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Free Thyroxine (FT4)

Published: July 27, 2026Updated: August 25, 2026

Free Thyroxine (FT4) measures the concentration of unbound thyroxine hormone in the blood, which is crucial for regulating metabolism and growth.

Quick summary

FT4 is a key indicator of thyroid function and metabolic activity.

Medically reviewed by WellAlly Editorial TeamLast reviewed August 25, 2026Editorial policy

Key facts at a glance

Units
ng/dL
Clinical utility
diagnostic
Result kind
quantitative
FDA-cleared
Roche FT4, Abbott FT4, Siemens FT4, Beckman FT4
interpretation: {"text":"May indicate hypothyroidism, iodine deficiency, or certain medications affecting thyroid function.","label":"Low FT4"}; {"text":"May suggest hyperthyroidism, thyroiditis, or excessive thyroid hormone replacement therapy.","label":"High FT4"}

Free Thyroxine (FT4) is a critical biomarker used to assess thyroid function. It represents the unbound, biologically active fraction of the thyroxine hormone, which is essential for maintaining metabolic homeostasis.

Thyroxine (T4) circulates in the blood in two forms: bound to proteins and free (unbound). The free form, FT4, is the active component that influences cellular metabolism, growth, and development.

Abnormal levels of FT4 can indicate thyroid disorders such as hyperthyroidism (elevated FT4) or hypothyroidism (decreased FT4). Monitoring FT4 levels is essential for diagnosing and managing these conditions.

Frequently asked questions

What is the normal range for FT4?
The normal reference range for FT4 is typically 0.8–1.8 ng/dL.
How is FT4 testing performed?
FT4 testing is done through a blood sample, usually taken from a vein in the arm.
  1. 1
    American Thyroid Association: Thyroid Function TestsSource
  2. 2
    Mayo Clinic: HypothyroidismSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

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Medically reviewed by WellAlly Editorial Team Last reviewed August 25, 2026. · Read our editorial policy