<P> The production of thyroxine and triiodothyronine is primarily regulated by thyroid - stimulating hormone (TSH), released by the anterior pituitary gland . TSH release in turn is stimulated by thyrotropin releasing hormone (TRH), released in a pulsatile manner from the hypothalamus . The thyroid hormones provide negative feedback to the thyrotropes TSH and TRH: when the thyroid hormones are high, TSH production is suppressed . This negative feedback also occurs when levels of TSH are high, causing TRH production to be suppressed . </P> <P> TRH is secreted at an increased rate in situations such as cold exposure in order to stimulate thermogenesis . In addition to being suppressed by the presence of thyroid hormones, TSH production is blunted by dopamine, somatostatin, and glucocorticoids . </P> <P> The thyroid gland also produces the hormone calcitonin, which helps regulate blood calcium levels . Parafollicular cells produce calcitonin in response to high blood calcium . Calcitonin decreases the release of calcium from bone, by decreasing the activity of osteoclasts, cells which break down bone . Bone is constantly reabsorbed by osteoclasts and created by osteoblasts, so calcitonin effectively stimulates movement of calcium into bone . The effects of calcitonin are opposite those of the parathyroid hormone (PTH) produced in the parathyroid glands . However, calcitonin seems far less essential than PTH, as calcium metabolism remains clinically normal after removal of the thyroid (thyroidectomy), but not the parathyroid glands . </P> <P> About 20,000 protein coding genes are expressed in human cells and 70% of these genes are expressed in the normal thyroid . Some 250 of these genes are more specifically expressed in the thyroid with about 20 genes being highly thyroid specific . The corresponding specific proteins are mainly involved in thyroid hormone synthesis, such as thyroglobulin, TPO and IYD, and expressed in follicular cells . Other proteins elevated in the thyroid are calcitonin related proteins such as CALCA and CALCB, expressed in the parafollicular c - cells . </P>

Which of the following does the thyroid gland use to make t3 and t4