Thyroid Hormone Regulation
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Thyroid Hormone Regulation

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Questions and Answers

What is the term used to describe advanced hypothyroidism?

  • Myxoedema (correct)
  • Hashimoto disease
  • Thyroid hormone resistance
  • Graves disease

    • What is the main cause of primary hyperthyroidism in adults?

  • Iodine deficiency
  • Graves disease (correct)
  • Hashimoto disease
  • Thyroid hormone resistance

    • What is the result of secondary hypothyroidism?

  • Increased thyroid hormone release
  • Increased TSH secretion
  • Decreased thyroid hormone release
  • Decreased TSH secretion (correct)

    • What is the term used to describe the condition where patients present with signs and symptoms of hypothyroidism without a decrease in circulating thyroid hormone levels?

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

    What is the primary function of the adrenal cortex?

    <p>Secretion of glucocorticoid, mineralocorticoid, and sex steroid hormones</p> Signup and view all the answers

    What is the cause of secondary hyperthyroidism?

    <p>Increased TSH levels due to pituitary adenomas</p> Signup and view all the answers

    What is the result of Hashimoto disease?

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

    What is the result of Graves disease?

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

    At physiologic levels, T4 is relatively inactive because:

    <p>It doesn't enter the cell nucleus at high enough concentrations to occupy the ligand-binding site of the thyroid hormone receptor</p> Signup and view all the answers

    What percentage of plasma T4 and T3 is bound to thyroxine-binding globulin (TBG)?

    <p>70%</p> Signup and view all the answers

    Which of the following hormones stimulates the production of TSH?

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

    What is the half-life of T3?

    <p>1 day</p> Signup and view all the answers

    What is the result of deficient thyroid hormone secretion?

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

    What percentage of each hormone circulates in its free form?

    <p>0.03%</p> Signup and view all the answers

    What is the main function of thyroxine-binding globulin (TBG)?

    <p>To bind to thyroid hormones</p> Signup and view all the answers

    Which of the following thyroid hormones binds more tightly to binding proteins?

    <p>T4</p> Signup and view all the answers

    What is the primary catecholamine secreted by the medulla?

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

    What is the effect of cortisol on protein metabolism in muscle?

    <p>Increased proteolysis</p> Signup and view all the answers

    What percentage of glucocorticoids are protein-bound in plasma?

    <p>90%</p> Signup and view all the answers

    What is the effect of oestrogen treatment on plasma CBG levels?

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

    What is the main stimulus to cortisol secretion?

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

    What is the effect of increased plasma cortisol levels on CRH secretion?

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

    What is the characteristic of cortisol levels in the diurnal rhythm?

    <p>Highest in the morning</p> Signup and view all the answers

    What is the effect of stress on CRH secretion?

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

    Where is TRH synthesized?

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

    What is the role of thyroglobulin in thyroid hormone synthesis?

    <p>It provides a scaffold for thyroid hormone synthesis</p> Signup and view all the answers

    What is the result of TSH receptor activation on thyroid follicular epithelial cells?

    <p>Stimulation of all steps involved in thyroid hormone synthesis</p> Signup and view all the answers

    What is the primary source of circulating T3?

    <p>Peripheral deiodination of T4</p> Signup and view all the answers

    What is the role of thyroid peroxidase in thyroid hormone synthesis?

    <p>It couples iodinated tyrosine residues to form T3 and T4</p> Signup and view all the answers

    What is the ratio of T4 to T3 in the plasma?

    <p>40:1</p> Signup and view all the answers

    Where does the majority of peripheral deiodination of T4 to form T3 occur?

    <p>Liver and kidney</p> Signup and view all the answers

    What is the term for the process of iodine binding to carbon 3 or 5 of the tyrosine residues on thyroglobulin?

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

    What is the primary function of aldosterone in the body?

    <p>To facilitate sodium reabsorption and potassium secretion</p> Signup and view all the answers

    What stimulates aldosterone production in the adrenal glands?

    <p>Renin-angiotensin system</p> Signup and view all the answers

    What is the name of the syndrome caused by overproduction of cortisol?

    <p>Cushing's syndrome</p> Signup and view all the answers

    What percentage of Cushing's syndrome cases are caused by pituitary adenoma?

    <p>70%</p> Signup and view all the answers

    What is the name of the syndrome caused by overproduction of aldosterone?

    <p>Conn's syndrome</p> Signup and view all the answers

    What is the name of the enzyme produced by the kidney in response to decreased blood volume?

