Monitoring Thyroid Replacement: Free T3 is NOT Used
For monitoring patients on thyroid replacement therapy, free T3 (FT3) measurement is not recommended and adds no clinical value—TSH and free T4 (FT4) are the appropriate tests. 1, 2
Why T3 Testing is Inappropriate for Monitoring
T3 levels remain normal even in over-replaced patients on levothyroxine, making this test unreliable for assessing adequacy of thyroid hormone replacement. 2, 3
Physiologic Basis for Avoiding T3 Testing
In patients taking levothyroxine, approximately 80% of circulating T3 comes from peripheral conversion of T4 to T3, not from the medication itself. 4
Patients on levothyroxine maintain normal T3 levels despite having elevated T4 levels that would indicate over-replacement, a phenomenon termed "chemical hyperthyroidism." 3
The T3-to-T4 ratio is significantly lower in levothyroxine-treated patients compared to both euthyroid and hyperthyroid individuals not on replacement therapy. 3
Even in severely over-replaced patients with fully suppressed TSH (<0.02 mIU/L) and markedly elevated free T4 (>27 pmol/L), T3 levels frequently remain within the normal reference range. 2
The Correct Monitoring Strategy
TSH is the primary test for monitoring thyroid replacement therapy, with free T4 used to interpret ongoing abnormal TSH levels. 1
Standard Monitoring Protocol
Monitor TSH every 6-8 weeks while titrating hormone replacement to achieve target TSH within the reference range (0.5-4.5 mIU/L). 1
Free T4 helps interpret ongoing abnormal TSH levels during therapy, as TSH may take longer to normalize than free T4. 1
Once adequately treated with a stable dose, repeat TSH testing every 6-12 months or if symptoms change. 1
When T3 Testing Has Limited Utility
T3 measurement is only potentially useful in specific hyperthyroid conditions, NOT in monitoring levothyroxine replacement. 5
T3 testing may be considered when TSH is suppressed (<0.01 μIU/mL) and free T4 is normal or low, to detect T3 thyrotoxicosis—but this represents endogenous hyperthyroidism, not levothyroxine monitoring. 5
Even in this narrow indication, T3 thyrotoxicosis is rare (0.5% of cases), and the test has limited utility in the vast majority of patients. 5
Critical Pitfalls to Avoid
Relying on normal T3 levels to exclude over-replacement is a dangerous error that can lead to continued excessive levothyroxine dosing with attendant risks of atrial fibrillation, osteoporosis, and cardiac complications. 2
Approximately 25% of patients on levothyroxine are unintentionally maintained on doses sufficient to fully suppress TSH, increasing risks for serious complications. 1
A normal T3 level provides false reassurance in over-replaced patients, as research shows that none of the over-replaced patients (with TSH <0.02 and T4 >27 pmol/L) had elevated T3. 2
The most discriminant T3 level for detecting over-replacement had only 58% sensitivity and 71% specificity, making it clinically unreliable. 2