Parathyroid Glands

Parathyroid Glands

The parathyroid glands are located on the posterior thyroid gland surface (FIGURE 16-14). There are usually four parathyroid glands: one superior and one inferior gland on each of the thyroid gland’s two lobes. They are covered in thin connective tissue capsules and appear yellowish-brown.

PTH is secreted by the cells of the parathyroid glands. Also called parathormone, it increases blood calcium concentration and decreases blood phosphate ion concentration to affect the bones, intestines, and kidneys. The most important hormone that controls blood calcium levels, PTH stimulates reabsorption of bone while causing kidney conservation of blood calcium and excretion of more phosphate ions in urine. It stimulates absorption of calcium from food in the intestines. PTH secretion is regulated by nega-tive feedback between the parathyroid glands and the blood calcium concentration. More PTH is secreted as blood calcium concentration drops and vice versa. PTH is important along with calcitonin for stable blood calcium concentration. Calcitonin decreases above-normal blood calcium concentrations, whereas PTH increases below-normal blood calcium concen-trations. Decreased levels of parathyroid hormone may result in convulsions.