• Active form of thyroid hormone - 3x more potent than T4
  • 20% of T3 secreted directly from the thyroid
  • 80% converted peripherally from T4
  • Often referred to as the prohormone due to low biological coactivity
  • Approximately 90% is converted in peripheral tissues to more active T3 form - principally in the liver and kidneys
  • Can also be converted to metabolically inactive reverse T3 (rT3)
    • Fasting increases the ratio of rT3:T3.