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

    What is the primary site of angiotensin-converting enzyme (ACE) activity?

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

    What is the effect of increased plasma potassium levels on aldosterone production?

    <p>It stimulates aldosterone production</p> Signup and view all the answers

    Study Notes

    Thyroid Hormone Synthesis

    • Thyroglobulin (Tg) plays a crucial role in the synthesis and storage of thyroid hormone.
    • Tg is a glycoprotein containing multiple tyrosine residues, which serves as a scaffold for thyroid hormone synthesis.
    • Iodine binds to carbon 3 or 5 of the tyrosine residues on Tg, resulting in the formation of monoiodinated tyrosine (MIT) and diiodinated tyrosine (DIT) residues.
    • The coupling of iodinated tyrosine residues, either of 2 DIT residues or of 1 MIT and 1 DIT residue, is catalyzed by the enzyme thyroid peroxidase (TPO) to form triiodothyronine (T3) or tetraiodothyronine (T4).

    Thyroid Hormone Release and Regulation

    • The thyroid gland releases more T4 than T3, resulting in 40-fold higher plasma concentrations of T4.
    • Most of the circulating T3 is formed peripherally by deiodination of T4, a process that involves the removal of iodine from carbon 5 on the outer ring of T4.
    • T4 acts as a prohormone for T3, and its deiodination occurs predominantly in the liver, kidney, and thyroid follicular epithelial cells.
    • Thyroid hormone synthesis is regulated by the hypothalamic-pituitary-thyroid axis, with TSH being the main stimulator of thyroid hormone synthesis.

    Thyroid Hormone Binding and Metabolism

    • Most of the plasma T4 and T3 is protein bound, mainly to thyroxine-binding globulin (TBG) and, to a lesser extent, transthyretin and albumin.
    • A small fraction of each hormone (0.03% of T4 and 0.3% of T3) circulates in its free form, which is the physiologically active form.
    • T4 has a lower metabolic clearance rate and a longer half-life (7 days) than T3 (1 day) due to its tighter binding to binding proteins.

    Disorders of the Thyroid Gland

    • Hypothyroidism is characterized by insufficient thyroid hormone action, resulting in weight gain, myopathy, menstrual disturbances, and other symptoms.
    • Two main types of hypothyroidism are distinguished: primary hypothyroidism (disease of the thyroid gland) and secondary hypothyroidism (decreased TSH secretion).
    • Hyperthyroidism is characterized by excessive functional activity of the thyroid gland, resulting in weight loss, tachycardia, nervousness, anxiety, and difficulty sleeping.
    • The most common cause of primary hyperthyroidism in adults is Graves' disease, an autoimmune thyroid disease.

    Adrenal Gland Hormones

    • The adrenal gland consists of two distinct tissues: the outer cortex and the inner medulla.
    • The cortex secretes glucocorticoids, mineralocorticoids, and sex steroid hormones, while the medulla secretes catecholamines, principally adrenaline.

    Glucocorticoid Secretion

    • Glucocorticoids have widespread metabolic effects on carbohydrate, fat, and protein metabolism.
    • In the liver, cortisol stimulates gluconeogenesis, amino acid uptake and degradation, and ketogenesis.
    • In adipose tissue, cortisol increases lipolysis, and in muscle, it increases proteolysis and amino acid release.

    Regulation of Glucocorticoid Secretion

    • The main stimulus to cortisol secretion is ACTH, which is regulated by negative feedback control, stress, and the diurnal rhythm of plasma cortisol.
    • The hypothalamic-pituitary-adrenal axis is the main regulatory system controlling cortisol secretion.

    Aldosterone Secretion

    • The principal physiological function of aldosterone is to conserve Na+, mainly by facilitating Na+ reabsorption and reciprocal K+ or H+ secretion in the distal renal tubule.
    • The renin-angiotensin system is the most important system controlling aldosterone secretion.
    • Aldosterone production is stimulated by the renin-angiotensin system, which is activated in response to changes in blood volume, renal perfusion pressure, and plasma K+ levels.

    Adrenocortical Hyperfunction

    • Hyperfunction of the adrenal cortex can lead to overproduction of cortisol (Cushing's syndrome) or aldosterone (Conn's syndrome).
    • Cushing's syndrome can be ACTH-dependent or ACTH-independent, and is caused by tumors that release either ACTH or cortisol.

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    This quiz covers the regulation of thyroid hormones, including the role of TRH, TSH, and the thyroid gland.

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