T3 Free

The T3, Free (FT3) test is a special blood test that checks for a type of thyroid hormone called triiodothyronine, which is not attached to any proteins that usually carry hormones in the blood. This test is important because it helps doctors figure out if someone has too much thyroid activity, known as hyperthyroidism, and to understand more about the thyroid hormone levels if there’s something unusual about the proteins that carry these hormones.

Doctors often use this test along with two other tests: one for thyroid-stimulating hormone (TSH, test code 899) and another for free T4 (FT4, test code 866). These tests together help doctors diagnose and manage hyperthyroidism effectively.

If a person has a very low TSH level, which can mean their thyroid is very active, or if they show signs that suggest hyperthyroidism, doctors might order the FT3 test. This is especially true if the FT4 test doesn’t show high levels. Sometimes, even if the FT4 levels are normal, the FT3 levels can be high. This situation can happen in about 10% of people who have hyperthyroidism because of conditions like Graves’ disease or a thyroid nodule that works on its own. This high FT3 can be an early clue that the disease is starting or coming back after treatment.

On the other hand, testing FT3 isn’t very helpful for finding out if someone has too little thyroid activity, or hypothyroidism, because FT3 levels usually don’t drop until after TSH and FT4 levels have already started to rise.