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Questions and Answers
What is a common symptom of hyperthyroidism?
Which substance can inhibit the synthesis of thyroid hormones?
What is the primary purpose of radioiodine treatment in hyperthyroidism?
In the context of thyroid hormone synthesis, what role does thyroglobulin play?
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Graves' disease is primarily characterized by which of the following?
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What is the primary role of propylthiouracil in thyroid hormone synthesis?
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Why does a decrease in thyroid hormone levels take several weeks to manifest after treatment with propylthiouracil?
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Which of the following describes the mechanism of action of radioiodine in treating hyperthyroidism?
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What is the Wolff-Chaikoff effect associated with the use of potassium iodide and iodine?
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What potential side effect may arise from using radioiodine therapy for hyperthyroidism?
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Propranolol is used in hyperthyroid patients primarily for which purpose?
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How long do the transient effects of potassium iodide remain effective before the thyroid gland becomes tolerant?
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What type of thyroid issue may occur if radioiodine treatment causes excessive damage to the thyroid gland?
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What is the primary purpose of using beta-blockers in patients with hyperthyroidism?
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Which symptom is commonly associated with hypothyroidism?
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What role does T4 play in thyroid hormone replacement therapy?
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Which treatment would be preferred for a patient experiencing a hypothyroid coma?
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What are common neurological effects seen in males with deleterious mutations in the MCT8 gene?
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Which treatment option is commonly used for hyperthyroidism when a malignant tumor is present?
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What is the result of excess cardiac dysfunction due to high doses of administered thyroid hormones?
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What is the inheritance pattern of MCT8 mutations in females?
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What is one of the primary mechanisms through which thioureylene drugs inhibit thyroid hormone synthesis?
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What is the primary mechanism behind the conversion of T4 to T3 in the brain?
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Which of the following is a common side effect of carbimazole or methimazole?
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In Graves disease, which signaling molecule is primarily involved in stimulating the thyroid gland?
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Which characteristic is typical of hyperthyroidism symptoms?
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What is a potential effect of radioiodine treatment for hyperthyroidism?
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Which drug class does propylthiouracil belong to?
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Which of the following accurately differentiates Graves disease from pituitary adenoma?
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Which component of thioureylene drugs is crucial for their antithyroid activity?
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What is a potential outcome of untreated hyperthyroidism?
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Which drug is often used in some countries as an alternative to carbimazole?
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What is one function of beta-adrenoceptor antagonists in the treatment of hyperthyroidism?
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In the context of Graves Disease, which of the following is NOT a common symptom associated with hyperthyroidism?
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Which process is primarily inhibited by antithyroid drugs in the synthesis of thyroid hormones?
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What is the primary mechanism of radioiodine treatment for hyperthyroidism?
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Which thyroid hormone is primarily responsible for increasing metabolism and promoting energy availability?
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What is a common secondary effect on cardiovascular health due to hyperthyroidism?
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How do thyroid hormones affect the Basal Metabolic Rate (BMR)?
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What role does T3 play in the CNS during development?
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What effect do thyroid hormones have on the use of macronutrients in the body?
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Which of these is a permissive effect of thyroid hormones on the sympathetic nervous system?
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In the absence of thyroid hormones, how does the thyroid receptor (TR) affect gene transcription?
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Study Notes
Thyroid Hormone Overview
- Thyroid hormone includes T3, T4, and rT3.
- T4 is secreted in larger quantities than T3.
- 1% of secreted thyroid hormone is rT3, which is inactive.
- T3 is more potent than T4.
- T4 is converted to T3 by a deiodinase enzyme.
- T3 binds to T3 receptors in cell nuclei.
- Calcitonin is secreted by parafollicular cells.
Thyroid Gland Structure
- C cells (parafollicular cells) secrete calcitonin.
- Thyroid epithelial cells (thyrocytes) are organized into follicles.
- Colloid is a glycoprotein.
- The thyroid secretes T3 and T4 and calcitonin.
- Thyroid follicles contain colloid.
- Follicular cells secrete thyroid hormone.
- A capsule of connective tissue surrounds the thyroid follicles.
- Capillaries are present in the connective tissue.
Thyroid Hormone Biosynthesis
- Thyroglobulin (TG) is synthesized in thyroid epithelial cells and secreted into the follicle lumen.
