The four parathyroid glands are two small endocrine structures usually located posterior to the thyroid gland. In most cases, each pair is found on both the right and left side of the thyroid gland, with each pair embedded in the superior and inferior poles of the thyroid gland. Occasionally, an individual may have five or six glands with one or more of these glands descending to the chest cavity. The primary function of parathyroid glands is to secrete parathyroid hormone (PTH), which plays a prominent role in maintaining a normal plasma calcium level.
CALCIUM PHYSIOLOGY AND REGULATION
While the vast majority of calcium in the normal adult body is stored in the bone as insoluble hydroxyapatite, the physiologically active form is found circulating in plasma, where calcium exists in both ionized and unionized forms. Ionized calcium is the active form and constitutes about 45% of total plasma calcium. The un-ionized form is mostly bound to albumin with a small percentage complexed with moieties found in plasma such as phosphate, bicarbonate, or citrate.
Three major hormones mediate the regulation of extracellular fluid calcium:
Parathyroid hormone (PTH)—It exerts its direct regulatory effect on bones and kidneys. On bones, PTH increases bone resorption by stimulating osteoclastic activities while inhibiting osteoblastic activities. This effect is the primary method for achieving a rapid increase in plasma calcium level. In the kidneys, it performs the following functions: it increases calcium reabsorption while increasing phosphate excretion and it potentiates the enzymatic conversion of 25-hydroxycholecalciferol into 1,25-dihydroxycholecalciferol (vitamin D3). The level of circulating ionized calcium, in a negative-feedback mechanism, controls the secretion of PTH. That is, high levels of ionized plasma calcium lead to inhibition of parathyroid gland and vice versa. Magnesium levels also have a role; low levels of magnesium result in increased PTH secretion.
Vitamin D (choleciferol)—It is obtained via nutrition or from skin through UV irradiation. This is the inactive form, which is converted to 25-hydroxycholecalciferol in the liver. During periods of hypocalcemia and hypophosphatemia, the kidneys convert this into active vitamin D3. Vitamin D3 facilitates increased intestinal and renal absorption of calcium and phosphate.
Calcitonin—It maintains the calcium level during periods of hypercalcemia by inhibiting osteoclastic bone resorption while increasing renal excretion of calcium.
Hyperparathyroidism (HPT) refers to oversecretion of PTH due to dysfunction intrinsic or extrinsic to parathyroid glands. Depending on the root cause, HPT can be classified as primary, secondary, or tertiary.
Primary HPT results from dysfunction intrinsic to the parathyroid gland, which causes oversecretion of PTH. The most common cause is parathyroid adenoma, which constitutes about 90% of all causes of primary HPT. The other causes are parathyroid hyperplasia and carcinoma both of which constitute about 9% and 1% of causes, respectively. Regardless of the etiology, the end result is hypercalcemia. Although most ...