Thyroid Gland Overview and Hormone Functions
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Questions and Answers

What is the primary mechanism for the formation of T3 in the body?

  • Deiodination of T4 in peripheral tissues such as the liver and kidneys. (correct)
  • Condensation of one DIT and one MIT molecule within the thyroid gland.
  • Direct synthesis from two DIT molecules in the thyroid follicle.
  • Secretion of T3 directly from the thyroid at a rate of 80 g/day.

    • Which plasma protein has the highest affinity for T4, despite having the smallest binding capacity?

  • Iodotyrosine deiodinase
  • Albumin
  • Prealbumin (Transthyretin)
  • Thyroxine-binding globulin (TBG) (correct)

    • What is the fate of the remaining DIT & MIT within the thyroid follicle that are not immediately used for T3 or T4 synthesis?

  • They are catabolized and excreted in urine.
  • They are secreted directly into the bloodstream.
  • They are stored indefinitely within the follicle for later use.
  • They are deiodinated to recover iodine and tyrosine for hormone synthesis. (correct)

    • Which of the following statements accurately describes the relationship between free and bound thyroid hormones in plasma?

    <p>Bound thyroid hormones act as a reservoir, while free hormones are biologically active and regulate TSH secretion. (C)</p> Signup and view all the answers

    If the production of reverse T3 (rT3) was blocked, how would T4 metabolism be affected?

    <p>T4 would only be converted to T3. (D)</p> Signup and view all the answers

    What is the primary function of thyroid metabolic hormones (T3 and T4) in the body?

    <p>To regulate lipid and carbohydrate metabolism (D)</p> Signup and view all the answers

    What is a potential consequence of hypothyroidism in adult life?

    <p>Physical slowing (D)</p> Signup and view all the answers

    Which of the following statements accurately describes the relationship between T3 and T4?

    <p>T4 is converted to T3 in peripheral tissues. (A)</p> Signup and view all the answers

    During thyroid hormone synthesis, what is the role of thyroglobulin?

    <p>It contains tyrosine residues that are iodinated. (A)</p> Signup and view all the answers

    What is the function of thyroid peroxidase enzymes in thyroid hormone production?

    <p>They mediate the oxidation and reaction of iodine with thyroglobulin. (C)</p> Signup and view all the answers

    How long does the colloid serve as a reservoir for thyroid hormones?

    <p>2 months (A)</p> Signup and view all the answers

    What is the first step in the synthesis of thyroid hormone production

    <p>Transport of iodine into colloid of the thyroid follicles (B)</p> Signup and view all the answers

    Which of the following best describes the structure of thyroglobulin?

    <p>A conjugated protein which includes a carbohydrate (C)</p> Signup and view all the answers

    Flashcards

    Thyroid gland

    Butterfly-shaped gland in front of the neck, largest endocrine gland.

    Metabolic hormones (T4 & T3)

    Thyroid hormones that regulate metabolism, O2 consumption, and influence body mass.

    Hypothyroidism

    Condition resulting in mental/physical slowing and cold intolerance.

    Hyperthyroidism

    Condition causing body wasting, nervousness, and heat production.

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    Thyroglobulin

    Glycoprotein containing tyrosine residues, serves as a thyroid hormone reservoir.

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    Iodination

    Process of adding iodine to tyrosine, essential for thyroid hormone synthesis.

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    Thyroid peroxidase

    Enzyme that mediates oxidation and iodination of tyrosine residues.

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    Colloid

    Fluid in thyroid follicles that stores thyroglobulin and thyroid hormones.

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    T4 Formation

    T4 is formed by the oxidative condensation of two DIT molecules.

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    T3 Formation

    T3 is produced by the condensation of one DIT and one MIT, with some reverse T3 formed as a trace.

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    Thyroid Hormone Distribution

    Thyroid hormones distribution in the follicle: T4 (35%), T3 (7%), DIT (33%), MIT (3%).

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    Plasma Protein Binding

    T4 is bound mostly by thyroxine-binding globulin, with only 0.02% free in plasma.

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    T4 Metabolism

    T4 is mainly metabolized into T3 (33%) and reverse T3 (45%), primarily in the liver and kidneys.

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    Study Notes

    Thyroid Gland

    • The thyroid gland is a butterfly-shaped structure located in the front of the neck.
    • It is one of the largest endocrine glands in the body.
    • It secretes metabolic hormones (T4 and T3) and calcitonin.
    • T4 is the primary secreted hormone; T3 is biologically active.

    Thyroid Hormone Synthesis and Secretion

    • Thyroid hormones are iodinated.
    • T4 (thyroxine) has four iodine atoms, T3 (triiodothyronine) has three.
    • Thyroid hormones are synthesized from tyrosine amino acids.
    • Steps in synthesis include iodine transport, oxidation, iodination of tyrosine, endocytosis of thyroglobulin, and thyroid hormone release.
    • Thyroglobulin is a glycoprotein containing tyrosine.
    • Thyroid peroxidase is a membrane-bound enzyme that oxidizes iodine and couples it with tyrosine.

    Thyroid Metabolic Hormones

    • Function:
      • Stimulate oxygen consumption in body cells.
      • Regulate lipid and carbohydrate metabolism.
      • Influence body mass and mentation.
    • Dysfunction:
      • In fetal/neonatal life: Absence or hypofunction leads to severe mental retardation and dwarfism.
      • In adulthood:
        • Hypothyroidism: Mental and physical slowing, cold intolerance.
        • Hyperthyroidism: Body wasting, nervousness, tachycardia, tremor, excess heat production.

    Thyroid Hormones in Normal Adult Humans

    • Secretion rates:
      • T4: 80 µg/day
      • T3: 4 µg/day
      • RT3: 2 µg/day
      • Most T₃ and RT₃ are formed in tissues by deiodination of T₄.
      • T₄ is conjugated and excreted.

    Thyroid Hormone Transport

    • Thyroid hormones are primarily bound to plasma proteins (TBG, prealbumin, albumin).
    • The bound form acts as a reservoir.
    • The free form of the hormone is biologically active.
    • Normal plasma levels of T4 are around 8 µg/dL, and T3 is about 0.15 µg/dL.

    Thyroid Hormone Metabolism

    • T₄ is deiodinated to T₃ in the periphery.
    • T₃ and T₄ participate in metabolism, and activation.

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    Thyroid Gland PDF

    Description

    This quiz covers the anatomy and physiology of the thyroid gland, including hormone synthesis and secretion. It explores the roles of T4 and T3 in metabolism and the consequences of thyroid dysfunction. Test your understanding of this vital endocrine gland's functions and processes.

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