A thyroxine test is a blood test that helps diagnose thyroid conditions. The thyroid is a small, butterfly-shaped gland at the base of your throat. Your thyroid makes hormones that control the way your body uses energy. These hormones affect your weight, heart, body temperature, muscle strength, and even your mood. In children, thyroid hormones affect growth, too.

Thyroxine, also known as T4, is a type of thyroid hormone. A T4 test measures the level of T4 in your blood. Too much or too little T4 can be a sign of thyroid disease.

There are two forms of T4 in your blood:

Free T4 is the active form of thyroxine hormone that enters your tissues where it's needed.

Bound T4 is thyroxine that attaches or binds to certain proteins which prevent it from entering your tissues.

T4 levels can be measured with either a free T4 test or a total T4 test:

A free T4 test directly measures the amount of free T4 in your blood. Medical experts believe this test provides more accurate information than a total T4 test, so it's used more often.

A total T4 test measures free and bound T4 together. A calculation may be done to figure out how much T4 is free. This is a less accurate way to check how well your thyroid is working.

Other names: free thyroxine, free T4, total T4 concentration, T4 index, thyroxine screen, free T4 concentration, thyroxine test by equilibrium dialysis

it used for?

A T4 test is used to check how well your thyroid is working. It's also used to help diagnose and monitor thyroid disease and conditions that may affect your thyroid, including:

Hypothyroidism, or underactive thyroid, which is when your thyroid gland doesn't make enough thyroid hormone to meet your body's needs

Hyperthyroidism, or overactive thyroid, which is when your thyroid gland makes more thyroid hormone than your body needs

Disorders of the pituitary gland, which is a gland at the base of your brain that makes TSH (thyroid stimulating hormone). TSH controls how much T4 your thyroid gland makes. If the results from a TSH test aren't normal, a T4 test can help diagnose whether the problem is with your thyroid or your pituitary gland.

In certain cases, a T4 test may be done as part of a group of tests called a thyroid panel. A thyroid panel includes a test for TSH as well as other thyroid tests.