Free T4
The unbound, active form of the main hormone produced by the thyroid.
| Typical reference range | 0.8–1.8 ng/dL |
Ranges vary by laboratory, method, age and sex: always use the range printed on your own report. Why labs differ.
What it measures
Thyroxine (T4) is the main hormone released by the thyroid. More than 99% is bound to carrier proteins; the small free fraction can enter cells, where it is converted to the more active T3.
Why it matters
Free T4 confirms and characterizes abnormal TSH results and distinguishes overt from subclinical thyroid disease. It is also used to monitor thyroid medication.
What a high result can mean
High free T4 indicates hyperthyroidism (Graves' disease, toxic nodules, thyroiditis) or excess levothyroxine. Biotin, recent levothyroxine dose and acute illness can affect results.
What a low result can mean
Low free T4 with high TSH indicates primary hypothyroidism; low free T4 with low or normal TSH suggests pituitary or hypothalamic disease.
What affects the result
If taking levothyroxine, testing before the daily dose gives more consistent results. Biotin supplements interfere with many immunoassays. Pregnancy and estrogen alter binding proteins and assay accuracy.
Units and conversion
Usually reported in ng/dL.
- From pmol/L: multiply by 0.078 to get ng/dL
- From ng/L: multiply by 0.1 to get ng/dL
Also called on lab reports
T4, Free, Free Thyroxine, T4 Free, FT4, Thyroxine, Free, Free T4, Direct, T4,Free(Direct), Thyroxine (T4), Free, Direct
Related tests: Thyroid
This page is educational and does not replace advice from a healthcare professional.