Free T3 — Normal Range & Interpretation

Clinical Context

Free T3 measures the unbound, biologically active form of triiodothyronine circulating in serum. Because only free hormone enters cells and binds nuclear receptors, Free T3 reflects true thyroid activity at the tissue level better than total T3. Clinicians order it to clarify hyperthyroidism when TSH is suppressed, to diagnose T3 toxicosis, and to assess the severity of thyrotoxic states.

Elevated Free T3 points to Graves disease, toxic multinodular goiter, toxic adenoma, early hyperthyroidism before Free T4 rises, T3 thyrotoxicosis, and exogenous T3 supplementation or factitious ingestion. Depressed Free T3 appears in advanced hypothyroidism, severe non-thyroidal illness (euthyroid sick syndrome), starvation, and in patients on high-dose glucocorticoids, beta blockers, or amiodarone, which impair peripheral T4-to-T3 conversion.

What the AANP exam actually tests on Free T3: the pattern recognition of T3 thyrotoxicosis — a suppressed TSH with normal Free T4 but elevated Free T3, classically seen in early Graves disease or autonomous nodules. Expect questions that pair Free T3 results with TSH and Free T4 to determine primary versus secondary thyroid disease. The exam also highlights that Free T3 falls last in hypothyroidism, making it a poor screening test, and that TSH remains the preferred initial study for thyroid assessment.

Related Labs

• TSH — mIU/L
• Free T4 — ng/dL
• Total T4 — μg/dL

Sources

• MedlinePlus