Thyroid Anatomy and Function Quiz
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Questions and Answers

What is the general location of the thyroid gland in the human body?

  • In front of the cricoid cartilage
  • Adjacent to the trachea
  • Above the hyoid bone
  • Below the thyroid cartilage (correct)

    • Which of the following best describes the structure of the thyroid gland?

  • Divided into four distinct sections
  • Comprises two lobes joined by an isthmus (correct)
  • A spherical structure without lobes
  • A single lobe with no isthmus

    • What additional structure may be present in about 50% of adults related to the thyroid gland?

  • Lateral lobe
  • Submandibular gland
  • Parathyroid glands
  • Pyramidal lobe (correct)

    • Which cartilage is located directly above the thyroid gland?

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

    What anatomical feature is immediately below the thyroid gland?

    <p>Cricoid cartilage</p> Signup and view all the answers

    What is the primary role of the sodium-iodide symporter in iodide trapping?

    <p>It facilitates the active transport of iodide into thyroid follicular cells.</p> Signup and view all the answers

    Which enzyme is responsible for the conversion of iodide (I-) to iodine (I2)?

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

    What is a potential effect of the inhibitory anion perchlorate (ClO4-) on iodide uptake?

    <p>It inhibits iodide uptake.</p> Signup and view all the answers

    What types of iodination can occur with tyrosines in the synthesis of thyroid hormones?

    <p>Monoiodinated (MIT) and diiodinated (DIT)</p> Signup and view all the answers

    What is the clinical significance of Pendred syndrome regarding thyroid function?

    <p>It leads to congenital hypothyroidism.</p> Signup and view all the answers

    What is the role of the thyroid hormone receptor (TR) in gene expression?

    <p>It binds to the thyroid response element of DNA, stimulating transcription.</p> Signup and view all the answers

    Which of the following is NOT a metabolic effect of thyroid hormone?

    <p>Decreased Na+/K+ ATPase activity</p> Signup and view all the answers

    What happens to fat metabolism when thyroid hormones are elevated?

    <p>Increased mobilization of lipids and decreased fat stores.</p> Signup and view all the answers

    How does thyroid hormone affect oxygen consumption in the body?

    <p>It increases oxygen consumption and mitochondrial number.</p> Signup and view all the answers

    What is one systemic effect of thyroid hormones on the cardiovascular system?

    <p>Increased heart strength and cardiac output with higher oxygen demand.</p> Signup and view all the answers

    What effect does increased gluconeogenesis have at high levels?

    <p>Degrades proteins</p> Signup and view all the answers

    Which of the following is NOT considered a benefit of adequate hormone levels for the central nervous system?

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

    Which action is specifically related to the development of the skeletal system?

    <p>Promotes calcification of bones</p> Signup and view all the answers

    How does effective hormone action impact intermediary metabolism?

    <p>Stimulates both synthetic and degradative pathways</p> Signup and view all the answers

    Which effect does growth hormone have on muscle contraction?

    <p>Increases rate and force of contraction</p> Signup and view all the answers

    What happens if the thyroglossal duct does not obliterate during embryonic development?

    <p>A thyroglossal duct cyst may manifest clinically.</p> Signup and view all the answers

    At what gestational week does the thyroid gland typically take its mature shape?

    <p>By the 7th to 8th week.</p> Signup and view all the answers

    Which thyroid hormone is considered biologically inactive?

    <p>Reverse T3 (rT3)</p> Signup and view all the answers

    Where are cystic masses commonly found in cases of thyroglossal duct cyst?

    <p>Just below the level of the hyoid bone.</p> Signup and view all the answers

    Which element is essential for the synthesis of thyroid hormones?

    <p>Iodide (I-)</p> Signup and view all the answers

    In which condition would one expect to see elevated Thyrotropin (TSH) levels?

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

    What is the likely consequence of ectopic thyroid tissue during development?

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

    What is the role of thyroglobulin in thyroid hormone synthesis?

    <p>It contains tyrosine residues necessary for hormone synthesis.</p> Signup and view all the answers

    What is the main product of T4 conversion in peripheral tissues?

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

    How is most of the circulating T3 produced?

    <p>Conversion of T4 in peripheral tissues</p> Signup and view all the answers

    What role does T4 serve in relation to T3?

