Anatomy And Physiology

Anatomy Physiology

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Despite the similarity of names, the thyroid and parathyroid glands have little in common apart from their proximity in the body and both being part of the endocrine system. The name of the parathyroid glands refers to their location on the rear surface of the thyroid gland.

Thyroid gland

The thyroid gland can be found just below the Adam's apple. The thyroid gland is unique in the human body: as it governs the rest of the endocrine system by adjusting the body's sensitivity to other hormones.

The thyroid's primary direct function is to regulate the body's metabolism through its production of triiodothyronine (T3) and thyroxine (T4). An overactive thyroid results in hyperthyroidism through overproduction of T3 and T4. In hyperthyroidism, many of the body's systems "speed up" to unhealthy levels, causing weight loss and other symptoms very similar to an overdose of adrenaline.  Conversely, an underactive thyroid results in hypothyroidism: in which the body's systems "slow down" to unhealthy levels, causing weight gain and general sluggishness.

Through its production of calcitonin, one of several hormones involved in calcium homeostasis, the thyroid also helps keep calcium levels stable. This function seems to be of less importance than the equivalent function of the ultimobranchial body in some other animals: but the full process in humans is still poorly understood.

The thyroid's intake of iodine to manufacture these hormones makes it particularly vulnerable to beta radiation from iodine-131. This radioisotope of iodine is produced in fairly large quantities (3% by weight) by uncontrolled fission, as is typical of atmospheric nuclear bomb testing and nuclear accidents. After it enters the thyroid, I-131 causes mutation and death in nearby cells through beta decay: which results in increased levels of thyroid cancer. Taking iodine pills after exposure to a nuclear accident protects against thyroid cancer by saturating the thyroid with safe iodine, thereby minimising the uptake of environmental I-131.

Parathyroid glands

The parathyroid glands' function is to regulate calcium homeostasis through their production of parathyroid hormone. Since serum (blood) phosphate and serum calcium normally exist in balance with each other, parathyroid hormone also governs kidney reabsorption of phosphate through negative feedback. Serum levels of calcium and phosphate also exist in balance with bone levels of calcium and phosphate.

The net effect of parathyroid hormone is thus to release calcium into the blood from the calcium reservoir in the bones: which also slightly lowers the serum phosphate level. Calcium homeostasis, including the occasional release of calcium into the blood, is necessary for proper neural function.

Too much production of the parathyroid hormone can result in hyperparathyroidism; while failure of the parathyroid glands causes hypoparathyroidism. These are conditions best caught through bloodwork. By the time symptoms develop, they can be serious. The mildest symptoms may be a pins-and-needles sensation, light muscle cramps, or a general feeling of sluggish confusion. If not caught early, hyperparathyroidism can result in renal failure or parathyroid cancer.

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