Order Total T3, Not Free T3
For assessing triiodothyronine levels, you should order total T3 rather than free T3 (FT3), as total T3 is the preferred test in clinical practice according to major thyroid guidelines. 1
Rationale from Guidelines
The 2011 American Thyroid Association and American Association of Clinical Endocrinologists guidelines explicitly state that "measurement of total T3 is frequently preferred in clinical practice" because assays for estimating free T3 are less widely validated than those for free T4 1. The 2004 JAMA guidelines similarly recommend measuring "either total T3 or FT3" when evaluating subclinical hyperthyroidism, acknowledging both as acceptable but not prioritizing FT3 2.
When to Order T3 Testing
Order T3 (total) in these specific scenarios:
- Suspected hyperthyroidism with suppressed TSH - particularly when free T4 is normal, to detect T3 thyrotoxicosis 1, 3
- Confirming hyperthyroidism diagnosis when TSH is <0.1 mIU/L 2
- Evaluating treatment response in patients with known hyperthyroidism 3
Critical Pitfall: When NOT to Order T3
Never order T3 (total or free) for monitoring levothyroxine dosing in hypothyroid patients - this is explicitly discouraged by the Endocrine Society's Choosing Wisely campaign 4. T3 levels bear little relation to thyroid status in patients on levothyroxine replacement, and normal T3 can be seen even in over-replaced patients 5.
Technical Considerations
Total T3 measurements are affected by protein binding (similar to total T4), so results must be interpreted in context of conditions that alter thyroid-binding globulin (TBG) - including pregnancy, estrogen therapy, hepatitis, and certain medications 1. However, this limitation is outweighed by the superior validation and reliability of total T3 assays compared to free T3 immunoassays 1.
While recent research suggests FT3 may occasionally better reflect clinical features than total T3 in discordant cases 6, and that FT3 reflex testing at TSH <0.01 μIU/mL may improve utility 7, these findings have not yet been incorporated into major society guidelines. The established standard remains total T3 measurement.