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Questions and Answers
What is the primary role of the sodium-iodide symporter (NIS) in thyroid follicular cells?
What is the primary role of the sodium-iodide symporter (NIS) in thyroid follicular cells?
Which substance inhibits iodide uptake in the thyroid gland?
Which substance inhibits iodide uptake in the thyroid gland?
What is the product of the iodination process involving thyroglobulin?
What is the product of the iodination process involving thyroglobulin?
Which enzyme is primarily responsible for the conversion of iodide to iodine during thyroid hormone synthesis?
Which enzyme is primarily responsible for the conversion of iodide to iodine during thyroid hormone synthesis?
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Pendred syndrome is associated with which of the following?
Pendred syndrome is associated with which of the following?
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What percentage of circulating T4 is found in a protein-free form?
What percentage of circulating T4 is found in a protein-free form?
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Which binding protein has the highest affinity for thyroid hormones?
Which binding protein has the highest affinity for thyroid hormones?
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What is the approximate concentration of T4 in plasma?
What is the approximate concentration of T4 in plasma?
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Compared to T4, how much more biologically active is T3?
Compared to T4, how much more biologically active is T3?
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Which of the following statements about the half-life of thyroid hormones is correct?
Which of the following statements about the half-life of thyroid hormones is correct?
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What is the structural unit of the thyroid gland?
What is the structural unit of the thyroid gland?
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Which cells in the thyroid gland are responsible for producing calcitonin?
Which cells in the thyroid gland are responsible for producing calcitonin?
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What artery is NOT involved in the arterial supply of the thyroid gland?
What artery is NOT involved in the arterial supply of the thyroid gland?
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What is the role of capillaries associated with the thyroid follicle?
What is the role of capillaries associated with the thyroid follicle?
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From which structure does the thyroid gland originate during embryonic development?
From which structure does the thyroid gland originate during embryonic development?
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Which nerve, when damaged, may lead to vocal cord paralysis?
Which nerve, when damaged, may lead to vocal cord paralysis?
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What anatomical feature connects the two lobes of the thyroid gland?
What anatomical feature connects the two lobes of the thyroid gland?
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At what point in gestation does the thyroid gland begin to develop?
At what point in gestation does the thyroid gland begin to develop?
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Which of the following roles is NOT associated with thyroid hormones?
Which of the following roles is NOT associated with thyroid hormones?
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What is the recommended daily intake of iodine for lactating women?
What is the recommended daily intake of iodine for lactating women?
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Which micronutrient is essential for thyroid hormone synthesis?
Which micronutrient is essential for thyroid hormone synthesis?
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What condition can arise due to severe iodine deficiency during pregnancy?
What condition can arise due to severe iodine deficiency during pregnancy?
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What is the primary function of thyroglobulin in thyroid hormone synthesis?
What is the primary function of thyroglobulin in thyroid hormone synthesis?
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Endemic goiter is often associated with which geographical condition?
Endemic goiter is often associated with which geographical condition?
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What is the role of Na+-K+ ATPase in relation to thyroid hormones?
What is the role of Na+-K+ ATPase in relation to thyroid hormones?
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Which step in the synthesis of thyroid hormones involves the transformation of iodide?
Which step in the synthesis of thyroid hormones involves the transformation of iodide?
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What happens to the thyroglossal duct if it does not atrophy?
What happens to the thyroglossal duct if it does not atrophy?
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Which week of gestation marks the final descent of the thyroid gland to its position anterior to the trachea?
Which week of gestation marks the final descent of the thyroid gland to its position anterior to the trachea?
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What is the primary function of T3 and T4 thyroid hormones?
What is the primary function of T3 and T4 thyroid hormones?
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What does the presence of reverse T3 (rT3) indicate?
What does the presence of reverse T3 (rT3) indicate?
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In the case of ectopic thyroid tissue, what typically occurs?
In the case of ectopic thyroid tissue, what typically occurs?
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Which of the following is a characteristic presentation of a thyroglossal duct cyst?
Which of the following is a characteristic presentation of a thyroglossal duct cyst?
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What is the significance of elevated Thyrotropin (TSH) levels in a patient?
What is the significance of elevated Thyrotropin (TSH) levels in a patient?
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What is the expected normal range for total Thyroxine (T4) levels?
