Free T4

Does this test have other names?

Free thyroxine test

What is this test?

This test measures the level of free T ₄, or free thyroxine, in your blood. A free T ₄ test is used to find out how well your thyroid gland is working.

T ₄ is 1 of 2 hormones produced by the thyroid, a butterfly-shaped gland in your neck. There are 2 forms of T ₄. Some T ₄ in your blood is bound to proteins, and some T ₄ circulates freely, or unbound from proteins. Testing for unbound T ₄ is more accurate than testing for bound T ₄. The free T ₄ test measures unbound T ₄.

The other thyroid hormone is triiodothyronine, or T ₃. T ₄ is changed to T ₃ so it becomes active and affects the tissues in the body. There are also 2 forms of T ₃, bound T ₃ and free T ₃.

These hormones help regulate your body’s metabolism. They go into action when prompted by thyroid stimulating hormone (TSH), which is released by the pituitary gland in your brain.

Why do I need this test?

You may need this test if your healthcare provider wants to find out whether you have a thyroid-related condition, such as hyperthyroidism, which means an overactive thyroid, or hypothyroidism, which means an underactive thyroid. If you do have a thyroid problem, you may have the test to keep track of changes or treatment.

Each condition has many different symptoms. If you have hyperthyroidism, you may often feel anxious and irritable, have trouble sleeping, and have an irregular or rapid heart rhythm. You may also feel quite tired and notice that you’re losing weight even though your appetite has increased. You may also have sweating and increased heat sensitivity, changes to bowel habits, and changes to menstrual patterns, such as a lighter flow.

If you have hypothyroidism, you may notice weight gain, even if you aren’t overeating. You may also be more sensitive to cold, have low energy, and notice changes to menstruation, such as heavier flow. You may also have muscle weakness, constipation, and dry skin and hair.