TABLE Parathyroid Hormone 21.8 Hormone Composition Source Major Functions Parathyroid hormone (PTH) Polypeptide containing 84 amino acids Principal (chief cells)a Increases blood calcium level in three ways: (1) promotes calcium release from bone (acting on osteoblasts via RANK-RANKL signaling system, it increases the relative number of osteoclasts); (2) acts on the kidney to stimulate calcium reabsorption by the distal tubule while  inhibiting phos-phate reabsorption in the proximal tubule; and (3)  increases formation of hormonally active 1,25-dihydroxycholecalciferol (1,25-(OH)2 vitamin D3) in the kidney, which promotes tubular reabsorption of calcium. aSome evidence suggests that oxyphil cells, which first appear in the parathyroid gland at about 4 to 7 years of age and increase in number after puberty, may also produce PTH.
calcium levels, they stimulate secretion of PTH; high levels of serum calcium inhibit its secretion. PTH functions at several sites:  Action on bone tissue. For years, the bone resorption has been considered the major effect of PTH action on bone. However, the actions of PTH on bone are more complicated. PTH acts directly and indirectly on several cell types. Receptors for PTH are found on osteoprogenitor cells, osteoblasts, osteocytes, and bone lining cells. Surprisingly, the bone-resorbing osteoclasts do not have PTH receptors; thus, they are indirectly activated by the RANKRANKL signaling mechanism of osteoblasts (page 228). The binding of PTH to its receptors on osteoblasts increases local RANK production and decreases osteoprotegerin (OPG) secretion. These changes then stimulate osteoclast differentiation, which leads to increased bone resorption and release of calcium and phosphates into the extracellular fluid. PTH also has an anabolic effect on bone that leads to increased bone mass; therefore, it is utilized in the treatment of osteoporosis (see Folder 8.2 in Chapter 8 on page 233).
 Kidney excretion of calcium is decreased by PTH stimulation of tubular reabsorption, thus conserving calcium.
 Urinary phosphate excretion is increased by PTH secretion, thus lowering phosphate concentration in the blood and extracellular fluids.
 Kidney conversion of 25-OH vitamin D3 to hormonally active 1,25-(OH)2 vitamin D3 is regulated primarily by PTH, which stimulates activity of 1-hydroxylase and increases the production of active hormone.
 Intestinal absorption of calcium is increased under the influence of PTH. Vitamin D3, however, has a greater effect than PTH on intestinal absorption of calcium.
PTH and calcitonin have reciprocal effects in the regulation of blood calcium levels.
Although PTH increases blood calcium levels, the peak increase after its release is not reached for several hours. PTH appears to have a rather slow, long-term homeostatic action. Calcitonin, however, rapidly lowers blood calcium levels and has its peak effect in about 1 hour; therefore, it has a rapid, acute homeostatic action.
The adrenal (suprarenal) glands secrete both steroid hormones and catecholamines. They have a flattened triangular shape and are embedded in the perirenal fat at the superior poles of the kidneys.