Hypothyroidism is a condition classified by an underactive thyroid gland—when the thyroid does not produce enough hormones. There are various treatments available depending on the cause of insufficient hormone production, but the standard of care is the same—initiating thyroid hormone replacement therapy.

To best understand the purpose of thyroid hormone replacement therapy, it helps to appreciate the role and interaction of T3 and T4—the two primary thyroid hormones. The full name of T3 is triiodothyronine, and T4's full name is tetraiodothyronine or thyroxine. T3 and T4 control your body's metabolism. If you don't have enough of these hormones, then your metabolism slows down. Your metabolic rate dictates how quickly you process food, how fast your heart beats, how much heat your body produces—and even how quickly you can think. In essence, T3 and T4 are in charge of how your body uses energy.