Thyroxine (T4) Test Test
The T4 test measures the level of thyroxine, a hormone made by your thyroid gland, in your blood. It helps check if your thyroid is working properly.
What this test measures
The T4 test measures the amount of thyroxine (T4) in your blood. T4 is the main hormone your thyroid makes. There are two forms: free T4 (active) and bound T4 (backup supply). A free T4 test measures only the active form, while a total T4 test measures both forms together.
What your results mean
Reference ranges vary by laboratory and by your age and sex. MedlinePlus does not publish a single normal range for this test — always read your result against the range printed on your own lab report and discuss it with your provider.
Higher-than-normal results
According to MedlinePlus, higher than normal T4 results may be a sign of hyperthyroidism, Graves' disease, certain stages of thyroiditis, toxic goiter, toxic thyroid nodule, too much iodine, a noncancerous pituitary tumor, taking too much thyroid hormone medicine, or a pituitary problem.
Lower-than-normal results
According to MedlinePlus, lower than normal T4 results may be a sign of hypothyroidism, Hashimoto's disease, certain stages of thyroiditis, a pituitary problem, too much or too little iodine, congenital hypothyroidism, or treatment for hyperthyroidism or thyroid cancer.
Frequently asked questions
What is the difference between free T4 and total T4?
Free T4 is the active form that enters your body tissues. Total T4 measures both free T4 and bound T4 (which is attached to proteins and stays in your bloodstream as backup).
Why is a T4 test often done with a TSH test?
A T4 test alone can't provide enough information to diagnose thyroid problems. TSH tells your thyroid how much hormone to make, so comparing both helps your provider understand your thyroid function.
What can cause high T4 levels?
High T4 may be caused by hyperthyroidism, Graves' disease, certain stages of thyroiditis, toxic goiter, toxic thyroid nodule, too much iodine, a noncancerous pituitary tumor, taking too much thyroid hormone medicine, or a pituitary problem.
What can cause low T4 levels?
Low T4 may be caused by hypothyroidism, Hashimoto's disease, certain stages of thyroiditis, a pituitary problem, too much or too little iodine, congenital hypothyroidism, or treatment for hyperthyroidism or thyroid cancer.
Do abnormal T4 levels always mean I have a medical condition?
No. Abnormal T4 levels don't always mean you have a medical condition that needs treatment. Many things can affect your T4 levels, including severe illness and certain medicines like birth control pills and steroids.
Related lab tests
Sources
- MedlinePlus: Thyroxine (T4) Test — NIH MedlinePlus
Last updated . Information is aggregated from official public sources and is not a substitute for professional medical care.