    <p>It acts as a prohormone.</p> Signup and view all the answers

    Which enzyme converts T4 to inactive rT3?

    <p>Inner-ring monodeiodinase</p> Signup and view all the answers

    What physiological effect does increased thyroid hormones have on TSH release?

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

    What triggers a decrease in TSH levels?

    <p>In increased thyroid hormone levels</p> Signup and view all the answers

    What is the outcome of excessive TSH levels?

    <p>Thyroid gland hypertrophy</p> Signup and view all the answers

    What happens to T4 when the cell has sufficient T3?

    <p>It is converted to rT3.</p> Signup and view all the answers

    Which gland is primarily responsible for TSH synthesis?

    <p>Anterior pituitary gland</p> Signup and view all the answers

    What effect does TRH have on the anterior pituitary gland?

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

    What is one significant role of growth hormone in the central nervous system?

    <p>Enhancing peripheral nerve reflex amplitude</p> Signup and view all the answers

    Which of the following actions is enhanced by growth hormone in relation to skeletal muscle?

    <p>Increased force and rate of contraction</p> Signup and view all the answers

    How does growth hormone impact intermediary metabolism?

    <p>It regulates both synthetic and degradative metabolic pathways</p> Signup and view all the answers

    What role does growth hormone play in the development of the skeletal system?

    <p>Stimulating the expression of growth factors in bone cells</p> Signup and view all the answers

    What specific effect does growth hormone have on general body growth during childhood?

    <p>It stimulates growth and maturation of various body tissues</p> Signup and view all the answers

    What is the primary reason thyroid hormones require carrier proteins in the bloodstream?

    <p>They are hydrophobic and need aid to travel in circulation.</p> Signup and view all the answers

    Which serum protein has the highest affinity for binding thyroid hormones?

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

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

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

    Which of the following statements regarding T3 and T4 is true?

    <p>T4 is secreted in greater amounts than T3.</p> Signup and view all the answers

    What is the typical half-life of T3 in circulation?

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

    What is the primary role of the sodium-iodide symporter in the thyroid follicular endothelial cells?

    <p>It actively transports iodide from the bloodstream into the cells.</p> Signup and view all the answers

    Which of the following describes the effect of TSH on iodide transport?

    <p>TSH enhances the uptake of iodide into the thyroid cells.</p> Signup and view all the answers

    Which condition is linked to Pendred syndrome?

    <p>Congenital hypothyroidism due to defective iodide efflux.</p> Signup and view all the answers

    What is the chemical process occurring with iodide in presence of H2O2 during thyroid hormone synthesis?

    <p>Iodide is oxidized to iodine (I2).</p> Signup and view all the answers

    How are tyrosine residues of thyroglobulin modified during iodination?

    <p>They can be either monoiodinated or diiodinated.</p> Signup and view all the answers

    What occurs if the thyroglossal duct does not fully obliterate during the development of the thyroid gland?

    <p>Formation of a thyroglossal duct cyst</p> Signup and view all the answers

    What does the presence of ectopic thyroid tissue along the thyroglossal duct commonly indicate?

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

    In a clinical assessment, what is the likely consequence for a child with hypothyroidism indicated by high Thyrotropin (TSH) levels?

    <p>Slowed growth and development</p> Signup and view all the answers

    What role does thyroglobulin play in thyroid hormone synthesis?

    <p>Acts as a storage form for T3 and T4</p> Signup and view all the answers

    What is the primary element necessary for the synthesis of thyroid hormones?

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

    What typically occurs when there is an excess of TSH in serum?

    <p>Increased production of T3 and T4</p> Signup and view all the answers

    Which structure is primarily responsible for the production of calcitonin in the thyroid gland?

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

    What is the significance of the thyroglossal duct during the development of the thyroid gland?

    <p>It connects the thyroid to the tongue.</p> Signup and view all the answers

    Which artery provides the primary blood supply to the thyroid gland?

    <p>Superior thyroid artery</p> Signup and view all the answers

    Which physiological impact could result from damage to the recurrent laryngeal nerve during thyroid surgery?

    <p>Vocal cord paralysis</p> Signup and view all the answers

    What is the embryological origin of the thyroid gland?

    <p>First pharyngeal arch</p> Signup and view all the answers

    What role do capillaries play in the function of the thyroid gland?

