Can this result diagnose a condition on its own?

No. This result should be interpreted with related hormones, symptoms, and clinical context.

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

Low Free T4 (FT4): What Does It Mean?

Low free T4 may occur with insufficient thyroid hormone availability; TSH helps distinguish primary from central patterns.

Test summary

Test name: Free T4 (FT4)
Abbreviations: FT4, Free T4
Category: Endocrinology / Thyroid
Panel: Thyroid Function Tests
Specimen: Serum / plasma
Common units: ng/dL, pmol/L

What does the test measure?

Measures the amount of thyroxine that is not bound to proteins and is available to enter tissues.

Why is the test ordered?

Used to clarify abnormal TSH results, evaluate thyroid function, and monitor thyroid disorders.

Information about a low result

Low free T4 may occur with insufficient thyroid hormone availability; TSH helps distinguish primary from central patterns.

Information about a high result

High free T4 may occur with thyroid hormone excess; thyroid medicine, thyroiditis, and other factors may affect the result.

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 range 1

>= 0.8 – <= 1.9 ng/dL

Population: General
Specimen: Serum / plasma

Example reference interval

MedlinePlus example. Laboratory-specific intervals and pregnancy context may differ.

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

Free T4 should not be interpreted alone; it is usually compared with TSH. Pregnancy, estrogen status, severe illness, and biotin supplements can affect results.

Related specialties

Endocrinology
Internal Medicine
Obstetrics and Gynecology
Pediatric Endocrinology

Frequently asked questions

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
Hashimoto's Disease — National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
U.S. government medical reference · 2026 · Hashimoto disease; TSH/T4/T3 and TPO antibody context · 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
Thyroxine (T4) Test — U.S. National Library of Medicine (MedlinePlus)
Government medical reference · 2024 · Total/free T4 testing and thyroid-function context · Verified: Aug 12, 2026
Free T4 test — U.S. National Library of Medicine (MedlinePlus)
Government medical encyclopedia · 2026 · Typical free-T4 reference interval; pregnancy/biotin/illness caveats · 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 Antibodies — U.S. National Library of Medicine (MedlinePlus)
Government medical reference · 2023 · TPOAb, TgAb and TRAb; autoimmune-thyroid context · Verified: Aug 11, 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