What is the expected normal range for total Thyroxine (T4) levels?
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What is the role of the thyroid hormone in carbohydrate metabolism?
What is the role of the thyroid hormone in carbohydrate metabolism?
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What effect does thyroid hormone binding have at the cellular level?
What effect does thyroid hormone binding have at the cellular level?
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Which of the following is NOT an effect of thyroid hormone on fat metabolism?
Which of the following is NOT an effect of thyroid hormone on fat metabolism?
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What is the impact of thyroid hormone on cardiovascular function?
What is the impact of thyroid hormone on cardiovascular function?
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Which process is stimulated by thyroid hormone as part of its metabolic effects?
Which process is stimulated by thyroid hormone as part of its metabolic effects?
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Which metabolic function is NOT directly stimulated by thyroid hormone?
Which metabolic function is NOT directly stimulated by thyroid hormone?
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What effect does thyroid hormone NOT have on the cardiovascular system?
What effect does thyroid hormone NOT have on the cardiovascular system?
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Which of the following effects is associated with hypothyroidism?
Which of the following effects is associated with hypothyroidism?
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How does thyroid hormone influence metabolic rate?
How does thyroid hormone influence metabolic rate?
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What mechanism mediates the transcriptional effect of thyroid hormone binding to its receptor?
What mechanism mediates the transcriptional effect of thyroid hormone binding to its receptor?
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What role does increased gluconeogenesis play in the body when present in higher excess?
What role does increased gluconeogenesis play in the body when present in higher excess?
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Which of the following is NOT an effect of growth hormone on development?
Which of the following is NOT an effect of growth hormone on development?
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What are essential functions of the respiratory system's increased respiration?
What are essential functions of the respiratory system's increased respiration?
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How does growth hormone impact skeletal muscle contraction?
How does growth hormone impact skeletal muscle contraction?
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Which process is stimulated directly by growth hormone in the body?
Which process is stimulated directly by growth hormone in the body?
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Considering the binding affinities, which protein has the lowest affinity for thyroid hormones?
Considering the binding affinities, which protein has the lowest affinity for thyroid hormones?
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Which thyroid hormone serves primarily as a prohormone for another hormone?
Which thyroid hormone serves primarily as a prohormone for another hormone?
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What is the biological activity relationship between T3 and T4?
What is the biological activity relationship between T3 and T4?
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What is the approximate half-life of Triiodothyronine (T3)?
What is the approximate half-life of Triiodothyronine (T3)?
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What is the primary consequence of severe iodine deficiency during pregnancy?
What is the primary consequence of severe iodine deficiency during pregnancy?
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Which characteristic is most associated with endemic goiter in iodine-deficient areas?
Which characteristic is most associated with endemic goiter in iodine-deficient areas?
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What percentage of the recommended daily intake of iodine is advised for adults?
What percentage of the recommended daily intake of iodine is advised for adults?
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Which of the following is a key building block required for the synthesis of thyroid hormones?
Which of the following is a key building block required for the synthesis of thyroid hormones?
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What is the role of thyroglobulin in thyroid hormone synthesis?
What is the role of thyroglobulin in thyroid hormone synthesis?
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Which tissue type is least affected by thyroid hormones?
Which tissue type is least affected by thyroid hormones?
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What is the primary method of preventing iodine deficiency in populations?
What is the primary method of preventing iodine deficiency in populations?
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Which of the following is true regarding the basal metabolic rate (BMR) and thyroid hormones?
Which of the following is true regarding the basal metabolic rate (BMR) and thyroid hormones?
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What happens to the thyroglossal duct during normal embryonic development?
What happens to the thyroglossal duct during normal embryonic development?
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Which of the following conditions can result from ectopic thyroid tissue?
Which of the following conditions can result from ectopic thyroid tissue?
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During which week of gestation does the thyroid gland assume its mature shape?
During which week of gestation does the thyroid gland assume its mature shape?
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In what context is reverse T3 (rT3) significant?
In what context is reverse T3 (rT3) significant?
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What can be a clinical manifestation of a thyroglossal duct cyst?
What can be a clinical manifestation of a thyroglossal duct cyst?
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What is the primary source of the building blocks for thyroid hormones?
What is the primary source of the building blocks for thyroid hormones?
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Which age group is most likely to present with symptoms of ectopic thyroid tissue?
