To understand the purpose of thyroid replacement therapy, you need to understand the interaction of T3 and T4—the two main thyroid hormones.
The thyroid gland is controlled by the pituitary gland, a small peanut-sized gland at the base of the brain. When the amount of thyroid hormones (T3 and T4) dip below optimal amounts in the body, the pituitary gland releases Thyroid Stimulation Hormone (TSH) which tells the thyroid gland to make more hormones.
The function of the thyroid gland is to take iodine, found in many foods, and convert it into thyroid hormone T4. The cells in the thyroid gland absorb iodine and combine it with the amino acid tyrosine to make T4. This T4 is then released into the bloodstream. T4 is not the active thyroid hormone; it has to be converted into T3 in your body in order to affect your metabolism. This conversion mostly occurs in your liver, kidneys, and muscles. However, this conversion will not be effective if you have issues such as a fatty liver or a sluggish liver or kidney problems. If due to such issues or other reasons for insufficient T3 production, you may feel tired, depressed or puffy, as well as other symptoms such as weight gain, dry skin and thinning scalp hair.
Every cell in your body depends upon thyroid hormones for regulation of their metabolism. A healthy thyroid gland generally produces about 80% T4 and about 20% T3, however, T3 possesses about four times the hormone “strength” as T4. T3 and T4 control your body’s metabolism. If you don’t have enough of them, then your metabolism slows down. Your metabolic rate dictates how quickly you process food, how fast your heart beats, how much heat your body generates and even how quickly you can think. In short, T3 and T4 are in charge of how your body uses energy.
Despite T3 being the stronger of the two, taking synthetic T4 hormone is considered the “standard” treatment for hypothyroidism, with the reasoning being that the body will take T4 and turn it into T3. However this is not always the case and it can often occur that when T4 is taken very little T3 is produced. This is why some people have little to no effect from taking common thyroid treatments as they are a synthetic T4 and their body is not effectively transforming it into T3.
In some cases the body actually converts T4 into Reverse T3. This increase in Reverse T3 typically intensifies the symptoms of low thyroid hormone disorder; most noticeably an increase in fatigue and weight gain.