Free T3 (triiodothyronine)

What is T3?

T3 (triiodothyronine) is one of two major hormones produced by the thyroid, a small, butterfly-shaped gland located at the base of the throat. T3 makes up less than 10% of what we call thyroid hormone, while T4 makes up the rest. T3, however, is about four times as strong as T4, and is thought to cause most, if not all, the effects of thyroid hormones.

The other major thyroid hormone is called T4 (thyroxine). Together they help to control the rate at which the body uses energy.

Why do we analyse free T3?

A free T3 test is used to assess how the thyroid gland is functioning. A test is usually used to help diagnose hyperthyroidism (overactive thyroid) and can also be ordered to monitor treatment in individuals with known thyroid disorders.

The T3 test is often carried out as part of an examination when a person has symptoms indicating hyperthyroidism, particularly if the free T4 level is not elevated.

Elevated or reduced levels of thyroid hormone indicate that there is an imbalance between the body's requirements and the supply of hormones, but it does not indicate exactly what is causing this surplus or shortage.

When a person is ill, the body's production of T3 from T4 is reduced. Most people who are ill enough to be admitted to hospital will have a low T3 or free T3 level.

If your thyroid produces higher quantities of T4 and T3, this causes symptoms indicating hyperthyroidism (overactive thyroid) such as:

  • Nervousness
  • Hand tremors
  • Weight loss
  • Sleeplessness
  • Swellings around dry, irritated eyes
  • In some cases, the person's eyes are unable to move normally and may appear to be staring. In other cases, the eyes may appear to be bulging

If the thyroid produces insufficient thyroid hormones, the person may display symptoms linked with hypothyroidism (underactive thyroid) such as:

  • Slower metabolism
  • Weight gain
  • Dry skin
  • Fatigue
  • Constipation

Levels of thyroid hormone in the blood may be low or high as a result of thyroid dysfunction. Or it could be due to insufficient or excessive TSH production due to a problem with the pituitary gland.

The most common causes of thyroid dysfunction are related to autoimmune diseases. Graves' disease causes hyperthyroidism, but it can also be caused by thyroid cancer or excessive production of TSH, for example. The effect of this can be detected and monitored by measuring free T3.

High levels of free T3

High levels of free T3 is evident early on in cases of hyperthyroidism (overactive thyroid). While the concentration may be normal for a long time in cases of hypothyroidism (underactive thyroid) however, concentration of T3 may be normal for a long time due to the increase in conversion from T4.

Also the concentration of free T3 remains unchanged in pregnancy or when an oestrogen supplement (such as the contraceptive pill) is taken.

Low levels of free T3

Low levels are seen late on in cases of hypothyroidism and in cases of severe general illness due to a reduction in conversion from T4.

Note: Reference intervals may differ during pregnancy.

More information about T3 (and T4)

Almost all the T3 (and T4) hormones in the blood are bound to protein. The rest is free (unbound) and is the biologically active form of the hormone.

T3 and T4 production is regulated by a complex system. When levels of thyroid hormones in the blood fall, the hypothalamus releases thyrotropin, which stimulates the pituitary gland and tells it to produce and release thyroid-stimulating hormone (TSH).

TSH then stimulates the thyroid to produce more thyroid hormones. Most of the hormone produced is T4. This hormone is relatively inactive and is converted into T3 in the liver and other tissues.


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