Thyroid and Parathyroid Hormones

Thyroid

• 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 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

Calcitonin

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