Synonyms
Free Thyroxine
Clinical Significance
Free T4 testing measures only the active form of thyroid hormone, which is unbound and can directly enter cells and affect them. Free T4 testing may be used to monitor response to treatment in people being treated for thyroid disorders.
Specimen
Serum
Stability
7 days at 4 to 8 °C
30 days at -20 °C
72 hours at ambient temperature
Reference Range
12 – 22 pmol/L
Interpretation
| THYROID-STIMULATING HORMONE (TSH) | FREE T4 | THYROID FUNCTION |
|---|---|---|
| Normal | Normal | Normal Thyroid Function |
| High | Normal | Subclinical Hypothyroidism |
| High | High | Hyperthyroidism related to Pituitary gland |
| High | Low | Hypothyroidism related to the Thyroid gland |
| Low | Normal | Subclinical Hyperthyroidism |
| Low | High | Hyperthyroidism related to the Thyroid gland |
| Low | Low | Hypothyroidism related to Pituitary gland |
Limitation
Measurement of free T4 during pregnancy may require special considerations due to chemical changes in the body. Total T4 and free T4 levels may also be misleading in patients who are very ill or malnourished because of low levels of the proteins that normally bind to T4.