What is thyroxine (t4)?

Thyroxine (T4) is the most common form of thyroid hormone that is released by the thyroid gland. Once it reaches the tissues, it is converted to another form of the thyroid hormone called triiodothyronine (T3), which is more active in regulating the body's chemical reactions.

In tests of thyroid function, a low T4 level may indicate the presence of low thyroid levels (hypothyroidism). Certain forms of liver or kidney disease may also cause a low T4 level, even though a thyroid problem may not be present.

I’m an endocrinologist – a hormone specialist – and you need to understand about the different types of thyroid hormones that are used to treat hypothyroidism, a condition where the thyroid hormone production becomes inadequate. Hypothyroidism leads to symptoms of cold hands and feet, weight gain, fatigue and poor concentration.

Thyroid hormones consist of either levothyroxine (T4) or triidothyronine (T3), which are produced in the thyroid gland. The thyroid gland is a butterfly shaped gland located in the front of the neck. T4 is produced in the thyroid gland while the majority of T3 is produced at the cells by conversion of T4 to T3. T4 is the storage form of thyroid while T3 is the activated form of thyroid. It is best to measure levels of free T4 and free T3, which measures only that amount of hormone that is freely available to the tissues. TSH is the pituitary hormone that regulates the hormone production by the thyroid gland.

The most frequently prescribed thyroid hormones are levothyroxine (Synthroid,),  triiodothyronine (Cytomel) and Armour Thyroid,  Armour is a made from the thyroid glands of cows and pigs and it contains both T4 and T3. Some patients do better with a combination of T4 and T3. Thyrolar is a combination of T4 and T3. Compounding pharmacists can also make up T3SR, a sustained release form of T3 that may lessen the palpitations that may occur with Cytomel. Armour Thyroid 65 mg is approximately equivalent to 100 mcg of Synthroid and 25 mcg of Cytomel. 

See an endocrinologist for expert advice on managing thyroid problems. For more information go to my website, www.drmarinajohnson.com