Thyroid function and bone turnover

Abstract

The mechanism of thyroid action on bone was studied in 15 patients with thyrotoxicosis and 14 patients with hypothyroidism. The patients were studied twice: when they were thyrotoxic or hypothyroid and when they had returned to a euthyroid state. Parameters of bone turnover showed a decrease when hyperthyroid patients became euthyroid: serum Ca (2.51 .+-. 0.04 vs. 2.38 .+-. 0.03 mmol/l, P < 0.05), acid phosphatase (11.7 .+-. 0.7 vs. 8.3 .+-. 0.4 U/l, P < 0.01), alkaline phosphatase (124 .+-. 11 vs. 98 .+-. 8 U/l, P < 0.05), the Ca/creatinine ratio (1.03 .+-. 0.31 vs. 0.43 .+-. 0.07, P < 0.01) and the hydroxyproline/creatinine ratio in the urine (69.9 .+-. 12 vs. 20.7 .+-. 2.4, P < 0.01). These parameters showed an increase when hypothyroid patients became euthyroid: serum Ca (2.36 .+-. 0.03 vs. 2.48 .+-. 0.04 mmol/l, P < 0.01), alkaline phosphatase (60 .+-. 4 vs. 84 .+-. 8 U/l, P < 0.05) and the hydroxyproline/creatinine ratio in the urine (15.9 .+-. 4.3 vs. 25.3 .+-. 3.2, P < 0.05). Changes in the Ca regulating hormones, parathyroid hormone, calcitonin and vitamin D metabolites, were not observed when hyperthyroid patients became euthryoid. When hypothyroid patients were treated a decrease in serum levels of 1,25-dihydroxyvitamin D (32.6 .+-. 4.6 vs. 17.9 + 2.5 ng/l, P < 0.01) was observed. Serum growth hormone levels decreased when hypothyroid patients became euthyroid (4.3 .+-. 0.5 vs. 2.6 .+-. 0.4 mU/l, P < 0.01). The possible mechanisms of thyroid action on bone are discussed. The presented findings are in accordance with a direct effect of thyroid hormones on bone in thyrotoxicosis. An additional factor could be somatomedin, that might also be involved in changes in bone turnover in hyper- and hypothyroidism.