How to Order and Send a Serum Free T3 (FT3) Test
Order free T3 (or total T3) only when TSH is suppressed (<0.45 mIU/L) and you need to evaluate for hyperthyroidism or T3 thyrotoxicosis—it has no role in monitoring hypothyroid patients on levothyroxine. 1
When to Order Free T3
Appropriate Clinical Scenarios
For suspected hyperthyroidism:
- When TSH is between 0.1-0.45 mIU/L: Measure FT4 and either total T3 or FT3 to exclude central hypothyroidism or nonthyroidal illness 1
- When TSH is <0.1 mIU/L: Repeat TSH along with FT4 and total T3 or FT3 within 4 weeks 1
- Most critical use: Diagnosing T3 thyrotoxicosis when TSH is suppressed but FT4 is normal or low 2, 3
Specialized situations where FT3 may be useful:
- Monitoring thyroid hormone replacement therapy in select cases
- Evaluating amiodarone-induced thyrotoxicosis
- Predicting outcome of antithyroid drug therapy in Graves' disease 3
When NOT to Order Free T3
Do not order FT3 for:
- Monitoring levothyroxine dosing in hypothyroid patients - this is explicitly recommended against by the Endocrine Society's Choosing Wisely campaign 4
- Diagnosing hypothyroidism - T3 levels are often normal in hypothyroidism due to TSH-driven hyperstimulation of remaining thyroid tissue 5, 3
- Routine thyroid screening
Practical Ordering Approach
The Algorithm
- Start with TSH - this is your screening test
- If TSH <0.45 mIU/L: Add FT4 and FT3 (or total T3)
- If TSH normal or high: FT3 adds no diagnostic value
Optimal TSH Cutoff for FT3 Testing
Recent evidence suggests TSH <0.01 mIU/L maximizes the utility of FT3 measurement:
- At TSH <0.01 mIU/L with normal/low FT4: 27.6% likelihood of detecting T3 thyrotoxicosis
- At TSH <0.3 mIU/L: only 10.3% likelihood
- All patients with newly diagnosed hyperthyroidism had TSH <0.01 mIU/L 2
The outpatient setting has higher yield (34% vs 14% in inpatients) when TSH <0.01 mIU/L 2
Sample Collection and Handling
- Timing: No specific fasting requirement for FT3
- For patients on levothyroxine: While FT4 should be collected before dosing (peaks 3.5 hours post-dose), this is less critical for FT3 monitoring since you shouldn't be using it for this purpose anyway 5
- Standard serum collection tube (red or tiger-top)
Critical Pitfalls to Avoid
The Most Common Error
Ordering FT3 to monitor hypothyroid patients on levothyroxine - this practice is so widespread that implementing electronic alerts reduced inappropriate ordering by >44% 4. T3 levels remain normal in hypothyroidism and provide no useful information for dose adjustment.
Method-Dependent Results
FT3 assays vary significantly between platforms, particularly at high concentrations. Different immunoassay methods (Abbott Alinity, Roche Cobas, Siemens Centaur) can give markedly different results 6. Be aware of your laboratory's specific method and reference ranges.
Pregnancy Considerations
While the guidelines discuss FT4 extensively in pregnancy (recommending total T4 instead), FT3 has even more limited utility in pregnancy and is generally not recommended 5.
The Bottom Line on Clinical Utility
FT3 testing has limited utility in the vast majority of patients 2. T3 thyrotoxicosis is relatively rare (0.5% of all thyroid tests in one large study), and most cases occur in patients with TSH <0.01 mIU/L 2. The test is most valuable when you have a suppressed TSH with normal or low FT4 and clinical suspicion of hyperthyroidism—not for routine monitoring or hypothyroidism evaluation.