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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