T4 (Total)
Thyroxine (T4) is one of the two main hormones the thyroid gland releases. Bound T4 attaches to proteins, which prevents it from entering the body's tissues, while free T4 enters the tissues where it is needed. A blood test that measures both free and bound T4 is called a total T4 test.
Why it matters: A T4 test is used to follow up an abnormal thyroid-stimulating hormone result, to help diagnose hyperthyroidism or hypothyroidism, to monitor T4 levels during thyroid hormone replacement therapy, to screen newborns for an underactive thyroid, and to evaluate conditions such as goitre, thyroid nodules and issues with the pituitary gland or hypothalamus. Higher-than-usual T4 could indicate thyrotoxicosis, which can result from hyperthyroidism, thyroid inflammation or taking excessive amounts of thyroid medicine. Lower-than-usual T4 usually indicates hypothyroidism, which has several causes including certain autoimmune diseases, poor iodine intake and the use of certain medicines. Providers most often use a free T4 test to assess thyroid function because it is more accurate, though in certain situations such as pregnancy a total T4 test might be necessary instead.
Source · Cleveland Clinic — what it is, why it matters, what moves it, related conditions
May increase with:
What is T4 (Total)?
Thyroxine (T4) is one of the two main hormones the thyroid gland releases. Bound T4 attaches to proteins, which prevents it from entering the body's tissues, while free T4 enters the tissues where it is needed. A blood test that measures both free and bound T4 is called a total T4 test.
What might a high or low T4 (Total) mean?
A T4 test is used to follow up an abnormal thyroid-stimulating hormone result, to help diagnose hyperthyroidism or hypothyroidism, to monitor T4 levels during thyroid hormone replacement therapy, to screen newborns for an underactive thyroid, and to evaluate conditions such as goitre, thyroid nodules and issues with the pituitary gland or hypothalamus. Higher-than-usual T4 could indicate thyrotoxicosis, which can result from hyperthyroidism, thyroid inflammation or taking excessive amounts of thyroid medicine. Lower-than-usual T4 usually indicates hypothyroidism, which has several causes including certain autoimmune diseases, poor iodine intake and the use of certain medicines. Providers most often use a free T4 test to assess thyroid function because it is more accurate, though in certain situations such as pregnancy a total T4 test might be necessary instead.
What is the typical reference range for T4 (Total)?
The general-population reference range shown here is 66 – 181 nmol/L. Reference ranges describe the general population and are not a personal target — discuss your results with your physician.
What can affect T4 (Total)?
It may be higher with: Pregnancy, Estrogen-containing birth control pills, because estrogen increases the proteins bound to T4.
Track your T4 (Total) over time
Upload your lab report and see where your values fall.
Get Started Free