When the parathyroid glands do not secrete sufficient PTH, the osteocytic reabsorption of exchangeable calcium decreases and the osteoclasts become almost totally inactive. As a result, calcium reabsorption from the bones is so depressed that the level of calcium in the body fluids decreases. Yet, because calcium and phos-phates are not being absorbed from the bone, the bone usually remains strong.
When the parathyroid glands are suddenly removed, the calcium level in the blood falls from the normal of 9.4 mg/dl to 6 to 7 mg/dl within 2 to 3 days, and the blood phosphate concentration may double. When this low calcium level is reached, the usual signs of tetany develop. Among the muscles of the body especially sen-sitive to tetanic spasm are the laryngeal muscles. Spasm of these muscles obstructs respiration, which is the usual cause of death in tetany unless appropriate treatment is applied.
Treatment of Hypoparathyroidism with PTH and Vitamin D. PTHis occasionally used for treating hypoparathyroidism. However, because of the expense of this hormone, because its effect lasts for a few hours at most, and because the tendency of the body to develop anti-bodies against it makes it progressively less and less effective, hypoparathyroidism is usually not treated with PTH administration.
In most patients with hypothyroidism, the administra-tion of extremely large quantities of vitamin D, to as high as 100,000 units per day, along with intake of 1 to 2 grams of calcium, keeps the calcium ion concentration in a normal range. At times, it might be necessary to admin-ister 1,25-dihydroxycholecalciferol instead of the nonac-tivated form of vitamin D because of its much more potent and much more rapid action. This can also cause unwanted effects, because it is sometimes difficult to prevent overactivity by this activated form of vitamin D.