Thyroid and Parathyroid Hormones


Thyroid and Parathyroid Hormones

  • Iodine-containing hormones produced by the thyroid gland
    • Thyroxine (T4)
      • secreted by the follicular cells of the thyroid gland
      • transported in blood
      • 99.95% is protein bound
        • primarily to thyroxine-binding globulin (TBG)
        • lesser extent to transthyretin and serum albumin.
      • prohormone and reservoir for active thyroid hormone triiodothyronine (T3)
        • converted as required in the tissues by deiodinases.
        • Type II deiodinase converts T4 into T3.
    • Triiodothyronine (T3)
      • one less iodine atom per molecule than Thyroxine
      • most powerful thyroid hormone
      • produced in smaller quantity
  • Increases metabolic activities of virtually all tissues of the body
    • influences physical development
  • Regulates brain, nerve, muscle function
  • Stimulates nearly all aspects of carbohydrate metabolism
    • Increases uptake of glucose by cells
    • Enhanced glucolysis
    • Enhanced gluconeogenesis
    • Increased rate of absorption from GI tract
    • Decreases cholesterol, phospholipids, and triglycerides in plasma
    • Increases free fatty acids
    • Extreme levels cause high basal metabolic rates


  • Parathyroid hormone
  • Regulates blood calcium levels by effects on bone modeling

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