Serum osteocalcin in patients taking L-thyroxine who have subclinical hyperthyroidism

Serum osteocalcin, an index of osteoblastic activity, is increased in hyperthyroidism. Serum osteocalcin levels are negatively correlated with bone density in patients with overt hyperthyroidism. Osteocalcin levels are also elevated in patients with multinodular goiter and subclinical hyperthyroidism. We therefore measured serum osteocalcin levels in patients taking T4 to determine if they correlated with the degree of TSH suppression. Despite an upward trend in serum osteocalcin measurements with decreasing TSH concentrations, there was no significant difference in serum osteocalcin among groups of patients with normal (0.5-5.0 mu/L), mildly reduced (0.1-0.5 mU/L), or undetectable serum TSH (less than 0.01 mU/L). However, a weak negative correlation was seen between serum TSH and osteocalcin concentrations (r = 0.29, slope = -0.28, P less than 0.05). Osteocalcin did not correlate with either serum free T4 or free T3 concentrations. Serum PTH concentrations were not different among the three patient groups. Our data suggest that osteocalcin is not a useful clinical marker for increased bone turnover in patients with subclinical hyperthyroidism due to T4 therapy. However, the trend towards higher osteocalcin levels in patients with suppressed serum TSH values, and the weak negative correlation between serum TSH and osteocalcin are consistent with findings of reduced bone density in these patients.