What Is Free T3 in a Blood Test?

Free T3 measures the unbound, biologically active form of triiodothyronine — the most metabolically active thyroid hormone.

Normal Range

• 2.3–4.2 pg/mL
• T3 is 3–5× more metabolically active than T4

T3 vs T4 vs Reverse T3

• T4: prohormone, produced by thyroid, must convert to T3
• T3: active hormone, drives metabolism
• Reverse T3: inactive, produced during stress/illness, blocks T3 receptors

When Is Free T3 Useful?

• Hyperthyroidism — T3 elevated when T4 is normal (T3 toxicosis)
• Monitoring T3-containing therapy (Cytomel, Armour)
• Assessing T4-to-T3 conversion problems
• Thyroid hormone resistance (rare)

When Is Free T3 Less Useful?

• Most hypothyroid patients — TSH + free T4 are sufficient
• Not a good screening test — fluctuates throughout the day