Thyroxine T4

T4 (thyroxine) and T3 (triiodothyronine) are the two main hormones produced by the thyroid gland. They play a vital part in numerous processes that are constantly ongoing in the body as they regulate the way cells store and use energy. These thyroid hormones can occur in a free or a bound form, which determines whether or not they have an impact on the body. Certain conditions can affect the thyroid gland, causing an imbalance of these hormones. In this article you can find out more about how T4 affects the body, which conditions can cause an imbalance, and how that imbalance can be tested.

T4 or thyroxine is an important hormone that is produced by the thyroid gland. The production of this hormone is regulated by the pituitary gland, which is a small gland located at the base of the brain. The pituitary produces a hormone called TSH (thyroid stimulating hormone), which stimulates the thyroid gland to produce T3 and T4 hormones. These hormones also give feedback to the pituitary gland, so that it knows when to slow down on the production of TSH.

T3 and T4 can both exist in a free or a bound form. When they occur in their bound form, they are bound to a protein. In their free form, they aren’t. It’s the free form of these hormones that are able to affect bodily functions. The thyroid mostly produces T4, which is inactive. T4 gets converted by the body into T3, which causes it to activate so that it can have its effect on various systems of the body. 1 2

The thyroid hormones work together to control the metabolism of our cells. This means that they control how the body uses and stores energy. This is an important function that regulates many systems and processes in the body. Thyroid hormones are especially important for: 3 4

• The regulation of the heart rate, respiratory rate and blood volume
• The regulation of heat production
• Oxygen delivery to the tissues
• The promotion of skeletal development
• An increase in wakefulness and alertness
• The regulation of the reproductive system in men and women
• The regulation of the gastro-intestinal function
• The regulation of the pituitary function
• Normal brain development - especially during pregnancy
• Cell function and metabolism

A T4 blood test is usually done as a part of a thyroid function test (TFT). This is a series of tests that are important to check the function of the thyroid gland. Thyroid function tests are also important for people with thyroid conditions to follow up on their condition and to see how well their medication is working. A thyroid function test usually consists of: 5

A TSH blood test

A T4 or free T4 blood test

A T3 or free T3 blood test

TSH levels will always be tested first, as this is the first hormone to show changes when there’s an imbalance. This test is also more reliable than T3 and T4 tests. 6

If you are experiencing symptoms of a thyroid condition and your TSH values fall outside of the normal range, then your doctor can look at your free T4 (FT4) value to check for hyperthyroidism and hypothyroidism. Your T4 levels vary according to your age, but they should normally be in the range of 12.0 - 30.0 pmol/L. Additionally, your free T3 can also be tested to check your thyroid function. 2 7 8

The results of your T4 blood test may be interpreted together with your TSH levels and your symptoms. As an important feedback loop exists between TSH and T3/T4, it’s essential to keep the association between these hormones in mind.

Low levels of T4 usually indicate hypothyroidism, a condition that causes the thyroid to be underactive. In the case of hypothyroidism, TSH will be high, as the pituitary gland will try to stimulate the thyroid to produce more thyroid hormone. If both your levels of T4 and TSH are low, this may indicate a condition of the pituitary gland. A mild form of hypothyroidism may result in high TSH and normal T4. 2 5

Various conditions may cause hypothyroidism. Common causes are: 5

• Hashimoto’s thyroiditis
• Thyroiditis
• Thyroid cancer
• Pituitary disease

If your levels of T4 are high in combination with low levels of TSH, this usually indicates hyperthyroidism, a condition that causes the thyroid to be overactive. The combination of low TSH and normal T4 points to mild hyperthyroidism. This form of hyperthyroidism is mild because the feedback system with the pituitary gland still makes up for the increase in thyroid hormone by producing less thyroid stimulating hormone. 5

Various conditions may be causing hyperthyroidism, such as: 5

• Graves’ disease
• Thyroiditis
• Thyroid cancer
• Thyroid nodules (multinodular goiter)

Q: Where is T4 converted to T3?

A: T4 gets converted to T3 in the liver, the kidneys, the muscles, the thyroid, the placenta, and parts of the brain.

Q: What is the difference between T3 and T4?

T3 and T4 are both hormones produced by the thyroid gland. T3 is the active form, while T4 is inactive.

Q: What does T4 measure?

A: A T4 test measures the amount of thyroxine in the blood. The total amount of T4 can be measured, as well as the amount of free T4.