514 VERTEBRATE LIFE AND ORGANIZATION 



known as Graves's disease, or exophthalmic goiter (Fig. 30.4 C). The 

 thyroid may be enhirged, or may be of nearly normal size, but it pro- 

 duces excessive amounts oi hormone, with a resulting increased basal 

 metabolic rate, increased production of heat, loss of weight, increased 

 heart rate and blood pressure, nervousness, and exophthalmos, or pro- 

 trusion of the eyeballs. Hyperthyroidism can be treated by surgical 

 removal of part of the thyroid, or by its destruction with x-rays or with 

 radioactive iodine. 



250. The Parathyroid Glands 



Embedded in or attached to the thyroid glands are small masses of 

 tissue called the parathyroid glands. There are usually two pairs 

 of jKirathyroids which develop embryologically as outgrowths of the 

 third and fourth pairs of pharyngeal pouches. Each gland consists of 

 solid masses and cords of epithelial cells, rather than of spherical 

 follicles as in the thyroid. The hormone secreted by the parathyroids, 

 called parathormone, is a protein, and was first extracted from para- 

 thyroid glands by Collip in 1925. It regulates, by mechanisms which 

 are not yet clear, the levels of calcium and phosphorus in the blood 

 and body fluids, and is essential for life. The complete removal of the 

 parathyroids results in death in a few days. Parathyroidectomy produces 

 a decreased concentration of calcium in the serum, a decreased excre- 

 tion of phosphorus, and a resulting increase in the amount of phos- 

 phorus in the serum. The animal is subject to muscular tremors, cramps 

 and convulsions, a condition known as tetany, which results from the 

 low level of calcium in the body fluids. An injection of a solution of 

 calcium stops the tetanic convulsions and further convulsions can be 

 prevented by repeated administration of calcium. 



Recent experiments indicate that there are two hormones secreted 

 by the parathyroid, both of which regulate calcium and phosphorus 

 concentrations in body fluids but by different mechanisms. One hor- 

 mone acts primarily on the kidney and leads to an increased excretion 

 of phosphorus; the other acts primarily on the cells within the bone 

 and regulates the deposition and dissolution of the bone salts. 



Parathyroid deficiencies are rare, occurring occasionally when the 

 glands are removed inadvertently during an operation on the thyroid, 

 or when degeneration results from an infection. The administration of 

 parathormone cannot be used for the long-term treatment of parathy- 

 roid deficiencies, for the patient becomes refractory to repeated injec- 

 tions of the extract. The deficiency can be treated successfully by a diet 

 rich in calcium and vitamin D and low in phosphorus. 



Hyperfunction of the parathyroid, induced by a tumor of the 

 gland, is characterized by high calcium and low phosphorus content of 

 the blood and by increased urinary excretion of both calcium and 

 phosphorus. The calcium comes at least in part from the bones and 

 soft, easily broken bones result. The increased level of calcium in the 

 body fluids eventually leads to deposits of calcium in abnormal places 

 —the kidney, intestinal wall, heart and lungs. 



