Thyroid Hormone Synthesis and Function Quiz
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Questions and Answers

What is the primary role of the sodium-iodide symporter (NIS) in thyroid follicular cells?

  • To synthesize thyroid hormones from iodide
  • To transport iodide from the colloid into the bloodstream
  • To stimulate the production of hydrogen peroxide
  • To actively transport iodide from the bloodstream against its gradient (correct)
  • Which substance inhibits iodide uptake in the thyroid gland?

  • Hydrogen peroxide (H2O2)
  • Sodium-iodide symporter
  • Thyroid peroxidase
  • Perchlorate ion (ClO4-) (correct)
  • What is the product of the iodination process involving thyroglobulin?

  • Thyroid-stimulating hormone (TSH)
  • Monoiodinated and diiodinated tyrosines (correct)
  • Iodide
  • Triiodothyronine (T3)
  • Which enzyme is primarily responsible for the conversion of iodide to iodine during thyroid hormone synthesis?

    <p>Thyroid peroxidase (TPO)</p> Signup and view all the answers

    Pendred syndrome is associated with which of the following?

    <p>Congenital hypothyroidism due to failure of iodide transport</p> Signup and view all the answers

    What percentage of circulating T4 is found in a protein-free form?

    <p>0.04%</p> Signup and view all the answers

    Which binding protein has the highest affinity for thyroid hormones?

    <p>Thyroxine binding globulin (TBG)</p> Signup and view all the answers

    What is the approximate concentration of T4 in plasma?

    <p>100 nmol/L</p> Signup and view all the answers

    Compared to T4, how much more biologically active is T3?

    <p>2 to 10 times more</p> Signup and view all the answers

    Which of the following statements about the half-life of thyroid hormones is correct?

    <p>rT3 has a half-life of 5 hours</p> Signup and view all the answers

    What is the structural unit of the thyroid gland?

    <p>Thyroid follicle</p> Signup and view all the answers

    Which cells in the thyroid gland are responsible for producing calcitonin?

    <p>C-Cells</p> Signup and view all the answers

    What artery is NOT involved in the arterial supply of the thyroid gland?

    <p>Brachiocephalic artery</p> Signup and view all the answers

    What is the role of capillaries associated with the thyroid follicle?

    <p>Transport of nutrients and hormones</p> Signup and view all the answers

    From which structure does the thyroid gland originate during embryonic development?

    <p>Thyroglossal duct</p> Signup and view all the answers

    Which nerve, when damaged, may lead to vocal cord paralysis?

    <p>Recurrent laryngeal nerve</p> Signup and view all the answers

    What anatomical feature connects the two lobes of the thyroid gland?

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

    At what point in gestation does the thyroid gland begin to develop?

    <p>Third week</p> Signup and view all the answers

    Which of the following roles is NOT associated with thyroid hormones?

    <p>Increasing synthesis of red blood cells</p> Signup and view all the answers

    What is the recommended daily intake of iodine for lactating women?

    <p>290 mcg</p> Signup and view all the answers

    Which micronutrient is essential for thyroid hormone synthesis?

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

    What condition can arise due to severe iodine deficiency during pregnancy?

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

    What is the primary function of thyroglobulin in thyroid hormone synthesis?

    <p>Facilitating the coupling of thyroid hormones</p> Signup and view all the answers

    Endemic goiter is often associated with which geographical condition?

    <p>Deficient soil iodide</p> Signup and view all the answers

    What is the role of Na+-K+ ATPase in relation to thyroid hormones?

    <p>Increasing metabolic rate and heat production</p> Signup and view all the answers

    Which step in the synthesis of thyroid hormones involves the transformation of iodide?

    <p>Iodide trapping</p> Signup and view all the answers

    What happens to the thyroglossal duct if it does not atrophy?

    <p>It forms a thyroglossal duct cyst.</p> Signup and view all the answers

    Which week of gestation marks the final descent of the thyroid gland to its position anterior to the trachea?

    <p>By the 8th week</p> Signup and view all the answers

    What is the primary function of T3 and T4 thyroid hormones?

    <p>They synthesize from iodide and tyrosine residues.</p> Signup and view all the answers

    What does the presence of reverse T3 (rT3) indicate?

    <p>It is a biologically inactive form of thyroid hormone.</p> Signup and view all the answers

    In the case of ectopic thyroid tissue, what typically occurs?

    <p>It develops along the path of the thyroglossal duct.</p> Signup and view all the answers

    Which of the following is a characteristic presentation of a thyroglossal duct cyst?

