thyroglobulin molecule, releasing free T4 and T3, which exit the cell membrane to enter the bloodstream. Less than 1 percent is unbound; the rest is bound by thyroxine binding globulins or other plasma proteins. These are gradually leached off the proteins and into target cells as free hormones. T3 and T4 are metabolic hormones because their hormones influence the basal metabolic rate of the body, which is the amount of energy the body uses at rest. When T3 and T4 bind to intracellular receptors on the mitochondria, they cause the breakdown of nutrients and the use of oxygen to make ATP. This is an inefficient process, with the formation of heat as a byproduct of these types of reactions. A high production of heat increases body temperature. Thyroid hormones are also important for fetal and childhood development; they continue to support normal neurological function in adults. Reductions in thyroid hormone can affect fertility, libido, and reproductive function. The presence of thyroid hormone is necessary for catecholamine sensitivity by upregulating the numbers of catecholamine receptors in the blood vessels. High hormone levels will increase body heat, increase the blood pressure, strengthen the heartbeat, and increase the heart rate. This is why thyroid disorders have several associated symptoms throughout the body. A lesser known hormone produced by the thyroid gland is called calcitonin. This is made by C cells, also called parafollicular cells between the follicles. The production of calcitonin is triggered by high blood calcium levels; it acts to decrease the calcium level in the bloodstream. It does this by inhibiting osteoclast activity, increasing osteoblast activity, increasing calcium loss in the urine, and decreasing the calcium absorption in the intestinal tract. Interestingly, it does not play a huge role in calcium homeostasis, as other molecules have a greater activity in doing this.
PARATHYROID GLANDS The parathyroid glands are four tiny glands found imbedded in the back surface of the thyroid gland, separated from the gland by a connective tissue capsule. There is more than one type of cell in the parathyroid gland; however, the chief cells are the ones that make the parathyroid
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