    <p>Deliver nutrients and transport hormones</p> Signup and view all the answers

    Which type of thyroid hormonal secretion is influenced by sympathetic innervation?

    <p>Hormone synthesis/secretion</p> Signup and view all the answers

    In which direction does the thyroid gland descend during embryonic development?

    <p>Anteriorly to the pharyngeal gut</p> Signup and view all the answers

    What is the primary role of T4 in the body?

    <p>Acts as a precursor to T3</p> Signup and view all the answers

    What happens to T3 levels when there is sufficient T3 in the cell?

    <p>The cell switches to the inner-ring enzyme and produces rT3</p> Signup and view all the answers

    What effect does excessive TSH levels have on the thyroid gland?

    <p>Increased size and activity of thyroid cells</p> Signup and view all the answers

    What enzyme is responsible for converting T4 to inactive rT3?

    <p>Inner-ring monodeiodinase</p> Signup and view all the answers

    How does increased thyroid hormone levels impact TSH release from the pituitary gland?

    <p>It causes TSH levels to decrease to nearly zero</p> Signup and view all the answers

    What is the predominant source of circulating T3 in the body?

    <p>Conversion of T4 in peripheral tissues</p> Signup and view all the answers

    How does TRH affect the anterior pituitary gland?

    <p>Stimulates TSH synthesis and release</p> Signup and view all the answers

    What is the overall impact of iodine deficiency on thyroid hormone levels?

    <p>Increased TSH and decreased T4</p> Signup and view all the answers

    What triggers the secretion of TSH from the anterior pituitary gland?

    <p>Low levels of thyroid hormones</p> Signup and view all the answers

    How does increased cAMP and Ca2+ affect the thyroid gland?

    <p>Enhances colloid uptake and hormone secretion</p> Signup and view all the answers

    Study Notes

    Thyroid Hormone Synthesis, Transport and Cellular Mechanism

    • The thyroid gland is located below the thyroid cartilage, behind the strap muscles.
    • It is a bilobed structure connected by an isthmus, situated below the cricoid cartilage.
    • Approximately 50% of adults have a pyramidal lobe; this is non-palpable and enlarges with disease.
    • Iodide trapping is a critical step in the process stimulated by TSH.

    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 Follicles

    • The functional unit of the thyroid gland is the thyroid follicle (or acinus).
    • Follicles are composed of follicular (epithelial) cells and a lumen filled with colloid.
    • Capillaries deliver nutrients and transport hormones.
    • Sympathetic innervation influences hormone synthesis and secretion.
    • Lymphatics drain excess fluid.
    • C-cells (para-follicular cells) produce calcitonin.

    Blood Supply and 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 (draining to internal jugular vein), brachiocephalic vein.
    • Innervation: recurrent laryngeal nerve (branch of vagus).
    • Damage to recurrent laryngeal nerve may cause vocal cord paralysis.
    • Emergency airway: cricothyrotomy through the cricothyroid membrane.

    Thyroid Gland Origin (Embryology)

    • The thyroid is the first endocrine gland to develop, approximately on day 24 of gestation/3rd week of gestation.
    • The thyroid gland arises from the first pharyngeal arch.
    • Thyroid precursor develops from a thyroglossal duct diverticulum in the floor of the pharynx.
    • The duct descends in the neck as an initially hollow structure solidifying to become bilobed and connected by an isthmus.
    • Initial descent is anterior to the pharynx.
    • Initially, still connected to the tongue by the thyroglossal duct, then bifurcates into two lobes.
    • Thyroglossal duct atrophies in the 7-10 week of gestation, with remnants sometimes persisting.
    • Ectopic thyroid tissue can arise along this path.
    • The thyroid gland origin diagram illustrates the embryological development of the thyroid.

    Thyroid Hormone Synthesis – Building Blocks

    • Iodine: A critical micronutrient in food (seafood, dairy, grains and vegetables). Recommended daily intake varies by age and pregnancy status (Kids 90-130 mcg, Adults 150 mcg, Pregnant 220 mcg, Lactating 290 mcg). Iodized salt is a significant source.
    • Thyroglobulin: Large glycoprotein produced by the rough ER of thyroid follicular epithelial cells. Stored in vesicles and exocytosed into the colloid. Composed of two subunits. Contains tyrosine residues sterically oriented for hormone production.
    • Tyrosine: An amino acid component.

