Thyroxine (T4) Test

Thyroxine is also called T4. T4 is a hormone that your thyroid gland makes. A T4 test measures the level of T4 in a sample of your blood.

Too much or too little T4 may be a sign of a thyroid problem. Your thyroid is a small, butterfly-shaped gland in the front of your neck. It makes hormones that control the way your body uses energy.

These hormones affect nearly every organ in your body and control many of your body's most important functions. For example, they affect your breathing, heart rate, weight, digestion, and mood. In children, thyroid hormones affect growth, too.

T4 is the main hormone that your thyroid makes. There are two forms of it in your blood: Free T4 is the active form of thyroxine hormone that enters your body tissues where you need it.; Bound T4 is thyroxine that attaches or binds to certain proteins which prevent it from entering your body tissues. It stays in your bloodstream as a 'backup supply' until your tissues need it.; T4 levels can be measured with either a free T4 test or a total T4 test: A free T4 test measures the amount of free T4 in your blood.

Medical experts believe this test is more accurate than a total T4 test, so it's used more often.; A total T4 test measures free and bound T4 together.; A T4 test alone can't provide enough information to diagnose thyroid problems. So, it's usually done with a TSH blood test. TSH stands for thyroid stimulating hormone.

It's a hormone made by your pituitary gland, a small gland at the base of your brain. TSH tells your thyroid how much hormone to make. Normally, if your T4 levels are too low, your pituitary makes more TSH to make your thyroid work harder.

If your T4 levels are too high, your pituitary stops making TSH.

Why this test is done

You may need a T4 test if:

• You had abnormal results on a TSH test.
• You have symptoms of hypothyroidism. Not having enough thyroid hormone slows down your body functions. The symptoms can vary from person to person and may include:
• Fatigue
• Weight gain
• Being very sensitive to cold
• Joint and muscle pain
• Dry skin
• Dry, thinning hair
• Heavy or irregular menstrual periods
• Fertility problems in women
• Slow heart rate
• Depression
• Constipation
• You have symptoms of hyperthyroidism. Having too much thyroid hormone speeds up your body functions. The symptoms can vary from person to person and may include:
• Weight loss, even though you may be eating more than usual
• Rapid or irregular heartbeat
• Feeling nervous or irritable
• Trouble sleeping
• Fatigue
• Shaky hands, muscle weakness
• Sweating or being very sensitive to heat
• Frequent bowel movements (pooping a lot) or diarrhea
• Goiter
• A member of your family has had thyroid disease. Thyroid disease tends to run in families.
• You're taking thyroid hormone medicine for hypothyroidism. A T4 test may be used to check how well your treatment is working.
• You have symptoms that could be caused by another thyroid condition or a problem with your pituitary gland.

How to prepare

In most cases, you don't need any special preparations for a T4 blood test. Certain medicines and supplements may affect your test results, so let your provider know what you're taking, but don't stop any medicines without talking with your provider.

If your provider has ordered more tests on your blood sample, you may need to fast (not eat or drink) for several hours before the test. Your provider will let you know if there are any special instructions to follow.

Sample type

Blood

Other names for this test

free thyroxine, free T4, total T4 concentration, Free T4 index (FTI), thyroxine screen, free T4 concentration, thyroxine test by equilibrium dialysis