T3
- Active form of thyroid hormone - 3x more potent than T4
- 20% of T3 secreted directly from the thyroid
- 80% converted peripherally from T4
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.