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Thyroid Function

Free Thyroxine (Free T4)

Normal range 0.8–1.8 ng/dL (10–23 pmol/L SI). Specimen: Serum.

Also known as: FT4.

0.8–1.8 ng/dLReference range

Reference range

Conventionalng/dL
0.81.8
SIpmol/L
1023
SpecimenSerum
Conventional range0.8–1.8 ng/dL
SI range10–23 pmol/L
Conversion factor×12.87 (conventional → SI)
PanelThyroid Function

Convert any Free Thyroxine (Free T4) result between units on the unit converter.

What is the normal range for Free Thyroxine (Free T4)?

The standard teaching reference range for Free Thyroxine (Free T4) is 0.8–1.8 ng/dL in conventional units, or 10–23 pmol/L in SI units. This value is measured in serum.

Source: Adult reference values compiled from Pagana & Pagana, Mosby's Diagnostic and Laboratory Test Reference, and Fischbach & Dunning, A Manual of Laboratory and Diagnostic Tests. Reference ranges vary by laboratory, method, age, sex, and population — always use the range printed on the actual result.

Above the range

Common causes of a high Free Thyroxine (Free T4)

  • Hyperthyroidism
  • Excess levothyroxine
  • Thyroiditis

Below the range

Common causes of a low Free Thyroxine (Free T4)

  • Hypothyroidism
  • Severe non-thyroidal illness

At the bedside

Nursing considerations

  • The unbound, active form; pair with TSH to confirm thyroid status.

Study aid

Your notes on Free Thyroxine (Free T4)

Same panel

More Thyroid Function tests

Free Triiodothyronine (Free T3)2.3–4.2 pg/mLThyroid Peroxidase Antibody (TPO Ab)< 35 (negative) IU/mLThyroid-Stimulating Hormone (TSH)0.4–4 mIU/LTotal Thyroxine (Total T4)5–12 µg/dLTotal Triiodothyronine (Total T3)80–200 ng/dL