary hyperthyroidism.

    <p>TSH</p> Signup and view all the answers

    Match the hormone with its effect:

    <p>TRH = Stimulates TSH release TSH = Stimulates thyroid hormone synthesis T4 = Regulates metabolism cAMP = Acts as a secondary messenger</p> Signup and view all the answers

    What is the primary effect of the Wolff-Chaikoff effect on the thyroid gland?

    <p>Suppresses thyroid hormone production</p> Signup and view all the answers

    Lugol's solution is a mixture used for long-term thyroid suppression.

    <p>False</p> Signup and view all the answers

    What is the initial preferred treatment for hypothyroidism?

    <p>Synthetic L-thyroxine (T4)</p> Signup and view all the answers

    ____ is used in emergencies to treat 'hypothyroid coma' due to its rapid action compared to T4.

    <p>T3</p> Signup and view all the answers

    Match the thyroid hormone with its role or effect:

    <p>T4 = Replacement therapy for hypothyroidism T3 = Emergency treatment for hypothyroid coma MCT8 = Transporter for T3 in neurons D2 = Converts T4 to T3 in glial cells</p> Signup and view all the answers

    Which of the following statements about Propranolol is true?

    <p>It is used to manage symptoms of hyperthyroidism.</p> Signup and view all the answers

    Males have two copies of the MCT8 gene, which can lead to thyroid dysfunction.

    <p>False</p> Signup and view all the answers

    What happens to T4 when it enters a glial cell?

    <p>It is converted to T3 by D2 deiodinase.</p> Signup and view all the answers

    Hypothyroid individuals may experience weight gain despite a ______ appetite.

    <p>poor</p> Signup and view all the answers

    What is a potential side effect of excessive thyroid hormone replacement therapy?

    <p>Cardiac dysfunction</p> Signup and view all the answers

    What is the primary cause of goitre worldwide?

    <p>Iodine deficiency</p> Signup and view all the answers

    Iodine intake affects thyroid hormone production and can lead to the development of goitre.

    <p>True</p> Signup and view all the answers

    What are the major clinical signs of Graves disease?

    <p>Exophthalmos and periorbital edema</p> Signup and view all the answers

    The 'iodide pump' of the thyroid gland is very effective, with up to ____% of ingested iodide being taken up.

    <p>30</p> Signup and view all the answers

    Match each condition with its characteristic feature:

    <p>Goitre = Enlarged thyroid gland Graves Disease = Weight loss despite increased appetite Hypothyroid Goitre = Thyroid gland growth without hyperthyroidism Pituitary adenoma = Excess TSH production</p> Signup and view all the answers

    Which hormone is typically found in low levels in patients with Graves disease?

    <p>TSH</p> Signup and view all the answers

    Hypothyroidism caused by hypothalamic or pituitary failure results in goitre.

    <p>False</p> Signup and view all the answers

    What physiological effects are associated with hyperthyroidism?

    <p>Increased heat production, excessive sweating, tremors, and altered heart rate.</p> Signup and view all the answers

    During conditions of insufficient iodide intake, the thyroid gland can grow from 25 g to ____ g.

    <p>250</p> Signup and view all the answers

    Which disease is characterized by goitre and causes difficulty in swallowing or breathing?

    <p>Graves Disease</p> Signup and view all the answers

    Study Notes

    Thyroid Hormone Overview

    • Thyroid hormone is crucial for metabolic processes.
    • The thyroid gland produces T4, T3, and rT3 hormones that regulate basal metabolic rate (BMR).

    Thyroid Hormone Structure and Function

    • 3,5,3',5'-tetraiodothyronine (L-thyroxine/L-T4) and 3,5,3'-triiodothyronine (L-triiodothyronine/L-T3) are critical thyroid hormones, with T4 being produced in greater amounts. T3 has higher potency than T4.

    • A small percentage of produced hormone is reversed T3 (rT3) which is inactive.

    • T4 is converted to T3 by a "deiodinase" enzyme in peripheral tissues.

    • T3 binds to T3 receptors (TRα and TRẞ) within the cell nucleus. This binding regulates mRNA synthesis and protein synthesis.

    • Calcitonin, secreted by parafollicular cells, is also a thyroid hormone.

    Thyroid Gland Structure

    • Thyroid follicles contain colloid.
    • Colloid is a glycoprotein.
    • Follicular cells secrete thyroid hormones.
    • Parafollicular cells secrete calcitonin.

    Thyroid Gland Products Synthesis

    • Thyroglobulin (TG/TRG), synthesized by thyroid epithelial cells, is secreted into the follicle lumen. TG comprises 134 tyrosines.

    • Iodide (I⁻) is avidly absorbed from blood via sodium-iodide symporter.

    • Iodide travels to the colloid along with thyroglobulin.

    • Thyroid peroxidase catalyzes the iodination and coupling sequential reactions to create thyroid hormones.

    • Thyroid epithelial cells ingest colloid via endocytosis and fuse with lysosomes that contain enzymes for thyroglobulin digestion.