    <p>50% presenting as midline cystic masses below the level of the hyoid bone.</p> Signup and view all the answers

    What is the significance of elevated Thyrotropin (TSH) levels in a patient?

    <p>It can suggest hypothyroidism.</p> Signup and view all the answers

    What is the expected normal range for total Thyroxine (T4) levels?

    <p>5-12 mcg/dl</p> Signup and view all the answers

    What is the role of the thyroid hormone in carbohydrate metabolism?

    <p>Enhance glycolysis and gluconeogenesis</p> Signup and view all the answers

    What effect does thyroid hormone binding have at the cellular level?

    <p>Induces transcription through its receptor</p> Signup and view all the answers

    Which of the following is NOT an effect of thyroid hormone on fat metabolism?

    <p>Decreased mobilization of lipids from adipose tissue</p> Signup and view all the answers

    What is the impact of thyroid hormone on cardiovascular function?

    <p>Increased heart strength and output</p> Signup and view all the answers

    Which process is stimulated by thyroid hormone as part of its metabolic effects?

    <p>Increase in oxygen consumption</p> Signup and view all the answers

    Which metabolic function is NOT directly stimulated by thyroid hormone?

    <p>Increased insulin resistance</p> Signup and view all the answers

    What effect does thyroid hormone NOT have on the cardiovascular system?

    <p>Decreased heart rate</p> Signup and view all the answers

    Which of the following effects is associated with hypothyroidism?

    <p>Increased plasma cholesterol</p> Signup and view all the answers

    How does thyroid hormone influence metabolic rate?

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

    What mechanism mediates the transcriptional effect of thyroid hormone binding to its receptor?

    <p>Addition of a coactivator and release of a co-repressor</p> Signup and view all the answers

    What role does increased gluconeogenesis play in the body when present in higher excess?

    <p>Stimulates the breakdown of proteins</p> Signup and view all the answers

    Which of the following is NOT an effect of growth hormone on development?

    <p>Promotes emotional dysregulation</p> Signup and view all the answers

    What are essential functions of the respiratory system's increased respiration?

    <p>Stimulates learning and memory capacity</p> Signup and view all the answers

    How does growth hormone impact skeletal muscle contraction?

    <p>Increases the speed and amplitude of contraction</p> Signup and view all the answers

    Which process is stimulated directly by growth hormone in the body?

    <p>Synthesis of structural and enzymatic proteins</p> Signup and view all the answers

    Considering the binding affinities, which protein has the lowest affinity for thyroid hormones?

    <p>Serum albumin</p> Signup and view all the answers

    Which thyroid hormone serves primarily as a prohormone for another hormone?

    <p>Thyroxine (T4)</p> Signup and view all the answers

    What is the biological activity relationship between T3 and T4?

    <p>T3 is significantly more biologically active than T4.</p> Signup and view all the answers

    What is the approximate half-life of Triiodothyronine (T3)?

    <p>1-3 days</p> Signup and view all the answers

    What is the primary consequence of severe iodine deficiency during pregnancy?

    <p>Significant cognitive delay and stunted growth</p> Signup and view all the answers

    Which characteristic is most associated with endemic goiter in iodine-deficient areas?

    <p>Enlargement of the thyroid gland</p> Signup and view all the answers

    What percentage of the recommended daily intake of iodine is advised for adults?

    <p>150 mcg</p> Signup and view all the answers

    Which of the following is a key building block required for the synthesis of thyroid hormones?

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

    What is the role of thyroglobulin in thyroid hormone synthesis?

    <p>To act as a precursor for thyroid hormone production</p> Signup and view all the answers

    Which tissue type is least affected by thyroid hormones?

    <p>Brain tissue</p> Signup and view all the answers

    What is the primary method of preventing iodine deficiency in populations?

    <p>Use of iodized table salt</p> Signup and view all the answers

    Which of the following is true regarding the basal metabolic rate (BMR) and thyroid hormones?

    <p>Thyroid hormones regulate BMR by increasing metabolic activity.</p> Signup and view all the answers

    What happens to the thyroglossal duct during normal embryonic development?

    <p>It may persist as a remnant.</p> Signup and view all the answers

    Which of the following conditions can result from ectopic thyroid tissue?

    <p>Thyroid hormone deficiency.</p> Signup and view all the answers

    During which week of gestation does the thyroid gland assume its mature shape?

    <p>7-8 weeks.</p> Signup and view all the answers

    In what context is reverse T3 (rT3) significant?

    <p>It is biologically inactive.</p> Signup and view all the answers

    What can be a clinical manifestation of a thyroglossal duct cyst?

    <p>Cystic masses below the hyoid bone.</p> Signup and view all the answers

    What is the primary source of the building blocks for thyroid hormones?

