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Thyroxine – Function, Levels, Side Effects, Dosage, Weight loss

Written by Dr.Mary

What is Thyroxine?

Thyroxine or Tetraiodothyronine (T4) is one of the three major hormones secreted by the thyroid gland. Thyroxine, in particular, is produced by the follicular cells of the thyroid gland.

Thyroxine Function

The hormone thyroxine is very important in the body’s metabolic function because it has effects in the functions of the different organs of the body.

  • In the liver, the hormone functions to increase glucose synthesis to pyruvate, conversion of fats to sugar, and hastens the release of cholesterol.
  • In the digestive system, thyroxine functions to increase the secretion of the digestive juices and promotes smooth muscle function and thus facilitate contractions for gastric motility.
  • In the skeletal and cardiac muscles, thyroxine also plays a major role in their functions. This hormone helps in the normal development of the human body as well as tone and function.
  • In the bones, thyroxine promotes bone maturation as the person age.
  • The hormone thyroxine also plays an important role in the normal development of the body systems during younger age and maintains its function throughout adulthood.
  • The reproductive system also benefits so much of this hormone. It increases the possibility of childbearing and promotes the secretion of milk by the breasts.
  • Thyroxine also plays an important role in the skin as it increases hydration, secretions of exocrine glands, hair growth.

Thyroxine Levels

The levels of the hormone thyroxine can be tested through the thyroxine test. There have been two ways of testing the thyroxine levels in the body: the total T4 test and the free T4 test. The former has been used for so many years in the diagnosis of hyperthyroidism and hypothyroidism, but it is said that total T4 tests can be affected by the level of proteins. On the other hand, free T4 test, which is a newer test, is not affected by the level of proteins. Free T4 is the active form of thyroxine; therefore, it is regarded by many as the accurate representation of thyroxine function. However, whichever of the two tests is needed, it is always done after or at the same time a TSH test is performed. The purpose of these tests is to determine whether the hormone’s feedback system is intact. Thus, a T4 and TSH tests is one strong combination of tests in establishing diagnosis in cases affecting the thyroxine levels.

Side Effects of Thyroxine

Thyroxine when used is considered to be a hormone replacement therapy. If the level used exceeds the needed amount will result to increased metabolic activity and lead to the following:

  • Palpitations
  • Chest pain
  • Muscle cramps
  • Sweating
  • Tremors
  • Diarrhea
  • Restlessness
  • Insomnia and headache

The reassuring thing about this situation, though, is that symptoms will usually respond to reduction in the dosage. There are also effects when thyroxine is used in conjunction with other medications. If the hormone is used in patients with diabetes, it is said that the need for insulin will most likely increase.

Thyroxine and Weight Loss

Thyroxine is considered to be a one of the hormones that synthesizes fats. Thyroxine increases the metabolism so if one is undergoing thyroxine replacement therapy, one feels more energetic, will burn more calories, and eventually, experience weight loss. Here’s another way how thyroxine contributes in losing weight.


  • It increases energy expenditure since the hormone makes the cells use up more energy.
  • Exercise is the most practical and easy way to increase the thyroid levels. Doing exercise will stimulate the production of thyroxine by the thyroid glands.
  • A healthy liver will have a great effect in the release of thyroxine by the thyroid gland. The liver is the main gland that stimulates the thyroid gland in producing the right amount of thyroxine needed by the body.

Thyroxine Dosage

The half-life of the hormone thyroxine runs for 7–10 days although it has much longer biological effect. The dosage will depend on body weight and age. Therefore, dosage will vary between children and adult. Children require larger doses of thyroxine compared to adults wherein the latter require about 1.6 microgram/kg/day2. Thus, adults will maintain level of 100–200 microgram/day to reach a safe level. In addition, the thyroxine level required in the elderly might also decline.

How to start thyroxine therapy?

The introduction of thyroxine therapy should be based on how long the hypothyroidism had existed and identify the health risks the patient may have such as coronary heart disease or heart failure. However, if a patient is otherwise healthy and has very minimal risks factors, then he or she can immediately undergo the therapy maintaining the level of 100–200 microgram/day.

For elderly patients and with identified heart disease can have a daily dose of 25 microgram for 3–4 weeks and should be reassessed before further increase of dosage to 25 microgram for 3–4 weeks until the predicted dose will be achieved. If the patient’s coronary symptoms are getting worse, manage and control the symptoms first before increasing the dosage and if a cardiac disease is strongly established, thyroxine dosage should be reduced.

For patients classified within these two, then it is safer to let them start a dosage of 50 micrograms/day and can be increased after 3–4 weeks.

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