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Questions and Answers
Where is the thyroid gland located in the body?
What is the main function of the follicular cells in the thyroid gland?
How many lateral lobes does the thyroid gland have?
What is the name of the nerves that lie close to the thyroid gland?
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What is the weight of the thyroid gland?
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What is the name of the cells found in the connective tissue of the thyroid gland?
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How many arteries and veins supply and drain the thyroid gland?
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What is the active form of the thyroid hormone?
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What is the function of Calcitonin?
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What is the primary role of the Sodium Iodine Symporter?
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What is the byproduct of the iodination of the side chains of tyrosine residues?
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What is the ratio of T3 to T4 production in the thyroid?
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What is the precursor to thyroid hormones?
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What is the process by which thyroglobulin is released into the lumen of the follicle?
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What is the result of removing the 5'-iodide from T4?
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What is the primary effect of Hashimoto's disease?
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What is the treatment for hypothyroidism?
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What is a common symptom of hypothyroidism in adults?
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What is the effect of treating hypothyroidism with excessive thyroxine?
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What is the result of removing the 3'-iodide from T4?
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What is the prevalence of Hashimoto's disease in the population?
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What is the most common form of hypothyroidism?
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What is the treatment for hypothyroidism?
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What is a symptom of hypothyroidism?
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What is the most common form of hyperthyroidism?
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What is a symptom of hyperthyroidism?
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What is the effect of hyperthyroidism on bone metabolism?
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What is the effect of hyperthyroidism on the heart?
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Where are T3 and T4 stored?
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What is the function of endocytosis in the thyroid gland?
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What percentage of T3 and T4 is free in circulation?
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What happens to thyroglobulin in the epithelial cells?
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Why does the amount of free T3/T4 decrease during pregnancy?
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What is the approximate half-life of T3 in circulation?
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What is the function of TBG, pre-albumin, and albumin?
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What is the effect of increased TBG on T3 and T4 levels?
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What happens to T3 and T4 after they are released from thyroglobulin?
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What is the approximate amount of T3 stored in the thyroid gland?
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Study Notes
Thyroid Hormone Conversion and Function
- T4 to T3 Conversion: T4 (thyroxine) is converted to T3 (triiodothyronine) by removing the 5'-iodide; T3 is 10 times more active than T4.
- Inactive Form: Removal of the 3'-iodide from T4 results in reverse T3 (rT3), which is inactive.
- Thyroid Hormones: Thyroid produces T4 and T3 derived from tyrosine with iodine. T4 is released in larger quantities but is converted to T3, the active hormone. Calcitonin, produced by parafollicular cells, regulates calcium metabolism.
Thyroid Disorders
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Hypothyroidism: Affects approximately 1% of the population, primarily women, often due to Hashimoto's disease, an autoimmune disorder characterized by:
- Follicular destruction or antibody interference with TSH receptors.
- Treatment typically involves oral thyroxine.
- Symptoms of Hypothyroidism: Include cold intolerance, weight gain, fatigue, bradycardia, muscle weakness, dry skin, hair loss, and a husky voice.
Anatomy of the Thyroid Gland
- Location: Positioned in the neck, anterior to the lower larynx and upper trachea, inferior to the thyroid cartilage, and highly vascularized.
- Structure: Comprises two lateral lobes connected by a central isthmus, weighs 15-20 grams, with a size of about 2-3 cm. Supplied by superior, middle, and inferior thyroid arteries and veins.
Cellular Structure
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Cell Types:
- Follicular cells: Organized in spherical structures (follicles) around a central lumen filled with colloid.
- Parafollicular (C) cells: Located in connective tissue, responsible for producing calcitonin.
Synthesis of T3 and T4
- Process: Involves transport of iodide into epithelial cells using the Sodium Iodine Symporter; synthesis of thyroglobulin within these cells followed by secretion.
- Iodination: Iodide is oxidized and incorporated into tyrosine residues forming MIT (monoiodotyrosine) and DIT (di-iodotyrosine), which couple to form T3 and T4 at a ratio of approximately 1:10.
Thyroid Function and Feedback Mechanism
- Hypothyroidism Signs: Characterized by decreased T4 and increased TSH levels.
- Hyperthyroidism: Often linked to Graves’ disease; autoimmune stimulation of TSH receptors leads to increased BMR, heat intolerance, tachycardia, and symptoms like exophthalmos and osteoporosis. Treated with carbimazole.
Storage and Transport of Thyroid Hormones
- Storage: T3 and T4 are stored extracellularly in the follicle lumen as part of thyroglobulin molecules, which can sustain the body for several months.
- Transport: T3 and T4 are hydrophobic, bound to proteins like thyroxine-binding globulin (TBG) and albumin. Less than 1% is free and biologically active; T3 has a shorter half-life compared to T4.
Impact of Pregnancy on Hormone Levels
- Estrogen Effects: Increases TBG synthesis during pregnancy leading to a decrease in free T3 and T4 levels, stimulating the thyroid gland due to decreased negative feedback on TRH and TSH production.
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Description
Learn about the structure and location of the thyroid gland, including its position in the neck, blood supply, and neighbouring nerves. Understand the dimensions and weight of the gland, and its composition of lateral lobes and central isthmus. Test your knowledge of thyroid gland anatomy!