Which age group is most likely to present with symptoms of ectopic thyroid tissue?
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What is the expected outcome if a patient has a significantly elevated Thyrotropin (TSH) level with low T4?
What is the expected outcome if a patient has a significantly elevated Thyrotropin (TSH) level with low T4?
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Which change in the TSH level indicates hyperthyroidism?
Which change in the TSH level indicates hyperthyroidism?
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Which condition is characterized by high levels of both TSH and TRH?
Which condition is characterized by high levels of both TSH and TRH?
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What is the normal range for Free Triiodothyronine (FT3) levels?
What is the normal range for Free Triiodothyronine (FT3) levels?
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In cases of TSH deficiency secondary to hypothyroidism, which of the following hormone levels would be expected?
In cases of TSH deficiency secondary to hypothyroidism, which of the following hormone levels would be expected?
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Which of the following best describes the normal relationship between TSH and Free Thyroxine (FT4)?
Which of the following best describes the normal relationship between TSH and Free Thyroxine (FT4)?
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What is the role of Thyroid Stimulating Hormone (TSH) in the hypothalamic-pituitary-thyroid axis?
What is the role of Thyroid Stimulating Hormone (TSH) in the hypothalamic-pituitary-thyroid axis?
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In the laboratory diagnosis of a patient with suspected thyroid issues, which test is considered the optimal screening test?
In the laboratory diagnosis of a patient with suspected thyroid issues, which test is considered the optimal screening test?
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Which of the following statements is true regarding the TSH normal range?
Which of the following statements is true regarding the TSH normal range?
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Study Notes
Thyroid Hormone Synthesis, Transport and Cellular Mechanism
- 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 Gland: Location and Structure
- Location: Situated below the thyroid cartilage, behind the strap muscles.
- Structure: Comprises two lobes joined by an isthmus, located below the cricoid cartilage.
- Further variation: Pyramidal lobe present in ~50% of adults; non-palpable, enlarges with disease.
Thyroid Gland & Follicles
- Functional unit: The thyroid follicle (or acinus).
- Components: Follicular (epithelial) cells and a lumen filled with colloid.
- Follicular cells: Deliver nutrients and transport hormones; sympathetic nervous system influences hormone synthesis/secretion, and lymphatic system drains excess fluid.
- Parafollicular cells (C cells): Produce calcitonin.
Thyroid Gland: Blood Supply & 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 → brachiocephalic vein.
- Innervation: Recurrent laryngeal nerve (branch of vagus); damage may cause vocal cord paralysis.
- Emergency airway: Cricothyrotomy through the cricothyroid membrane.
Thyroid Gland Origin - Embryology
- Development: The thyroid is the first endocrine gland to develop, on about the 24th day of gestation (3rd week).
- Origin: Arises from the 1st pharyngeal arch.
- Early development: Begins as a diverticulum (thyroid diverticulum) in the floor of the pharynx.
- Descent: Descends in the neck as an initially hollow structure, later solidifying and becoming bilobed, connected by an isthmus.
- Later development (5 weeks): Initial descent anterior to pharyngeal gut; still connected to the tongue by the thyroglossal duct; eventually bifurcates into two lobes.
- Further development (7th week): Forms mature shape and location anterior to trachea; thyroglossal duct obliterates by 7-10 weeks, but remnants may persist.
- Ectopic tissue: Ectopic thyroid tissue can arise along this pathway.
Ectopic Thyroid Gland
- Lingual Thyroid: A 4-year-old girl may have slowed growth with elevated TSH (55 ulU/ml) and low T4 (3.5 mcg/dL). (Example)
Thyroid Hormone: An Overview
- Synthesis: Thyroid hormones (T3 and T4) are synthesized from iodide (I) and tyrosine residues of thyroglobulin.
- Types of thyroid hormones: Tyrosine, Thyroxine (T4), Triiodothyronine (T3) and Reverse T3 (rT3).
What is Thyroid Hormone?
- Essential functions: Critical for brain development, skeletal function, and growth in infants; Regulates metabolic activity (except brain/spleen/testes); increases basal metabolic rate and oxygen consumption, increasing heat production by stimulating Na+-K+ ATPase; affects virtually all tissues.
Thyroid Hormone Synthesis - Building Blocks
- Ingredients: Iodine, thyroglobulin, and tyrosine.
