Free T3 (free triiodothyronine)

Where T3 comes from

Your thyroid gland makes mostly T4 (thyroxine) and a smaller amount of T3. Much of the T3 your body uses is actually made outside the thyroid, in the liver, kidneys and other tissues, which strip one iodine atom off T4 to create the more potent T3.

T3 sets the pace of metabolism in nearly every tissue. It affects heart rate, body temperature, digestion, energy use, cholesterol and how quickly your thoughts and reflexes run.

Free versus total

Almost all T3 in the blood is attached to carrier proteins. A total T3 test measures bound and unbound hormone together. A free T3 test aims to measure only the unbound portion. That sounds more relevant, but in practice free T3 assays are less dependable; MedlinePlus notes that experts generally regard total T3 as the more accurate measurement, and the American Thyroid Association describes free T3 as often unreliable.

When T3 testing helps

TSH is the best first test of thyroid function, usually paired with free T4. T3 adds the most value in a few situations:

• Confirming hyperthyroidism or judging how severe it is, since T3 is often high.
• Spotting 'T3 toxicosis,' an overactive state in which T3 is raised but free T4 is normal.
• Monitoring treatment of an overactive thyroid.
• Helping work out pituitary causes when TSH and T4 don't fit the usual pattern.

Why it's rarely useful for an underactive thyroid

In hypothyroidism, T3 is usually the last value to fall, because the body ramps up conversion of T4 to T3 to compensate. A normal T3 therefore doesn't rule out an underactive thyroid, and a low T3 on its own is often caused by something else.

Serious illness, fasting, rapid weight loss and some medicines can lower T3 even when the thyroid is healthy, a pattern sometimes called non-thyroidal illness. Reverse T3 testing, which is popular in some wellness settings, isn't considered clinically useful for diagnosing hypothyroidism in people who aren't hospitalized.

Getting a clean result

Biotin supplements can distort many thyroid tests, so the American Thyroid Association suggests stopping biotin for two days before blood is drawn. Estrogen, from birth control pills, hormone therapy or pregnancy, raises binding proteins and pushes total T3 and T4 up without changing thyroid function. Tell your clinician about every medicine and supplement you take.

Thyroid symptoms such as fatigue, weight change, feeling cold, hair thinning and low mood overlap heavily with menopause, low testosterone, iron deficiency and poor sleep. That overlap is a good reason to interpret thyroid results as a set rather than chasing any single number.