**4.2. Hyperthyroidism**

Hyperthyroid patients present with an increased bone turnover and a risk for osteoporosis. The activity of osteoblasts and osteoclasts are increased, the latter predominates favoring resorption, negative balance of calcium, and bone loss (Melsen & Mosekilde, 1977; Mosekilde et al, 1990). Thyrotoxicosis in adults is a recognized cause of high-bone-turnover osteoporosis. Reduced bone mineral density was noted in hyperthyroid patients with an increased susceptibility to fragility fracture (Mosekilde et al, 1990; Vestergaard et al, 2002, 2005).

In both, clinical and subclinical hyperthyroidism, elevation of markers of bone turnover and decreased BMD have been reported (Kumeda et al, 2000; Heemstra et al, 2006; Lee et al, 2006). Previous studies that investigated impact of thyroid dysfunction on BMD and fracture risk did not provide conclusive results. Recently published population studies indicate association of endogenous subclinical hyperthyroidism with an increased fracture risk (Bauer et al, 2001; Jamal et al, 2005; Vadiveloo et al, 2011). Consequences of the hyperthyroid status (overt and subclinical) on bone turnover, BMD and fracture risk will be discussed and compared with the data in healthy population.