- Iodide (I−) is taken up from blood via a sodium-iodide symporter (iodine trap).
- Synthesis of thyroid hormones involves iodination and coupling of tyrosine residues on thyroglobulin by thyroid peroxidase.
- Thyroid epithelial cells ingest colloid, fuse with lysosomes which contain digestive enzymes, and thyroid hormones diffuse into the blood by carrier proteins like TBG (major).
- Transthyretin (minor, deliveries to the CNS); and albumin (minor)
Iodine Distribution and Turnover
- 400 µg of iodide is ingested and excreted daily.
- Daily minimum requirement: 150 µg for adults, 90-120 µg for children, and 200 µg for pregnant women.
- About 70-80 µg of iodide is taken up daily by the thyroid gland.
- The thyroid gland stores about 7500 µg of iodide in iodothyronines.
- 70–80 µg is released daily. (1% of total)
Thyroid Hormone Action (Mechanism)
- TR functions as a heterodimer with the Retinoic Acid X receptor (RXR).
- TR-RXR binds to thyroid response element (TRE) on the target gene.
- In the absence of TH, TR−RXR represses gene transcription through the recruitment of a corepressor complex.
- When TH is present, the corepressor complex is released and coactivator complexes increase histone acetylation to promote transcription.
Physiological Effects of Thyroid Hormones
- Effects on cellular differentiation and development, especially in the nervous system.
- Effects on metabolic pathways and use of carbohydrates, lipids, and proteins.
- Accelerated metabolism by increasing carbohydrate, fat, and protein turnover to provide adequate cellular energy.
- Increased oxygen consumption and generation of heat.
- Regulates basal metabolic rate and body temperature.
- Promotes growth and maturation of the central nervous system (especially essential during gestation).
- Stimulates the autonomic nervous system and catecholamine action, increasing β-adrenergic receptor numbers and cAMP production in the heart and increasing the response to catecholamine.
- Affects cardiovascular and respiratory effect by altering the expression of ryanodine channels in the sarcoplasmic reticulum to enhance Ca2+ release. Also enhances sensitivity of adrenoceptors (especially B1 receptors)
Regulation of Thyroid Hormone Secretion
- TRH (thyrotropic releasing hormone) from the hypothalamus stimulates the anterior pituitary.
- TSH (thyroid stimulating hormone) from the anterior pituitary stimulates the thyroid.
- Peripheral tissues use/metabolize T4 and T3.
- Low BMR, cold, trauma, and stress trigger TRH release, which stimulates the release of TSH and subsequently, T3/T4.
- Glucocorticoids and estrogens affect the thyroid hormone levels by regulating TRH and TSH receptors.
- Iodide uptake, peroxidase, thyroglobulin synthesis are temporarily inhibited when the serum iodine uptake is high (Wolff-Chaikoff effect).
Thyroid Disorders and Treatment
- Hyperthyroidism (overactive thyroid): Autoimmune disease (Graves Disease), benign tumor, excessive TSH production.
- Hypothyroidism (underactive thyroid): Inflammatory/Autoimmune disease (Hashimoto disease), hypothalamic/pituitary dysfunction, dietary iodine deficiency.
-
Treatment of Hyperthyroidism:
- Surgery (partial or complete removal of gland).
- Medications: Thioureylene compounds, iodine-containing preparations, and beta-adrenergic antagonists (propranolol).
- Treatment of Hypothyroidism: L-thyroxine (T4), L-triiodothyronine (T3), or L-iothyronine, used as "replacement therapy."
MCT8 Mutations
- Males with one copy of the MCT8 gene and a deleterious mutation can have Allan-Herndon-Dudley syndrome.
- Affected males experience neurological abnormalities, failure to deliver TH to foetal brain areas, and global developmental delay.
- Affected females with one mutated copy of the gene have only mild thyroid phenotype and no neurological effect with a 50% chance of passing the mutation onto a child.
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Description
This quiz covers the basics of thyroid hormones, including T3, T4, and rT3, alongside their biosynthesis and the structure of the thyroid gland. Understand the roles of various cells and the processes involved in hormone production. Test your knowledge on the anatomy and physiology of the thyroid.
