Parathyroid and Thyroid Hormones in the Development of Immobilization Osteoporosis

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Abstract
The experiment was designed to determine whether the osteoporosis observed with immobilization is the result of local factors or develops in response to circulating hormones, which are more effective in the immobilized part. The hormones considered were those from the thyroid and parathyroid glands. Thirty adult dogs were divided into 4 groups: intact, parathyroidectomized, thyroidectomized and thyro-parathyroidectomized. The right hind limbs were immobilized in a single hip spical cast, which was replaced at weekly intervals. After 3 to 12 weeks, the tibia and 1 metatarsal were removed from the immobilized and nonimmobilized limbs of each animal, and quantitative microradiographic studies were carried out; the degree of porosity, bone formation and bone resorption were measured in the mid-shaft of these 2 bones. The pCO2 and pO2 and pH of arterial and venous tibial blood were measured in selected animals. The results showed that after 3 weeks there were both roentgenologic evidence and microradiographic evidence of osteoporosis in the immobilized limb of intact animals; the increase in bone resorption continued for 8 weeks at which time bone formation rose and showed further increase at 12 weeks. Osteoporosis developed only in the intact animals; this indicated that the osteoporosis of immobilization depends on the presence of parathyroid and thyroid glands. The results further indicated that bone resorption or increased bone turnover produces a change in blood metabolites as evidenced by increased pCO2 and decreased pH of bone blood in the immobilized limbs of the intact animals. No such changes appeared in the control limbs or in the immobilized limbs of the animals lacking parathyroids or thyroids or both. There appears to be a local factor produced by disuse which may act by increasing the sensitivity of the bone to normal circulating levels of thyroid and parathyroid hormones; alternatively, a local change in the disused area may increase the effectiveness of the hormones in their action on normal bone. Further experiments are being carried out in an attempt to discover which of these alternatives is more likely.

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