Hypothyroidism occurs when the thyroid gland does not produce enough thyroid hormone. Thyroid hormone is a chemical messenger used by the brain to control many functions in the body including metabolism, which is how the cells use energy. The standard treatment for hypothyroidism is hormone replacement therapy.
About T3 and T4
The thyroid gland produces two main types of thyroid hormone which are known as T3 (triiodothyronine) and T4 (tetraiodothyronine). These two hormones control your body’s metabolism. Being low on thyroid hormone means your metabolism is slower than normal. This can affect how your heart beats, how you process food, how your body handles heat, and how well you can think.
The basic hormone produced by the thyroid gland is T4. When T4 enters your bloodstream and interacts with other chemicals it releases one iodine atom and becomes T3. T3 is generally the more active and stronger thyroid hormone. T3 carries messages from the brain to other cells to regulate the metabolism.
Hypothyroidism treatment provides extra thyroid hormone to the body. If your thyroid is still functioning but not keeping up, your hormone replacement dose may be lower than if your thyroid is not functioning at all. The U.S. Food and Drug Administration (FDA) regulates thyroid hormone replacements, which are only available by prescription.
Your doctor will prescribe the lowest dose possible to bring your hormone level into the normal range. Too much thyroid hormone replacement can cause other symptoms, including irregular heartbeat.
Synthetic thyroid replacement
Thyroid hormone replacement is available as a synthetic (lab-created) hormone which is most commonly called levothyroxine. This is considered by many to be the standard hypothyroidism treatment. Levothyroxine contains only the T4 hormone.
Proponents of synthetic replacement believe the body will convert T4 to T3 to complete the replacement process. Manufacturers claim that synthetic T4 is identical to the hormone produced by the human thyroid gland and claim it is superior to animal hormone replacements because the dose is more consistent.