When to Order T3 Testing
T3 testing is rarely indicated in the routine evaluation of hypothyroidism and should NOT be ordered for monitoring levothyroxine replacement therapy. 1
Primary Indications for T3 Testing
Suspected Hyperthyroidism with Discordant Results
- Order T3 when TSH is undetectable (<0.1 mIU/L) AND free T4 is normal, as T3 is a sensitive marker of endogenous hyperthyroidism 2
- This pattern may indicate T3-predominant hyperthyroidism (T3 toxicosis) 2
Central (Secondary) Hypothyroidism Monitoring
- Use free T3 alongside free T4 to monitor central hypothyroidism, as TSH cannot be used reliably in pituitary or hypothalamic disease 2
- TSH levels are inappropriately normal or low despite inadequate thyroid hormone in central hypothyroidism 2
Confirming Subclinical Thyroid Dysfunction
- Measure T3 when defining subclinical hypothyroidism: elevated TSH with normal T4 AND normal T3 confirms the diagnosis 3
- Measure T3 when defining subclinical hyperthyroidism: low TSH with normal T4 AND normal T3 confirms the diagnosis 3
When NOT to Order T3
Monitoring Levothyroxine Replacement
- T3 measurement adds nothing to the assessment of T4 over-replacement in primary hypothyroidism 1
- Normal T3 levels can be seen even in over-replaced patients with suppressed TSH and elevated free T4 1
- T3 levels bear little relation to thyroid status in patients on levothyroxine replacement 1
Initial Evaluation of Suspected Primary Hypothyroidism
- TSH is the preferred initial test, with free T4 added if TSH is abnormal 2
- T3 is not needed for diagnosis or treatment decisions in primary hypothyroidism 1
Routine Follow-up of Treated Hypothyroidism
- TSH is the most important parameter to monitor adequacy of replacement therapy in primary hypothyroidism 2
- T3 testing does not guide dose adjustments and should be avoided 1
Special Circumstances
Combination T4/T3 Therapy Assessment
- T3 levels may be measured when patients are on combination therapy containing liothyronine to assess adequacy 4
- However, this remains controversial and is not standard practice 5
Myxedema Coma
- Both T4 and T3 levels may be measured in this medical emergency, though treatment should not be delayed for results 4
Critical Pitfalls to Avoid
- Do not order T3 to exclude over-replacement in patients on levothyroxine - a normal T3 provides false reassurance, as over-replacement can occur with normal T3 levels 1
- Do not use T3 to guide levothyroxine dose adjustments - TSH and free T4 are the appropriate tests 2, 1
- Do not order T3 reflexively with every thyroid panel - it adds cost without clinical benefit in most situations 1