Thyroid function and bone turnover

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 calcium (2.51 +/- 0.04 vs 2.38 +/- 0.03 mmol/l, P less than 0.05), acid phosphatase (11.7 +/- 0.7 vs 8.3 +/- 0.4 U/l, P less than 0.01), alkaline phosphatase (124 +/- 11 vs 98 +/- 8 U/l, P less than 0.05), the calcium/creatinine ratio (1.03 +/- 0.31 vs 0.43 +/- 0.07, P less than 0.01) and the hydroxyproline/creatinine ratio in the urine (69.9 +/- 12 vs 20.7 +/- 2.4, P less than 0.01). These parameters showed an increase when hypothyroid patients became euthyroid: serum calcium (2.36 +/- 0.03 vs 2.48 +/- 0.04 mmol/l, P less than 0.01), alkaline phosphatase (60 +/- 4 vs 84 +/- 8 U/l, P less than 0.05) and the hydroxyproline/creatinine ratio in the urine (15.9 +/- 4.3 vs 25.3 +/- 3.2, P less than 0.05). Changes in the calcium regulating hormones, parathyroid hormone, calcitonin and vitamin D metabolites, were not observed when hyperthyroid patients became euthyroid. 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 less than 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 less than 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.