Triiodothyronine, commonly called T3, is the most biologically actve hormone produced by the thyroid gland. However it only accounts for about 15-20% of the total hormone production of the thyroid with the majority being thyroxine (T4). T4 though is converted to T3 in the tissues. T3 is between 30 and 40 times more biologically active and accounts for most of the effects on the body attributed to thyroid hormones.

Synthetic T3 is known generically as liothyronine or by the brand name Cytomel. Traditionally the drug of choice for the treatment low thyroid function (hypothyroidism) has been thyroxine but T3 is sometimes added if this is ineffective alone. It is given orally in tablet form.

Hypothyroidism is usually tested for using blood levels of thyroid stimulating hormone (TSH) and thyroxine (T4). Some physicians may also ask for levels of T3 and autoantibodies to be checked. Autoantibodies are proteins produced by the immune system that attack the patient's own thyroid gland, damaging it and making it unable to function correctly.

Symptoms of hypothyroidism include:

• Fatigue
• Trouble getting up in the morning
• Weight gain
• Poor concentration and memory (mental sluggishness)
• Depression
• Low body temperature (hands and feet always cold)
• Headaches
• Menstrual problems
• Low Blood pressure and heart rate
• High cholesterol
• Thinning hair
• Hoarse, gravelly voice

Because thyroid hormones affect the function of every cell in the body and the body's metabolic rate the symptoms of low thyroid function are many and varied. The list above is by no means comprehensive.

Hypothyroidism has been implicated in conditions including chronic fatigue syndrome, fibromyalgia and depression and T3 is sometimes used as part of an overall treatment strategy for them.

Some doctors (and patients from experience) believe that to effectively treat hypothyroidism T3 needs to be included in the treatment since it is T3 that is mainly responsible for the effects of thyroid hormones on the body. It is T3 rather than T4 that enters the cells and binds to receptors there to increase metabolic rate. It is suggested that some patients may not be able to convert T4 to T3 effectively and that this is the reason for their hypothyroidism. One reason for this could be selenium deficiency since this mineral is required by the enzyme (5'-deiodinase) which carries out the conversion.

Timed-release versions of T3 (available from compounding pharmacies) are often recommended as T3 levels in the body fall quickly. With regular T3 medication careful monitoring of symptoms and regular dosing is required.

To learn more see the following book:

Your Thyroid and How to Keep it Healthy - by Dr. Barry Durrant-Peatfield

Please note: T3 is a prescription medication that should only be used under the supervision of your doctor. See side-effects.

Thyroxine (T4) is the major hormone produced by the thyroid gland. It accounts for approximately 80-85% of the hormone production of the thyroid with the majority of the remainder being triiodothyronine (T3).

Thyroxine is often considered to be the storage form of thyroid hormone, providing a reservoir in the blood, from which T3 can be made when the body requires it. T3 is between 30 and 40 times more biologically active and accounts for most of the effects on the body attributed to thyroid hormones.

Synthetic thyroxine known generically as levothyroxine (or l-thyroxine) or by brand names including Synthroid and Levothroid is traditionally the drug of choice for the treatment low thyroid function (hypothyroidism). It is given orally in tablet form.

Hypothyroidism is usually tested for using blood levels of thyroid stimulating hormone (TSH) and thyroxine (T4). Some physicians may also ask for levels of T3 and autoantibodies to be checked. Autoantibodies are proteins produced by the immune system that attack the patient's own thyroid gland, damaging it and making it unable to function correctly.

Symptoms of hypothyroidism include:

• Fatigue
• Trouble getting up in the morning
• Weight gain
• Poor concentration and memory (mental sluggishness)
• Depression
• Low body temperature (hands and feet always cold)
• Headaches
• Menstrual problems
• Low Blood pressure and heart rate
• High cholesterol
• Thinning hair
• Hoarse, gravelly voice

Because thyroid hormones affect the function of every cell in the body and the body's metabolic rate the symptoms of low thyroid function are many and varied. The list above is by no means comprehensive.

Hypothyroidism has been implicated in conditions including chronic fatigue syndrome, fibromyalgia and depression and thyroxine is sometimes used as part of an overall treatment strategy for them.

Please note: Thyroxine is a prescription medication that should only be used under the supervision of your doctor. See side-effects.