Free T4 (thyroxine)
Also known as: FT4, free thyroxine, T4, thyroxine
The unbound fraction of thyroxine, the main hormone the thyroid gland secretes. Measured with TSH, it tells whether a thyroid problem is mild or severe and whether it originates in the thyroid or the pituitary.
- Also known as
- FT4, free thyroxine, T4, thyroxine
- Units
- pmol/L (UK) or ng/dL (US); 12.9 pmol/L = 1 ng/dL
- Measured by
- Thyroid function tests, TSH test
What it is and what it does
The thyroid releases mostly T4, with a smaller amount of the more active T3; tissues convert T4 to T3 as needed. Over 99% of T4 travels bound to carrier proteins, so laboratories measure the free fraction, which is what acts on cells and is not distorted by pregnancy or the pill, both of which raise the binding proteins. A low free T4 with a high TSH is primary hypothyroidism; a low free T4 with a low or normal TSH points to pituitary disease; a high free T4 with a suppressed TSH is hyperthyroidism. Free T4 changes more quickly than TSH after starting or adjusting treatment and is the value followed in pituitary disease, where TSH cannot be trusted.
Why it may be higher
- Hyperthyroidism (Graves disease, toxic nodular goitre, thyroiditis)
- Excess levothyroxine
- Amiodarone, heparin (assay effect), biotin interference, acute psychiatric illness (transient)
- Rare TSH-secreting pituitary tumours or thyroid hormone resistance
Why it may be lower
- Primary hypothyroidism
- Pituitary or hypothalamic disease
- Severe non-thyroidal illness (sick euthyroid pattern), some antiepileptic drugs and high-dose steroids
- Under-treatment or malabsorption of levothyroxine
A high or low value can be associated with several different situations. The result is interpreted with other results, symptoms, history and clinical findings.
What can affect the result
- Timing of the levothyroxine dose (a sample soon after a dose reads higher)
- Biotin and some antibodies interfere with immunoassays
- Severe illness, pregnancy (trimester-specific ranges)