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Gland structure and naming
The parathyroid glands produce parathyroid hormone which controls calcium levels. There are usually four parathyroid glands which are located in the neck,physically near the thyroid gland. The parathyroid glands have no functional connection to thyroid physiology. The name parathyroid refers to "para" (close to) the "thyroid." These glands are usually less than one centimeter in size and are usually located behind the thyroid gland. Post mortem exams show that four normal glands are found in 91% of people, three glands are found in 5%, and five glands in 4% (1,2). The anatomic location of parathyroid glands varies widely due to the embryonic origin of the glands from the 3rd and 4th pharyngeal pouches with eventual migration to the lower neck. The superior parathyroid glands are usually located on the posterior margin of the upper 2/3 of the thyroid lobes. The inferior parathyroids are more variably located: 40% are within 1cm of the inferior margin of the thyroid gland, 17% are abutting the inferior thyroid, 26% in the superior portion of the thymus, and 2% are in the upper mediastinum in the chest. (3)
Normal parathyroid gland function
The parathyroid glands are similar to the thermostat in your house. A thermostat is given a preset normal range at which to keep the temperature and adjusts whether hot or cold air is pumped into the house to keep the temperature normal. The parathyroid glands are given a preset normal range at which to keep blood calcium concentrations and adjust whether parathyroid hormone levels are increased to decreased to keep the calcium concentration normal. If the blood calcium level increases to the higher end of normal, the parathyroid glands reduce the level of PTH secretion into the blood which will reduce calcium level. If calcium levels begin to drop, the parathyroid glands release more calcium into the blood which will increase blood calcium concentrations. This regulation occurs minute to minute. The most common abnormality of the system is hyperparathyroidism.
Parathyroid hormone function
The parathyroid hormone (PTH is the abbreviation) controls the level of calcium in the blood. The parathyroid hormone has a number of functions.
1) Bone resorption: this is the release of calcium from bones into the blood.
2) Causes the kidneys keep calcium in the blood instead of letting it go out in the urine, thus increasing blood calcium levels. Also causes the kidneys to lose phosphate in the urine, thus lowering blood phosphate levels. Phosphate in the blood usually binds calcium.
3) Causes an increase in the activated form of vitamin D (1,25 dihydroxy vitamin D) which then allows the gastrointestinal system to better absorption calcium from the diet in the blood. Thus, there is an increased in blood calcium levels.
Calcium function
Calcium is a mineral whose concentration in the blood is closely regulated, usually 8.5 to 10.2 mg/dl. Blood calcium serves important functions in the body mainly as a electrical current and partially as a building block. Calcium concentrations determine the electrical currents in our nervous system and muscles. In the nervous system calcium levels allow communication between nerves and in the muscle calcium controls muscle contractility (muscle strength). Calcium is the main component of bones, thus becoming the main building block for proper bone strength.
References
1. Gilmore JR et al. The gross anatomy of parathyroid glands. J Pathol. 46: 133 (1938)
2. Alveryd A. Parathyroid glands in thyroid surgery. Acta Chir Scand. 389:1 (1968)
3. Akerstrom G et al. Surgical anatomy of human parathyroid glands. Surgery 95:14 (1984)
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