Iodine
- Importance: Essential micronutrient found in seafood, dairy products, and vegetables.
- Recommended daily intake (IOM): Kids (90-130 mcg); Adults (150 mcg); Pregnant women (220 mcg); Lactating women (290 mcg).
Thyroid Hormone Mechanisms and Control
- Iodide trapping: Active transport of iodide from bloodstream against gradient, stimulated by TSH, inhibited by perchlorate; Pendrin transports iodide into colloid. (details on specific transporters)
- Oxidation/Organification: Iodide is converted to iodine (I2), catalyzed by thyroid peroxidase (TPO).
- Coupling: Formation of T3 and T4 (through coupling of MIT and DIT) (details on specific coupling reactions).
- Endocytosis: Thyroglobulin containing T3 and T4 is endocytosed into the follicular cell.
- Proteolysis: Lysosomal proteases digest thyroglobulin, releasing T3 and T4.
- Transport: T3 and T4 are bound to proteins (TBG, TBPA, albumin) to travel in the bloodstream. (Details of protein binding for T4 and T3).
- Conversion: T4 is converted into T3 by peripheral tissues (liver and kidney) via deiodinases. (Mechanisms of deiodination).
- Regulation: TRH from hypothalamus triggers TSH release from the anterior pituitary stimulating thyroid hormone synthesis and release. Negative feedback loop regulates these processes., (exact negative feedback loop).
Thyroid Response to TSH
- Effects: Increased cAMP, calmodulin, and kinase activity, Increased uptake of colloid, Increased liberation of T3 and T4, Increased production/uptake of thyroglobulin, Increased Iodination of thyroglobulin, Increased size/activity of thyroid cells. (More detail on the specific response pathways)
Inhibitor of Thyroid Hormone Synthesis
- Important factors: Iodine availability, integrity of thyroid gland/tissue, conversion of T4→T3, and proper regulation (details).
- Possible Inhibitors: Drugs, goitrogens, and specific examples.
Thyroid Hormone Action
- Mechanism: T3 binds to the thyroid receptor (TR), forming a complex that binds to thyroid response elements in DNA, affecting gene transcription, including additional details on the role of RXR (retinoid X receptor).
- Functions: Metabolic effects (carbohydrate and lipid metabolism, temperature), Cardiovascular effects (cardiac output), CNS effects (development), growth effects, and intermediary effects on carbohydrate, lipid, and protein metabolism. (More in-depth description of each function).
Tests of Thyroid Function
- TSH: Measures pituitary secretion of TSH in response to T4 and T3 feedback.
- Free T4: Assesses free unbound T4 levels, including specific units (pmol/L).
- Free T3: Measures free unbound T3 levels.
- Total T4: Measures bound and free T4.
- Total T3: Measures bound and free T3.
TSH Inverse Relationship with T4
- Variation: Wide variation in individual TSH:T4 “setpoints” (and more detail).
- Usefulness: Specific TSH:T4 setpoint for each individual.
TSH Assay in Healthy Patients
- Optimality: TSH assay is the optimal screening test for ambulatory healthy patients to evaluate thyroid status (hypothyroid/euthyroid/hyperthyroid), along with specific ranges.
Thyroid Disorders
- Disorders: Primary hypothyroidism (more details on specific symptoms), Graves' disease (autoimmune mimicking TSH stimulation), TSH deficiency (secondary hypothyroidism), further defining each, including the signs and symptoms.
- Patterns of TSH, T4, T3, FT4, FT3: Each disorder results in characteristic patterns in the assessed values.
Case Presentations (Examples)
- Case 1 (Hyperthyroidism): A 46-year-old woman with elevated total T4 (256 nmol/L), free T4 (34.6 pmol/L), and low TSH (<0.01 mU/L) (Examples with specific test results and interpretations).
- Case 2 (Hypothyroidism): A 72-year-old woman with low free T4 (<5 pmol/L) and elevated TSH (150 mU/L). (Examples with specific test results and interpretations)
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Description
Test your knowledge on thyroid hormone synthesis and the roles of various components within the thyroid gland. This quiz covers topics such as the sodium-iodide symporter, iodide uptake inhibitors, and the structure of thyroid tissues. Challenge yourself with questions related to the production and function of thyroid hormones like T3 and T4.