Calcitonin

Promotes mineralization and lowers blood Ca concentration in children, but usually has little effect in adults; may prevent bone loss in pregnant and lactating women

Calcitriol

(Vitamin D)

Promotes intestinal absorption of Ca and phosphate; reduces urinary excretion of both; promotes both resorption and mineralization; stimulates osteoclast activity

Cortisol

Inhibits osteoclast activity, but if secreted in excess (Cushing disease), can cause osteoporosis by reducing bone deposition (inhibiting cell division and protein synthesis), inhibiting growth hormone secretion, and stimulating osteoclasts to resorb bone

Estrogen

Stimulates osteoblasts and adolescent growth; prevents osteoporosis

Growth hormone

Stimulates bone elongation and cartilage proliferation at epiphyseal plate; increases urinary excretion of Ca but also increase intestinal Ca absorption, which compensates for the loss

Insulin

Stimulates bone formation; significant bone loss occurs in untreated diabetes mellitus

Parathyroid hormone

Indirectly activates osteoclasts, which resorb bone and raise blood Ca concentration; inhibits urinary Ca excretion; promotes calcitriol synthesis

Testosterone

Stimulates osteoblasts and promotes protein synthesis, thus promoting adolescent growth and epiphyseal closure

Thyroid hormone

Essential to bone growth; enhances synthesis and effects of growth hormone, but excesses can cause hypercalcemia, increased Ca excretion in urine, and osteoporosis

Growth Factors

At least 12 hormonelike substances produced in bone itself that stimulate neighboring bone cells, promote collagen synthesis, stimulate epiphyseal growth, and produce many other effects

Vitamin A

Promotes glycosaminoglycan (chondroitin sulfate) synthesis

Vitamin C

(Ascorbic acid)

Required for collagen synthesis, bone growth, and fracture repair

Vitamin D

Normally functions as a hormone

(see calcitriol)