Triiodothyronine: What is it? How is it controlled? Related Disorders, Test T3, Effects of High Levels and Low Levels of this Hormone

It is a hormone that is produced by the thyroid gland.

It is the most potent thyroid hormone and affects almost all body processes, such as metabolic rate, body temperature, growth, digestive functions, muscle control, development, brain function, maintenance of bones, and heart rate.

Triiodothyronine is also known as T3 and liothyronine.

Approximately 20% of triiodothyronine is secreted into the bloodstream directly by the thyroid gland. The remaining 80% is produced from the conversion of thyroxine by organs such as the liver and kidneys.

How is triiodothyronine controlled?

The production and release of thyroid hormones, thyroxine, and triiodothyronine, are controlled by a feedback loop that involves the hypothalamus, the pituitary gland, and the thyroid gland.

Possible thyroid disorders include:

  • Hyperthyroidism: when your thyroid produces too much thyroid hormone.
  • Hypopituitarism: when the pituitary gland does not produce average amounts of pituitary hormones.
  • Primary or secondary hypothyroidism: when your thyroid does not produce moderate quantities of thyroid hormones.
  • Thyrotoxic periodic paralysis: when your thyroid produces high levels of thyroid hormones, resulting in muscle weakness.

A thyroid disorder can cause a wide range of symptoms. For example, you may have mental problems such as anxiety or physical issues such as constipation and menstrual irregularities.

Other possible symptoms include:


  • Weakness and fatigue.
  • Difficulty in sleeping.
  • Increased sensitivity to heat or cold.
  • Loss or gain of weight.
  • Dry or swollen skin
  • Dry eyes, irritated, swollen, or bulging.
  • Hair loss.
  • Hand tremors.
  • Incrise of cardiac frecuency.

If you already have confirmation of a thyroid problem, your doctor may use a T3 test to see if there has been any change in your condition.

Sometimes your doctor may also order a T4 test or a TSH test. TSH, or thyroid-stimulating hormone, is the hormone that stimulates the thyroid to produce T3 and T4.

Testing the levels of one or both hormones can help your doctor understand ​​what is happening.

What is a T3 test?

The thyroid gland is found in the neck, just below Adam’s apple. The thyroid creates hormones and controls how your body uses energy and your body’s sensitivity to other hormones.

The thyroid produces a hormone called triiodothyronine, known as T3. It also has a hormone called thyroxine, known as T4. These hormones regulate your body’s temperature, metabolism, and heart rate.

Most of the T3 in your body binds to proteins. The T3 that does not bind to the protein is called free T3 and circulates unbound in your blood. The most common type of T3 test, known as the T3 total test, measures both kinds of T3 in the blood.

By measuring T3 in your blood, your doctor can determine if you have a thyroid problem.

Preparation for a T3 test

You must inform your doctor about all the medications you are currently taking, as some may affect the results of the T3 test.

Some medications that can affect your T3 levels include:

  • Medications related to the thyroid.
  • Steroids
  • Birth control pills or other medicines that contain hormones, such as androgens and estrogens.

Procedure for a T3 test

The T3 test involves drawing blood. The blood will then be tested in a laboratory.

Typically, average results range from 100 to 200 nanograms per deciliter (ng / dL).

A typical result of the T3 test does not necessarily mean that your thyroid is working perfectly. Measuring your T4 and TSH can help your doctor determine if you have a thyroid problem despite the typical result of T3.

What do the abnormal results of the T3 test mean?

Because thyroid functions are complicated, this single test may not give your doctor any definitive answers about what is wrong.

However, abnormal results can help orient them in the right direction. Your doctor may also take a T4 or TSH test to get a clearer picture of your thyroid function.

Abnormally high levels of T3 are expected in pregnant women and people with liver disease. If your T3 test also measured the level of free T3, your doctor may rule out these conditions.

What happens if there are high levels of triiodothyronine?

If you are not pregnant or have liver disease, high levels of T3 may indicate thyroid problems, such as:

High levels of T3 may also indicate high levels of protein in the blood. In rare cases, these elevated levels could indicate thyroid cancer or thyrotoxicosis.

What happens if there are low levels of triiodothyronine?

Abnormally low levels of T3 may indicate hypothyroidism or starvation. It could also suggest that you have a long-term illness since T3 levels decrease when you are sick.

If you are sick enough to be hospitalized, your T3 levels are likely below. This is one of the reasons why doctors do not routinely use the T3 test as a thyroid test. Instead, they often use it together with the T4 and TSH test to understand ​​how the thyroid works.