The thyroid gland has a strong iodine uptake capacity, with about 25% of the iodine taken up for every flow of blood through the thyroid gland. The thyroid gland is also the most iodine-rich tissue in the body, containing 80% of the total iodine in the body, with iodine concentrations 25 times higher than in the blood. These cells combine iodine and tyrosine to produce the thyroid hormones: tetraiodothyronine (T4) and triiodothyronine (T3), which are released into the bloodstream for systemic control of metabolism. The normal thyroid gland produces about 80% of T4 and 20% of T3, but T3 is three to four times more potent than T4. The thyroid gland is under the control of the pituitary gland. The pituitary gland is a small gland only about the size of a peanut, located at the base of the brain. If blood levels of T3 and T4 fall too low, the pituitary gland will produce thyroid-stimulating hormone (TSH), which stimulates the thyroid gland to produce more of the hormone. Under the influence of TSH, the thyroid gland produces and secretes large amounts of T3 and T4 to raise their levels in the blood. The pituitary gland is sensitive to this and subsequently reduces TSH production. The pituitary gland itself is regulated by another gland (the hypothalamus). The hypothalamus produces TSH-releasing hormone (TRH), which informs the pituitary gland (which releases TSH) to stimulate the thyroid gland (which produces and secretes thyroid hormone). We can compare the thyroid gland to a furnace, the pituitary gland to a thermostat, and the thyroid hormones to heat. When the room gets cold (thyroid hormone levels in the blood drop), the thermostat starts working (TSH rises) and the furnace produces more heat (thyroid hormone). The room temperature returns to normal and the thermostat stops working. The hypothalamus can be compared to the person who controls the thermostat and from whom the pituitary gland learns what level the thyroid gland should be at.