Free T4 (Free Thyroxine, FT4)
Definition
T4 (thyroxine) is a hormone produced by the thyroid gland that regulates the body's energy use. It exists in the blood in two forms:
- Free T4, the active form that enters tissues
- Bound T4, bound to proteins, remaining in circulation as a reserve
The free T4 test measures only the active form; it is considered more accurate and is used more often. The total T4 test measures both together.
Purpose
Diagnosis of hypothyroidism or hyperthyroidism. Investigation of thyroid nodules, goiter, or pituitary disorders. Screening newborns for congenital hypothyroidism. Monitoring thyroid medication effectiveness.
Results
- Low T4, hypothyroidism, Hashimoto's disease, thyroiditis, congenital hypothyroidism, or the effect of previous hyperthyroidism treatment
- High T4, hyperthyroidism, Graves' disease, toxic goiter, pituitary tumor, excessive iodine intake, or medication overdose
Normal Values
0.8 to 1.9 ng/dL or 10.3 to 24.5 pmol/L.
Normal reference ranges may vary slightly between laboratories.
Risks and Limitations
The T4 test alone does not provide enough information to diagnose thyroid problems. It must be interpreted together with TSH and other thyroid tests. Severe illness and medications (birth control pills, steroids) affect results.