When should T3 (triiodothyronine) levels be tested?

When to Test T3 (Triiodothyronine)

T3 testing should be reserved for specific clinical scenarios when TSH is suppressed (<0.01 mIU/L) and free T4 is normal or low, primarily to diagnose T3 thyrotoxicosis—it has no role in routine thyroid screening or in assessing levothyroxine over-replacement. 1, 2

Initial Thyroid Assessment

• Start with TSH alone as the initial screening test for suspected thyroid dysfunction, which has 98% sensitivity and 92% specificity in confirming thyroid disease 3, 1
• Add free T4 only if TSH is abnormal to distinguish primary from central thyroid disorders 1
• T3 measurement is not part of initial thyroid screening and should not be ordered routinely 1, 4

Specific Indications for T3 Testing

T3 Thyrotoxicosis (Primary Indication)

• Measure T3 when TSH is suppressed (<0.01 mIU/L) AND free T4 is normal or low to evaluate for T3 toxicosis 3, 1, 2
• This scenario occurs in only 0.5% of newly diagnosed hyperthyroidism cases 2
• The likelihood of detecting T3 thyrotoxicosis increases dramatically at lower TSH cutoffs: 10.3% at TSH <0.3 μIU/mL versus 27.6% at TSH <0.01 μIU/mL 2
• T3 testing is more clinically useful in outpatient settings (34% positive rate) compared to inpatient settings (14% positive rate) when TSH <0.01 μIU/mL 2

Symptomatic Hyperthyroidism with Minimal Free T4 Elevation

• Consider T3 in highly symptomatic patients with suppressed TSH but only minimal free T4 elevations 3
• This helps distinguish overt hyperthyroidism from subclinical disease 3

When NOT to Test T3

Levothyroxine Over-Replacement

• Do not use T3 to assess levothyroxine over-replacement in hypothyroid patients—it adds no diagnostic value 1, 4
• In over-replaced patients on levothyroxine, T3 levels remain normal despite biochemical hyperthyroidism (suppressed TSH, elevated free T4) 4
• TSH and free T4 are the only tests needed to assess adequacy of thyroid hormone replacement 1, 4

Routine Screening

• T3 has no role in screening for thyroid dysfunction in asymptomatic patients 1
• Even in confirmed thyroid disease, T3 testing has limited clinical utility outside the specific scenarios above 1

Hypothyroidism Diagnosis

• Do not use T3 to diagnose hypothyroidism—TSH and free T4 are the essential tests 1
• In subclinical hypothyroidism, T3 levels are normal by definition 3

Common Clinical Pitfalls

• Avoid ordering T3 reflexively with TSH and free T4—this leads to unnecessary testing in 98.4% of cases 2
• Do not be falsely reassured by a normal T3 in a patient with suppressed TSH and elevated free T4 on levothyroxine; over-replacement can still be present 4
• Recognize that transient TSH suppression occurs with medications, non-thyroidal illness, and recovery from thyroiditis—do not order T3 based on a single abnormal TSH 1
• In patients with Down syndrome, elderly patients, and postpartum women at higher risk for thyroid dysfunction, TSH remains the appropriate initial test, not T3 3

Optimal Testing Algorithm

1. Order TSH first for any suspected thyroid dysfunction 1
2. If TSH is abnormal, add free T4 1
3. Only add T3 if: TSH <0.01 μIU/mL AND free T4 is normal/low AND clinical suspicion for hyperthyroidism exists 1, 2
4. Consider implementing reflex testing protocols where T3 is automatically added only when TSH <0.01 μIU/mL with normal/low free T4 to reduce unnecessary testing 2