T3 test explained

Triiodothyronine (T3) is one of the two main hormones produced by the thyroid gland, the other being thyroxine (T4). The thyroid makes less T3 than T4, but T3 is biologically more active: most of the T3 in the blood is formed in the peripheral tissues, where T4 is converted into T3. T3 acts on almost every tissue in the body — it regulates the metabolic rate, the heart rhythm, body temperature, and muscle and nerve function.

In the blood, T3 travels mostly bound to proteins, while the small free fraction (free T3, or fT3) is the biologically active one and is measured by a separate test. This test usually measures total T3 — the protein-bound and the free fraction together. A T3 test is used to assess thyroid function, especially when TSH is altered or hyperthyroidism is suspected, and it is often done together with TSH and free T4, so that disorders arising in the thyroid can be told apart from those arising in the pituitary gland.

• When a TSH result is altered (low or high) and thyroid function needs to be clarified
• When hyperthyroidism is suspected — with anxiety, irritability, palpitations, sweating, weight loss, tremor or intolerance of heat
• When diagnosing or monitoring Graves' disease or a toxic nodular goitre
• To assess whether the cause of hyperthyroidism is in the thyroid or in the pituitary gland (together with TSH and free T4)
• To monitor the course of treatment in thyroid disease
• When TSH is normal but the symptoms still raise a suspicion of a thyroid function disorder

A raised T3 most often indicates hyperthyroidism — an overactive thyroid. The most common causes are Graves' disease, a toxic nodular goitre, liver disease and the use of thyroid hormone medication. It is always worth assessing the result together with TSH and free T4, so that it is clear whether the problem is in the thyroid or in the pituitary gland.

• Hyperthyroidism (an overactive thyroid)
• Graves' disease
• Toxic nodular goitre
• Liver disease
• Use of thyroid hormone medication

Symptoms that can go with a raised T3

• Anxiety, irritability
• Weight loss despite an increased appetite
• Tremor (shaking)
• Muscle weakness
• Increased sweating, intolerance of heat
• An irregular heart rhythm
• More frequent bowel movements
• An enlarged thyroid
• Insomnia, tiredness

What is usually looked at with a raised T3

• Assessing the result together with TSH and free T4, so that it is clear whether the cause is in the thyroid or in the pituitary gland
• Whether there are signs of Graves' disease or of a toxic nodular goitre
• Liver function markers, if liver disease is suspected
• Whether thyroid hormone medication is being taken and whether the dose chosen for it is the right one (to be discussed with a doctor)

A low T3 can indicate hypothyroidism, but it also often arises from a severe illness or from starvation, when the body reduces its metabolic rate (the so-called non-thyroidal illness syndrome). Other causes are thyroiditis and Hashimoto's disease. The result has to be assessed together with TSH and free T4.

• Hypothyroidism (an underactive thyroid)
• Hashimoto's disease
• Thyroiditis (inflammation of the thyroid)
• A severe acute illness or a state of starvation (the body temporarily reduces its T3 production)

What is usually looked at with a low T3

• Assessing the result together with TSH and free T4
• Whether there are signs of Hashimoto's disease or of another autoimmune thyroid disorder
• Whether there was a severe acute illness at the time of the test that could have temporarily affected the result
• When to repeat the test, so that the change over time can be seen

Related pages

Related pages on this site: TSH, free T4 (FT4), free T3 (FT3), which thyroid tests you need, how to prepare for thyroid tests and how to read blood test results.