    Thyroid Hormone Synthesis – Steps

    • Iodide trapping (stimulated by TSH): iodide is actively transported from the bloodstream to thyroid follicular cells against its concentration gradient using iodide symporter (Na+/K+ ATPase).
    • Oxidation/Organification: iodide (I-) is converted to iodine (I2) by thyroid peroxidase (TPO) in the presence of hydrogen peroxide (H₂O₂).
    • Coupling: formed products are monoiodotyrosine (MIT) and diiodotyrosine (DIT). Iodine (I₂), combines with tyrosine residues of thyroglobulin (catalyzed by TPO).
    • Endocytosis: stored colloid is pinocytosed into follicular cells.
    • Proteolysis: lysosomal proteases digest thyroglobulin and release T3 and T4. MIT and DIT are degraded.
    • Transport: T4 and T3 diffuse across the basolateral membrane into capillaries and enter the circulation.

    Thyroid Hormone Synthesis - Overview

    • Thyroid hormones (T3 and T4) are synthesized from iodide and tyrosine residues of thyroglobulin.
    • Reverse T3 (rT3): a biologically inactive form, found in significant amounts.

    Transport

    • Thyroid hormones are hydrophobic and need carrier proteins to travel in the bloodstream.
    • T4 is bound more tightly than T3 to serum binding proteins.
    • Affinity of these serum proteins is highest for thyroxine-binding globulin (TBG) followed by thyroxine-binding prealbumin (TBPA), and then albumin. Only a small fraction of T3 and T4 (~0.015% and 0.33% respectively) are unbound in the blood.

    Conversion of Iodothyronines

    • The peripheral tissues (liver and kidneys) convert T4 to T3 by deiodinase enzyme.
    • T3 is 2-10 times more active than T4.
    • Total serum T3 is about 2% of T4.
    • T4 acts as a prohormone for the active T3 after deiodination.

    Thyroid Hormone Regulation

    • Thyroid is under hypothalamic-pituitary control.
    • Negative feedback loop: TRH, TSH, T3, and T4 through the hypothalamic-pituitary-thyroid axis. Hypothalamus releases TRH. Anterior pituitary releases TSH. Thyroid gland releases T3 & T4. Factors controlling thyroid hormone synthesis include the availability of iodine, integrity of the gland, and the conversion of T4 to T3.

    Thyroid Stimulating Hormone (TSH)

    • A glycoprotein synthesized in the anterior pituitary.
    • Secretion occurs in pulses with slightly higher levels at night.
    • TSH stimulates thyroid hormone synthesis and increases the size and vascularity of the thyroid gland.
    • Increased thyroid hormone levels have a negative feedback effect on TSH release from the pituitary.
    • Decreased thyroid hormone levels lead to an increase in TSH release.

    Thyroid Response to TSH

    • Increase in cAMP, Ca²⁺, calmodulin activity, and increase in protein kinase activity.
    • Increase in colloid uptake, Increased liberation of T3 and T4, Increased production of Thyroglobulin, Increased iodine uptake, Increased iodination of thyroglobulin
    • Increased size and activity of thyroid cells, Increased number of thyroid cells (growth).

    Inhibitors of Thyroid Hormone Synthesis

    • Drugs and goitrogens inhibit thyroid hormone synthesis.
    • Iodine and various drugs block iodide trapping and other steps.

    Thyroid Function Tests

    • Key tests are used to measure thyroid function: TSH, free T4, free T3, total T4, and total T3. Normal ranges for these tests are included in a separate section.

    Thyroid Disorders

    • Primary hypothyroidism: Low TSH, low free T4, and free T3.
    • Graves' Disease: High TSH, low free T4 and free T3.
    • TSH deficiency: high TRH, high TSH but low free T4 and free T3.

    Summary of Actions

    • Thyroid hormones have various effects on growth and development, body systems, and intermediary metabolism. Summary of actions also included.

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    Test your knowledge on the thyroid gland's anatomy and physiology with this quiz. It covers the thyroid's location, structure, hormonal function, and related syndromes. Perfect for students studying human anatomy or endocrinology.

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