Triodothyronine: Definition, Function, T3 Test, Regulation and Symptoms of High and Low Hormone Levels

It is one of the two main hormones produced by the thyroid gland, a small butterfly-shaped organ that extends over the trachea at the base of the throat.

The other primary thyroid hormone is called thyroxine (T4).

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

The function of triiodothyronine (T3)

Both hormones (T3 and T4) help control the speed at which the body uses energy.

Thyroid hormone participates in many physiological processes of the body, especially in the regulation of growth and development, the control of metabolism, and the law of body temperature.

Participates in a negative feedback system by inhibiting the secretion of the hormone thyrotropin in the pituitary gland.

T3 constitutes less than 10% of what we call thyroid hormone, while T4 does the rest. T3, however, is approximately four times stronger than T4 and is thought to cause most, if not all, of the effects of thyroid hormones.


Why can a free T3 test be required?

A free T3 test is used to evaluate how the thyroid gland works. It is usually to help diagnose hyperthyroidism (overactive thyroid) and can also be ordered to control treatment in people with known thyroid disorders.

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

T3 regulation

A feedback loop regulates the system of production of thyroid hormone.

Then, when thyroxine and triiodothyronine levels increase in the thyroid hormones, they prevent the release of both the thyrotropin-releasing hormone from the hypothalamus and the thyroid-stimulating hormone from the pituitary gland.

This system allows the body to maintain a constant level of thyroid hormones.

When the levels of thyroid hormones in the blood decrease, the hypothalamus releases thyrotropin, which stimulates the pituitary gland and tells it to produce and release the 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 becomes T3 in the liver and other tissues.

Levels of Triyodotironin

Normal levels of T3 in the blood are between 1.07 and 3.37 nmol / L. However, these values ​​decrease with age.

Normal levels of FT3 in the blood range between 3 and 8.5 nmol / L.

Elevated or reduced thyroid hormone levels indicate an imbalance between the body’s requirements and the hormone supply. Still, it does not tell exactly what is causing this excess or shortage.

When a person is sick, the production of Triyodotironina from the body from T4 is reduced. Most sick people to enter the hospital will have a low level of T3 or free T3.

Symptoms of high and low levels

What happens if I have high levels of triiodothyronine?

Thyrotoxicosis is the name of the condition in which people have too much thyroid hormone in the bloodstream.

It can result from hyperactivity of the thyroid gland (hyperthyroidism) for conditions such as Graves’ disease, thyroid inflammation, or a benign tumor.

Thyrotoxicosis can be recognized by a goiter, which is a swelling of the neck due to an enlarged thyroid.

Other symptoms of thyrotoxicosis:

  • Heat intolerance.
  • Weightloss.
  • Increase in appetite
  • Increase in bowel movements
  • Irregular menstrual cycle.
  • Accelerated or irregular heart rhythm.
  • Palpitations
  • Fatigue.
  • Irritability.
  • Temblor.
  • Thinning
  • Hair loss.
  • Retraction of the eyelids: resulting in a ‘fixed gaze’ appearance.

What happens if I have low levels of triiodothyronine?

Hypothyroidism is the term for producing too little thyroid hormone by the thyroid gland.

This may be due to autoimmune diseases (such as Hashimoto’s disease), a deficient iodine intake, or some medications.

Because thyroid hormones are essential for physical and mental development, hypothyroidism not treated before birth and during childhood can cause learning problems and reduced growth.

Hypothyroidism in adults produces a decrease in body functions and can trigger the following symptoms:

  • Fatigue.
  • Intolerance to cold temperatures.
  • Low heart rate
  • Weight gain.
  • Reduced appetite
  • Bad memory.
  • Depression.
  • The rigidity of the muscles.
  • Reduced fertility