    <p>Iodide and tyrosine residues.</p> Signup and view all the answers

    Which age group is most likely to present with symptoms of ectopic thyroid tissue?

    <p>Newborns.</p> Signup and view all the answers

    What is the expected outcome if a patient has a significantly elevated Thyrotropin (TSH) level with low T4?

    <p>Hypothyroidism is indicated.</p> Signup and view all the answers

    Which change in the TSH level indicates hyperthyroidism?

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

    Which condition is characterized by high levels of both TSH and TRH?

    <p>Primary hypothyroidism</p> Signup and view all the answers

    What is the normal range for Free Triiodothyronine (FT3) levels?

    <p>2.3 - 4.2 pg/mL</p> Signup and view all the answers

    In cases of TSH deficiency secondary to hypothyroidism, which of the following hormone levels would be expected?

    <p>N or ↓ TSH, ↓ T4, ↓ T3</p> Signup and view all the answers

    Which of the following best describes the normal relationship between TSH and Free Thyroxine (FT4)?

    <p>Inverse relationship</p> Signup and view all the answers

    What is the role of Thyroid Stimulating Hormone (TSH) in the hypothalamic-pituitary-thyroid axis?

    <p>Stimulates T4 and T3 secretion from the thyroid</p> Signup and view all the answers

    In the laboratory diagnosis of a patient with suspected thyroid issues, which test is considered the optimal screening test?

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

    Which of the following statements is true regarding the TSH normal range?

    <p>Normal range is 0.5-4.5 mU/L</p> Signup and view all the answers

    Study Notes

    Thyroid Hormone Synthesis, Transport and Cellular Mechanism

    • Learning Outcomes: Describe the structure and location of the thyroid; Describe the origin of the thyroid gland and its functional relationships; Explain the mechanisms and control of thyroid hormone synthesis; Outline the biochemical and clinical functions of thyroid hormones.

    Thyroid Gland: Location and Structure

    • Location: Situated below the thyroid cartilage, behind the strap muscles.
    • Structure: Comprises two lobes joined by an isthmus, located below the cricoid cartilage.
    • Further variation: Pyramidal lobe present in ~50% of adults; non-palpable, enlarges with disease.

    Thyroid Gland & Follicles

    • Functional unit: The thyroid follicle (or acinus).
    • Components: Follicular (epithelial) cells and a lumen filled with colloid.
    • Follicular cells: Deliver nutrients and transport hormones; sympathetic nervous system influences hormone synthesis/secretion, and lymphatic system drains excess fluid.
    • Parafollicular cells (C cells): Produce calcitonin.

    Thyroid Gland: Blood Supply & Innervation

    • Arterial supply: Superior thyroid artery (from external carotid), inferior thyroid artery (from thyrocervical trunk).
    • Venous drainage: Superior and middle thyroid veins; inferior thyroid vein → brachiocephalic vein.
    • Innervation: Recurrent laryngeal nerve (branch of vagus); damage may cause vocal cord paralysis.
    • Emergency airway: Cricothyrotomy through the cricothyroid membrane.

    Thyroid Gland Origin - Embryology

    • Development: The thyroid is the first endocrine gland to develop, on about the 24th day of gestation (3rd week).
    • Origin: Arises from the 1st pharyngeal arch.
    • Early development: Begins as a diverticulum (thyroid diverticulum) in the floor of the pharynx.
    • Descent: Descends in the neck as an initially hollow structure, later solidifying and becoming bilobed, connected by an isthmus.
    • Later development (5 weeks): Initial descent anterior to pharyngeal gut; still connected to the tongue by the thyroglossal duct; eventually bifurcates into two lobes.
    • Further development (7th week): Forms mature shape and location anterior to trachea; thyroglossal duct obliterates by 7-10 weeks, but remnants may persist.
    • Ectopic tissue: Ectopic thyroid tissue can arise along this pathway.

    Ectopic Thyroid Gland

    • Lingual Thyroid: A 4-year-old girl may have slowed growth with elevated TSH (55 ulU/ml) and low T4 (3.5 mcg/dL). (Example)

    Thyroid Hormone: An Overview

    • Synthesis: Thyroid hormones (T3 and T4) are synthesized from iodide (I) and tyrosine residues of thyroglobulin.
    • Types of thyroid hormones: Tyrosine, Thyroxine (T4), Triiodothyronine (T3) and Reverse T3 (rT3).

    What is Thyroid Hormone?

    • Essential functions: Critical for brain development, skeletal function, and growth in infants; Regulates metabolic activity (except brain/spleen/testes); increases basal metabolic rate and oxygen consumption, increasing heat production by stimulating Na+-K+ ATPase; affects virtually all tissues.

