Thyroxine is a thyroid hormone that is produced and secreted into the blood by the thyroid gland, a butterfly shaped organ that sits at the base of the neck.


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What is thyroxine?


Thyroxine is a thyroid hormone that is produced and secreted by the thyroid gland.

Thyroxine which is also known as T4 is a vital hormone that plays a role in regulating the metabolism and digestion, the formation of heart tissue and muscle, the maintenance of the bones and brain development.

Thyroxine (T4) is a prohormone and can be converted to its bioactive form triiodothyronine (T3), once it reaches its target tissue, which might be the liver, kidney, brain, muscle or heart.

Both thyroxine and triiodothyronine are produced and released by the thyroid gland. If you are living with thyroid issues, it is likely that the thyroid gland is releasing too much or too little thyroxine and/or triiodothyronine. Low levels of thyroxine or triiodothyronine can change the way your body processes fat which can cause high cholesterol and clogging of your arteries which could lead to angina and a heart attack.

If the thyroid gland produces too little thyroxine and higher levels of thyroid stimulating hormone (TSH), you may be living with a condition called hypothyroidism, also known as an underactive thyroid.

If the thyroid gland produces too much of your thyroid hormones, you may be living with a condition called hyperthyroidism, also known as an overactive thyroid.



What is the difference between free thyroxine and bound thyroxine?


The difference between free thyroxine and bound thyroxine is that free thyroxine travels from the thyroid gland through the bloodstream to the tissues that need it, whereas bound thyroxine attaches to proteins that prevent it from moving into the cells of tissues.

There is a higher volume of bound T4 than free T4 in the body. About 99% of T4 is bound as opposed to “free”.

As we know, thyroxine (T4) can be converted into triiodothyronine (T3) outside of the thyroid gland, most notably in the liver.

In sum: The major difference between free thyroxine and bound thyroxine is that free thyroxine travels through the body to the tissues that need it whereas bound thyroxine attaches to proteins and is therefore prevented from entering the tissues.


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Written by Hannah Kingston | Medically Reviewed by Gwen Murphy, PHD, MPH