Updated: 8/2/2020

Thyroid Physiology

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Introduction
  • The thyroid produces hormones that contain iodine
    • T4 is converted to active T3 and inactive reverse T3 (rT3) in peripheral tissue
    • T3 is more potent than T4
    • rT3 is inactive
    • the thyroid controls the body’s metabolic rate
Physiology
  • Functions of T3
    • Brain maturation
    • Bone growth
    • Beta-adrenergic
    • Basal metabolic rate
    • Blood glucose
    • Breakdown lipids (↑ lipolysis)
  • Synthesis
    • synthesized in thyroid gland and stored in follicles
    • thyroid peroxidase catalyzes oxidation, organification, and coupling
    • T4 is converted to rT3 via 5'-deiodinase, which removes an iodine from the inner ring of T4
    • T4 is converted to T3 in peripheral tissues via 5’-deiodinase, which removes an iodine from the outer ring of T4
      • inhibited by steroids, beta-blockers, and propylthiouracil
  • Regulation of thyroid hormones
    • production of thyroid hormones
      • thyrotropin-releasing hormone (TRH) release from the hypothalamus stimulates thyroid-stimulating hormone (TSH) release from the pituitary gland
      • TSH then stimulates thyroid follicular cells to produce T3/T4
      • In cases of papillary thyroid carcinoma, patients are often treated with levothyroxine after thyroidectomy such that TSH levels are suppressed, in order to prevent TSH stimulation of any remaining malignant cells 
    • negative feedback loop
      • free T3/T4 causes ↓ TRH secretion and ↓ sensitivity of pituitary to TRH
    • bound T3/T4 are inactive
      • thyroxine-binding globulin (TBG) binds to T3/T4, making them inactive

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