Can this result diagnose a condition on its own?

No. Hormone tests are interpreted together with symptoms, medical history, and other laboratory results.

Which reference range should I use?

Use the reference interval provided by the laboratory that performed your test.

When should I discuss the result with a healthcare professional?

Results that are clearly outside range, persistent, or inconsistent with symptoms should be reviewed by a healthcare professional.

Information about a laboratory test

Free T3 (FT3): What Does This Blood Test Measure?

Measures the unbound, free form of T3 in the blood.

Test summary

Test name: Free T3 (FT3)
Abbreviations: FT3, Free T3
Category: Endocrinology / Thyroid
Panel: Thyroid Function Tests
Specimen: Serum / plasma
Common units: pg/mL, pmol/L

What does the test measure?

Measures the unbound, free form of T3 in the blood.

Why is the test ordered?

Often used to complement TSH and free T4, particularly when hyperthyroidism is suspected.

Information about a low result

Low T3 can occur in some hypothyroid patterns but may also fall during severe non-thyroidal illness; it does not confirm hypothyroidism by itself.

Information about a high result

High T3 may occur with hyperthyroidism, and the combination with TSH and free T4 helps interpret the pattern.

Reference and clinical context

Use the reference interval printed on your own laboratory report first. Hormone results may vary with the assay, age, sex, pregnancy, menstrual-cycle phase, and sampling time.

Reference ranges may vary by laboratory, method used, age, sex, pregnancy status, and clinical context. The reference range on your own laboratory report takes precedence.

Interpretation notes

A normal T3 result alone does not rule out thyroid disease. TSH and free T4 are generally more informative.

Related specialties

Endocrinology
Internal Medicine
Pediatric Endocrinology

Frequently asked questions

More information about this test

Sources

Thyroid Tests — National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
U.S. government medical reference · 2026 · TSH, T4, T3 and thyroid-antibody testing; interpretation as a group · Verified: Aug 11, 2026
Graves' Disease — National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
U.S. government medical reference · 2026 · Graves disease; thyroid-stimulating immunoglobulin / receptor-antibody context · Verified: Aug 11, 2026
Triiodothyronine (T3) Tests — U.S. National Library of Medicine (MedlinePlus)
Government medical reference · 2023 · Total/free T3 testing and thyroid-function context · Verified: Aug 12, 2026
Thyroid Function Tests — American Thyroid Association
Professional society patient guidance · 2026 · TSH, free T4 and T3 interpretation; free vs total hormone context · Verified: Aug 11, 2026