Free T3 (FT3) test

Triiodothyronine (T3) is one of the two main hormones made by the thyroid gland; it regulates metabolism, heart rate, body temperature and how energy is used. Most of the T3 in blood is bound to proteins (mainly to thyroxine-binding globulin) and is inactive, while free T3 (FT3) is the unbound, biologically active part that cells can use straight away. This is why an FT3 test is considered more accurate than total T3: its result is not affected by changes in protein levels, for example during pregnancy or while taking hormonal contraceptives.

Most of the T3 in the body is formed in peripheral tissues (the liver and the kidneys), where thyroxine (T4) is converted into T3; only a small part is released directly from the thyroid gland. FT3 is most often tested together with TSH and free T4 when the diagnosis of hyperthyroidism needs to be clarified, or to monitor treatment when the TSH and FT4 results are unclear. Which tests of this group usually go together is set out in thyroid tests.

• When the TSH result is low and hyperthyroidism (an overactive thyroid) is suspected — FT3 helps confirm the diagnosis, especially when FT4 is still normal (T3 toxicosis)
• When there are signs of hyperthyroidism: palpitations, weight loss, tremor, anxiety, heat intolerance, sweating
• To monitor treatment in people already being treated for an overactive or an underactive thyroid
• When the TSH and FT4 results contradict each other or are unclear, to clarify the state of the thyroid
• When a pituitary disorder is suspected and TSH does not reflect the real state of the thyroid

A raised FT3 most often indicates hyperthyroidism (an overactive thyroid). Less often the cause is taking thyroid hormone medicine, liver disease or, rarely, T3 toxicosis. Pregnancy and hormonal contraceptives can raise total T3, but they affect free T3 less. TSH and FT4 help tell the cause apart. Possible causes:

• Hyperthyroidism (for example Graves' disease) — an overactive thyroid
• Taking thyroid hormone medicine, or too high a dose of it
• Liver disease
• T3 toxicosis — a rare condition in which only T3 is raised while T4 is normal
• Pregnancy or hormonal contraceptives (these more often affect total T3 rather than free T3)

Symptoms that can go with a raised FT3:

• Palpitations, a fast pulse
• Weight loss despite a normal appetite
• Tremor
• Anxiety, irritability
• Sweating, heat intolerance

What a doctor usually looks at alongside a raised FT3:

• The TSH and FT4 results — together they help establish whether the problem is in the thyroid gland or in the pituitary gland
• Whether thyroid hormone medicine is being taken, or whether its dose has been adjusted recently
• Thyroid antibodies (for example TSH receptor antibodies), if an autoimmune cause is suspected
• When to repeat the test, so that a change over time can be seen

A low FT3 most often indicates hypothyroidism (an underactive thyroid), but it can also appear in serious illness, in starvation or because of inflammation of the thyroid. Since the body can temporarily reduce T3 production during a serious illness as a protective measure, a low FT3 without other signs does not always mean a thyroid problem. Possible causes:

• Hypothyroidism (for example Hashimoto's disease) — an underactive thyroid
• A serious acute or chronic illness (“low T3 syndrome”) — a temporary response of the body to stress, not a thyroid disorder
• Starvation or severe undernutrition
• Inflammation of the thyroid (thyroiditis)

Symptoms that can go with a low FT3:

• Tiredness, weakness
• Cold intolerance
• Weight gain
• Constipation
• Dry skin, hair loss

What a doctor usually looks at alongside a low FT3:

• The TSH and FT4 results — they help separate a thyroid disorder from a temporary response of the body to a serious illness
• Whether there is a co-existing serious illness, starvation or marked weight loss at the time of the test
• Thyroid antibodies, if an autoimmune cause is suspected
• When to repeat the test to assess the change