The thyroid gland uses iodine from the diet to make thyroid hormone (thyroxine). Thyroid hormone is stored in the thyroid gland and released into the blood stream as needed by the body. The release of thyroid hormone is controlled by the pituitary gland which makes a hormone called thyroid stimulating hormone (i.e. TSH) that stimulates the thyroid to make and release more thyroid hormone. When there is enough or too much thyroid hormone in the body, the TSH level goes down. (Figure 2) Thyroid hormone has a number of different effects. Overall, the thyroid can be thought of as controlling the body’s metabolism. Thyroid hormone increases the heart rate and how forcefully the heart squeezes. It also affects how fast food moves through your GI tract, bone loss, and how fast sugar is made and used up in the body. All of these effects are increased in patients who have hyperactive thyroids (hyperthyroidism) and decreased in underactive thyroids (hypothyroidism).