When should T3 (triiodothyronine) levels be checked?

When to Check T3 Levels

T3 levels should be measured only when TSH is suppressed (particularly <0.01 mIU/L) and free T4 is normal or low, primarily to diagnose T3 thyrotoxicosis in symptomatic patients. 1, 2

Primary Screening Approach

• TSH is the recommended first-line test for evaluating suspected thyroid dysfunction, with 98% sensitivity and 92% specificity 1
• If TSH is abnormal, measure free T4 next to distinguish between subclinical and overt thyroid dysfunction 1
• T3 measurement is not part of routine thyroid screening and has limited utility in most clinical scenarios 1, 2

Specific Indications for T3 Testing

T3 Thyrotoxicosis (Primary Indication)

• Check T3 when TSH is suppressed (<0.01 mIU/L) and free T4 is normal or decreased in a symptomatic patient 2, 3
• This represents true T3 thyrotoxicosis, where free T3 may be elevated despite normal T4 4
• The likelihood of detecting T3 thyrotoxicosis increases dramatically at lower TSH cutoffs: only 10.3% at TSH <0.3 mIU/L versus 27.6% at TSH <0.01 mIU/L 2
• T3 thyrotoxicosis is relatively rare, occurring in only 0.5% of newly diagnosed hyperthyroid cases 2

Clinical Context Matters

• T3 testing has higher utility in outpatient settings (34% positive rate) compared to inpatient settings (14% positive rate) when TSH <0.01 mIU/L 2
• All patients with newly diagnosed hyperthyroidism had TSH <0.01 mIU/L, making this the most clinically relevant cutoff 2

When NOT to Check T3

Levothyroxine-Treated Patients

• T3 measurement adds no value in assessing levothyroxine over-replacement in hypothyroid patients 5
• Patients on levothyroxine replacement can have elevated T4 levels while remaining clinically euthyroid with normal T3 levels 5, 6
• The T3/T4 ratio is significantly lower in levothyroxine-treated patients compared to those with endogenous hyperthyroidism 6
• Normal T3 levels can be misleading and may falsely reassure clinicians about over-replacement when TSH is suppressed and free T4 is elevated 5

Routine Hypothyroidism Evaluation

• T3 testing provides no additional benefit in diagnosing primary hypothyroidism 7
• TSH alone is sufficient for monitoring adequacy of thyroid hormone replacement in primary hypothyroidism 3

Practical Algorithm

1. Start with TSH for all suspected thyroid dysfunction 1
2. If TSH is abnormal, add free T4 1
3. Only add T3 if:
   • TSH is <0.01 mIU/L (undetectable) AND
   • Free T4 is normal or low AND
   • Patient is symptomatic AND
   • Patient is in outpatient setting (higher yield) 2, 3

Common Pitfalls to Avoid

• Do not routinely order T3 as part of a thyroid panel—it leads to unnecessary testing in 98.4% of cases 2
• Do not use T3 to assess levothyroxine dosing adequacy—it will not reflect over-replacement and may provide false reassurance 5
• Do not check T3 with only mildly suppressed TSH (0.1-0.3 mIU/L)—the yield is too low to justify testing 2
• Avoid checking T3 in hospitalized patients unless there is strong clinical suspicion, as the positive predictive value is lower 2