Low normal TSH levels are associated with low bone mineral density in healthy postmenopausal women.

OBJECTIVE

Hyperthyroidism is accompanied by low bone mass. Because the reference range of TSH levels is defined statistically, some individuals with low normal TSH levels may have mild hyperthyroidism and reduced bone mass. We therefore determined whether serum TSH levels correlate with bone mineral density (BMD).

DESIGN

A cross-sectional hospital-based survey. Participants Nine hundred and fifty-nine healthy postmenopausal women.

MEASUREMENTS

We measured BMD at the lumbar spine and femoral neck using dual energy X-ray absorptiometry, and serum TSH concentrations using immunoluminometry.

RESULTS

BMD at the lumbar spine and femoral neck increased with TSH level (P for trend < 0.001 at both sites). Even after adjustment for age, years since menopause and body mass index, subjects with low normal TSH levels (0.5-1.1 mU/l) had significantly lower BMDs at the lumbar spine (0.863 +/- 0.009 g/cm2 vs 0.900 +/- 0.009 g/cm2, P = 0.004) and femoral neck (0.660 +/- 0.006 g/cm2 vs 0.683 +/- 0.006 g/cm2, P = 0.006) than those with high normal TSH levels (2.8-5.0 mU/l), as well as a 2.2-fold increased risk of osteoporosis (95% confidence interval: 1.2-4.0).

CONCLUSION

These results suggest that low normal TSH levels may not be physiological for postmenopausal women and, during treatment of hypothyroidism, may not be adequate for avoiding osteoporosis.