Guidelines for Ordering T3 (Triiodothyronine) Testing

Indications Based on TSH and Free T4

Order a T3 measurement when serum TSH is suppressed and free T4 is normal or borderline, to identify T3 thyrotoxicosis in which only T3 is elevated. 1, 2
If TSH is suppressed (<0.1 mIU/L) and free T4 is elevated, T3 testing is unnecessary because hyperthyroidism is already confirmed by the elevated free T4. 1
When TSH is suppressed (<0.1 mIU/L) and free T4 is normal or low, perform T3 testing to distinguish T3 thyrotoxicosis from subclinical hyperthyroidism. 1, 2, 3
Do not order T3 when TSH is within the reference range; a normal TSH essentially excludes hyperthyroidism except for rare TSH‑producing pituitary adenomas or thyroid‑hormone resistance. (American College of Endocrinology & American Association of Clinical Endocrinologists) 4, 3

Use total T3 rather than free T3 for clinical evaluation because free‑T3 assays have greater methodological variability and are less well validated. (American College of Endocrinology & American Association of Clinical Endocrinologists) 4, 3

Pregnancy: Order T3 only when TSH is suppressed and free T4 is normal to assess for T3 thyrotoxicosis. (American College of Endocrinology & American Association of Clinical Endocrinologists) 5
Elderly patients: In individuals with TSH < 0.1 mIU/L and normal free T4, obtain free T3 to differentiate T3 hyperthyroidism from subclinical hyperthyroidism, influencing cardiovascular and bone‑health management. (Annales d’endocrinologie) 6
Immune‑checkpoint inhibitor therapy: In highly symptomatic patients with minimal free T4 elevation, T3 measurement can help identify thyrotoxicosis due to immune‑related thyroiditis. (Journal of Clinical Oncology) 7