[Bone and hormones. Effects of parathyroid hormone on the bone]

ACTION OF PTH: Parathyroid hormone (PTH) stimulates both bone formation and bone resorption. It also stimulates synthesis of 1,25-dihydroxyvitamin D3 which increases intestinal absorption of calcium and phosphorus and stimulates bone resorption. In bone tissue, PTH acts mainly on osteoblasts which, unlike osteoclasts, carry PTH receptors. PTH also exerts an indirect action on osteoclasts, increasing their number and activity. IMPACT OF INSUFFICIENT PTH: In the absence of PTH, bone remodeling is greatly reduced. Increased bone density has been observed in patients with hypoparathyroidism studied 10 years after total thyroidectomy. IMPACT OF EXCESS PTH: Chronic excess of PTH accelerates bone loss. In primary hyperparathyroidism, reduction of bone density to the order of 10% has been observed, which corresponds to one standard deviation compared with normal subjects of the same age and sex. The hypothesis of an increased risk of fractures remains unresolved. ADMINISTRATION OF PTH: Intermittent rather than continuous subcutaneous administration appears to be necessary to observe the anabolic effects of PTH on bone. The conjunction of favorable anabolic effects demonstrated in animal models, concerning both bone density and mechanical properties, and early studies in humans, provide converging arguments similar to those observed in preclinical investigations, suggest that PTH could be a promising treatment for osteoporosis.