II. Physiology
- See Thyroid Anatomy
- Images
III. Interpretation: Normal Levels
- Triiodothyronine (T3): 80-180 ng/dl
- Free T3: 230-619 pg/dl
IV. Mechanism: Measurement
- Total Triiodothyronine (Total T3)
- Radioimmunoassay measures bound and Free T3
- Affected by TBG levels as well as other factors
- Free Triiodothyronine (Free T3) measures only active T3
- Not affected by Thyroid Binding Globulin (TBG)
- Total Triiodothyronine is preferred
- Free T3 represents 0.3% of total T3
- False Hypothyroidism if impaired T4 to T3 conversion
- Systemic illness
- Fasting
- Surgical stress
- Drugs
- Prednisone
- Propylthiouracil
- Amiodarone
- Radiopaque contrast
- Propranolol