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.