    • Free thyroid hormones diffuse into the blood and bind to carrier proteins (TBG, transthyretin, albumin).

    Iodine Distribution and Turnover

    • Approximately 400 µg of iodine is consumed and excreted daily.

    • Adults require 150 µg iodine a day.

    • For pregnant women, it is 200 µg.

    • The thyroid gland stores a substantial amount of iodine (7500 µg) as iodothyronines, with 70-80 µg released daily.

    • The high iodine storage supports body against deficiencies for extended periods.

    Thyroid Hormone Receptors

    • Thyroid hormones act as heterodimers with Retinoic Acid X receptor (RXR).

    • TR-RXR complexes bind to Thyroid Response Elements (TREs) on the target genes.

    • In the absence of TH, TR-RXR represses gene transcription through recruitment of a corepressor complex (containing histone deacetylase (HDAC)).

    • When TH is present, corepressor complexes are released and coactivator complexes (including histone acetyltransferase (HAT)) are recruited. HAT complexes promote transcription by increasing histone acetylation.

    Physiological Effects of Thyroid Hormones

    • Thyroid hormones regulate cellular differentiation and development, mainly in the nervous system.
    • Thyroid hormones control the metabolism of carbohydrates, lipids, and proteins. These actions are interconnected across the body.

    Physiological Effects on Metabolism and Growth

    • Thyroid hormones enhance basal metabolic rate (BMR), increase oxygen consumption, and boost heat production.

    • During development and growth, thyroid hormones stimulate neural growth and central nervous system maturation.

    • Adequate thyroid hormones after birth are necessary for subsequent mental and physical development.

    Physiological Effects on Cardiovascular System

    • Thyroid hormones modify Ca2+ channels in the sarcoplasmic reticulum and enhance adrenoceptor sensitivity by noradrenaline.
    • Thyroid hormones accelerate metabolic responses to starvation, as well as to autonomic nervous system signals.

    Effects on the Nervous System

    • Thyroid hormones enhance wakefulness, alertness and responsiveness to stimuli.

    Thyroid Hormone Secretion Regulation

    • TRH (thyrotropic releasing hormone) from the hypothalamus stimulates the anterior pituitary to release TSH (thyroid stimulating hormone).

    • TSH subsequently stimulates the thyroid gland to produce T3 and T4. Circulating T3 and T4 levels regulate the negative feedback loop governing TRH and TSH.

    • Factors such as low BMR, cold, trauma, and stress can increase TRH and TSH release.

    • Estrogens and glucocorticoids impact thyroid hormone regulation.

    Abnormalities of Thyroid Function

    • Hyperthyroidism can occur due to autoimmune disease (Graves' disease), benign thyroid tumor, or excessive TSH secretion.
    • Hypothyroidism symptoms involve autoimmune disease (Hashimoto's disease) and/or hypothalamic /pituitary dysfunction.

    Goitre

    • An enlarged thyroid gland, often caused by iodine deficiency, can result in hypothyroidism or hyperthyroidism.
    • Grave's disease or excess TSH from the pituitary can also cause goitre.

    Hyperthyroidism - Graves Disease

    • Characteristic of Graves' disease is weight loss despite increased food intake. (increased BMR and increased heat production).
    • Other symptoms: increased heart rate, difficulty swallowing/breathing, exophthalmos, and periorbital edema.
    • Diagnosis via elevated serum free/total T3/T4 and low serum TSH levels.
    • The disease is primarily caused by autoantibodies (thyroid stimulating immunoglobulin (TSI)) stimulating the thyroid gland.
    • The disorder is considered secondary endocrine, as the problem is pituitary suppression by T3/T4.

    Treatment of Hyperthyroidism

    • Surgery: Removal of the gland is an option, especially if the gland is enlarged or malignant.

    • Drugs:

    • Thioureylenes (propylthiouracil, carbimazole, methimazole): Inhibit thyroid hormone synthesis; they initially decrease thyroid hormone synthesis immediately, but effects can take weeks due to existing gland hormone stores.

    • Iodine-containing preparations (radioiodine): Radioactive iodine is ingested to target the thyroid gland and destroy it, a more permanent solution. A risk is causing hypothyroidism or thyroid damage.

    • ADRENORECEPTOR antagonists: Used to counteract the overactive sympathetic nervous system (e.g. Propranolol) , managing symptoms like increased heart rate and arrhythmias.

    Treatment of Hypothyroidism

    • Replacement therapy: Synthetic T4 (levothyroxine) or T3 is used.
    • Synthetic hormones restore healthy hormone levels.

    MCT8 Mutations

    • MCT8 defects severely hinder the delivery of T3 to the brain.
    • This results in specific types of neurodevelopmental disorders in males, and milder thyroid phenotypes in females.

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    Description

    This quiz provides an in-depth overview of thyroid hormones, including their structure, function, and the role of the thyroid gland in metabolic processes. Test your knowledge on the different types of thyroid hormones, their interactions, and how they regulate various body functions.

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