    Thyroid Hormone Synthesis - Building Blocks

    • Ingredients: Iodine, thyroglobulin, and tyrosine.

    Iodine

    • Importance: Essential micronutrient found in seafood, dairy products, and vegetables.
    • Recommended daily intake (IOM): Kids (90-130 mcg); Adults (150 mcg); Pregnant women (220 mcg); Lactating women (290 mcg).

    Thyroid Hormone Mechanisms and Control

    • Iodide trapping: Active transport of iodide from bloodstream against gradient, stimulated by TSH, inhibited by perchlorate; Pendrin transports iodide into colloid. (details on specific transporters)
    • Oxidation/Organification: Iodide is converted to iodine (I2), catalyzed by thyroid peroxidase (TPO).
    • Coupling: Formation of T3 and T4 (through coupling of MIT and DIT) (details on specific coupling reactions).
    • Endocytosis: Thyroglobulin containing T3 and T4 is endocytosed into the follicular cell.
    • Proteolysis: Lysosomal proteases digest thyroglobulin, releasing T3 and T4.
    • Transport: T3 and T4 are bound to proteins (TBG, TBPA, albumin) to travel in the bloodstream. (Details of protein binding for T4 and T3).
    • Conversion: T4 is converted into T3 by peripheral tissues (liver and kidney) via deiodinases. (Mechanisms of deiodination).
    • Regulation: TRH from hypothalamus triggers TSH release from the anterior pituitary stimulating thyroid hormone synthesis and release. Negative feedback loop regulates these processes., (exact negative feedback loop).

    Thyroid Response to TSH

    • Effects: Increased cAMP, calmodulin, and kinase activity, Increased uptake of colloid, Increased liberation of T3 and T4, Increased production/uptake of thyroglobulin, Increased Iodination of thyroglobulin, Increased size/activity of thyroid cells. (More detail on the specific response pathways)

    Inhibitor of Thyroid Hormone Synthesis

    • Important factors: Iodine availability, integrity of thyroid gland/tissue, conversion of T4→T3, and proper regulation (details).
    • Possible Inhibitors: Drugs, goitrogens, and specific examples.

    Thyroid Hormone Action

    • Mechanism: T3 binds to the thyroid receptor (TR), forming a complex that binds to thyroid response elements in DNA, affecting gene transcription, including additional details on the role of RXR (retinoid X receptor).
    • Functions: Metabolic effects (carbohydrate and lipid metabolism, temperature), Cardiovascular effects (cardiac output), CNS effects (development), growth effects, and intermediary effects on carbohydrate, lipid, and protein metabolism. (More in-depth description of each function).

    Tests of Thyroid Function

    • TSH: Measures pituitary secretion of TSH in response to T4 and T3 feedback.
    • Free T4: Assesses free unbound T4 levels, including specific units (pmol/L).
    • Free T3: Measures free unbound T3 levels.
    • Total T4: Measures bound and free T4.
    • Total T3: Measures bound and free T3.

    TSH Inverse Relationship with T4

    • Variation: Wide variation in individual TSH:T4 “setpoints” (and more detail).
    • Usefulness: Specific TSH:T4 setpoint for each individual.

    TSH Assay in Healthy Patients

    • Optimality: TSH assay is the optimal screening test for ambulatory healthy patients to evaluate thyroid status (hypothyroid/euthyroid/hyperthyroid), along with specific ranges.

    Thyroid Disorders

    • Disorders: Primary hypothyroidism (more details on specific symptoms), Graves' disease (autoimmune mimicking TSH stimulation), TSH deficiency (secondary hypothyroidism), further defining each, including the signs and symptoms.
    • Patterns of TSH, T4, T3, FT4, FT3: Each disorder results in characteristic patterns in the assessed values.

    Case Presentations (Examples)

    • Case 1 (Hyperthyroidism): A 46-year-old woman with elevated total T4 (256 nmol/L), free T4 (34.6 pmol/L), and low TSH (<0.01 mU/L) (Examples with specific test results and interpretations).
    • Case 2 (Hypothyroidism): A 72-year-old woman with low free T4 (<5 pmol/L) and elevated TSH (150 mU/L). (Examples with specific test results and interpretations)

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    Description

    Test your knowledge on thyroid hormone synthesis and the roles of various components within the thyroid gland. This quiz covers topics such as the sodium-iodide symporter, iodide uptake inhibitors, and the structure of thyroid tissues. Challenge yourself with questions related to the production and function of thyroid hormones like T3 and T4.

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