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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

• 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

• 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.

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!