How does the Thyroid Work?
The thyroid gland produces two main hormones, thyroxine (T-4) and triiodothyronine (T-3). They maintain the rate at which your body uses fats and carbohydrates, help control your body temperature, influence your heart rate, and help regulate the production of protein. Your thyroid gland also produces calcitonin, a hormone that regulates the amount of calcium in your blood.
The rate at which T-4 and T-3 are released is controlled by your pituitary gland and your hypothalamus — an area at the base of your brain that acts as a thermostat for your whole system. The hypothalamus signals your pituitary gland to make a hormone called thyroid-stimulating hormone (TSH).
Your pituitary gland then releases TSH - the amount depends on how much T-4 and T-3 are in your blood. Finally, your thyroid gland regulates its production of
hormones based on the amount of TSH it receives.
How does Levothyroxine Work?
Some forms of thyroid replacement combine both T3 and T4 (such as natural thyroid replacement made from pig thyroids). However, because the body converts T4 into T3 as needed, most people can successfully take just T4 (such as levothyroxine). This is the most common type of thyroid replacement.