Most of the thyroxine (T4) in the blood is attached to a protein called thyroxine-binding globulin. Less than 1% of the T4 is unattached. A total T4 blood test measures both bound and free thyroxine. Free thyroxine affects tissue function in the body, but bound thyroxine does not.
Thyroid hormones are made by the thyroid gland. The thyroid gland makes and releases two thyroid hormones:thyroxine (T4) and triiodothyronine (T3). Thyroid hormones affect every cell and all the organs of the body. Too much thyroid hormone speeds things up and too little thyroid hormone slows things down. They:
Control the rate at which your body burns calories . This affects whether you gain or lose weight.
Can slow down or speed up your heartbeat.
Can raise or lower your body temperature.
Change how fast food moves through your digestive tract.
Affect muscle strength.
Control how quickly your body replaces dying cells.
The pituitary gland and the thyroid gland work together. The pituitary gland (located near the base of the brain) makes, stores, and releases thyroid-stimulating hormone (TSH). When TSH is secreted by the pituitary gland, it causes the thyroid gland to release more T3 and T4. A high TSH level means there is not enough thyroid hormone, and a low TSH level means there is too much.