Let us go through a run down on thyroid metabolism. First, the hypothalamus sends Thyroid Releasing Hormone (TRH) to the pituitary gland. Second, the pituitary gland releases Thyroid Stimulating Hormone (TSH) to thyroid gland. Third, TSH stimulates Thyroid Peroxidase (TPO) activity to use iodine to create T4 and T3 hormones. From here it can get confusing so pay attention…93% is T4 and only 7% is T3. Although the thyroid gland secretes only a little T3 , it happens to be the predominant and most active form the body can use. This means the majority of T4 has to be converted to T3 which occurs mainly in the liver. Of all the T4 the thyroid secretes, the body uses only about 60% of it. Some T4 is converted in reverse T3 (rT3), which happens to be a form the body can not use. Twenty percent becomes active in the intestines in the presence of a sufficient amount of healthy gut flora. This could take us into a whole other discussion about the use of antibiotics and the lack of use of probiotics. We will have this discussion at a later time but hopefully you can see why antibiotic use can dampen thyroid function, based off the simple fact that 20% of healthy thyroid hormone function depends heavily on healthy gut flora. The remaining T4 is converted to T3 in peripheral tissues.