Hormones are either small proteins (peptide hormones) or organic chemicals. The "classic" hormones are made in glands and travel to target organs via the bloodstream. Many other local hormones are made by one kind of cell, secreted into the surrounding fluid, to affect another kind of cell - - - or even to affect other cells of the same type. This page concentrates on the classic hormones made in the endocrine glands (parathyroid, thyroid, ovaries, testes, adrenal glands, and pituitary gland).
The following graphics review the bone cell lineage and the effects of the major systemic hormones. Click on the "next" arrow to see the flash movie.
Parathyroid Hormone (PTH) is a peptide hormone produced by the parathyroid glands. It binds to receptors in the bone and kidney. A decrease in serum calcium concentration and an increase in serum phosphorous concentration stimulate PTH secretion. PTH also:
* stimulates osteoclastic bone resorption indirectly to release calcium from bone.
* stimulates bone formation that is coupled to bone resorption.
* increases renal tubular reabsorption of calcium.
* stimulates the renal production of 1,25 dihydroxyvitamin D to increase calcium absorption from the intestine.
* enhances renal phosphate and bicarbonate excretion.
Structure of PTH 1-37 from National Center for Biotechnology Information.
Calcitonin is a peptide hormone produced by cells within the thyroid gland. Calcitonin secretion is stimulated by high blood calcium concentrations, and it acts as a physiologic antagonist to PTH. Osteoclasts have receptors for calcitonin, but the effects are transient. Calcitonin also:
* inhibits osteoclast resorption
* delays calcium absorption from the intestine
* increases calcium urinary excretion
Structure of calcitonin from National Center for Biotechnology Information.
1,25 dihydroxyvitamin D is an active hormone which is produced by the kidney, under the control of PTH, from precursors of dietary vitamin D intake and UV skin-production of vitamin D. It is not really a vitamin, but the name was given many years ago, before anybody knew the function of this molecule. Vitamin D receptors are present in bone, kidney, intestines, and other cells. The chemical name is 1,25-dihydroxy-cholecalciferol, and it:
Gonadal steroids are produced by the ovaries and testes and are very important in maintaining bone balance. They are also important in normal growth and development and in the development of peak bone mass. The mechanism of action is unclear but receptors for estrogen and androgen are found in bone.
Androgens (such as testosterone):
Growth Hormone (GH) is a growth promoting hormone produced by the pituitary gland. It is "anabolic", which means it stimulates bone formation. Growth hormone also:
Thyroid hormone is produced by the thyroid gland. Bone cells have receptors for thyroid. This hormone also:
Glucocorticoid (also called cortisol) is produced by the adrenal gland. Bone cells have receptors for glucocorticoid. This hormone is absolutely essential for life, but excess levels cause multiple deleterious effects on the skeleton. This steroid hormone: