Parathyroid Glands
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The parathyroid glands are located on the poste-rior thyroid gland surface.
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.
