Effect of replacement doses of thyroxine on bone mineral density.

INTRODUCTION

Hyperthyroidism is associated with a reduction in bone mineral density (BMD). Suppressive doses of thyroxine (T4), inducing subclinical hyperthyroidism, have been reported by some investigators to reduce BMD. Little work has been done on replacement doses of T4.

AIM

The aim was to investigate the effect of replacement doses of T4 on BMD.

STUDY DESIGN

Cross-sectional study of hypothyroid patients on long-term T4 replacement doses, comparing those who had primary hypothyroidism with those who were previously hyperthyroid.

PATIENTS

Fifty women on replacement doses of T4 for more than 5 years were recruited. Twenty-five were treated for primary (group 1) and 25 for radioiodine-induced hypothyroidism (group 2). They were well matched for age, menstrual status, smoking history, body mass index (BMI), dose and duration of T4 replacement as well as thyroid status.

MEASUREMENTS

BMD was assessed by dual energy X-ray absorptiometry. Free T4 (FT4), FT3 as well as ultrasensitive TSH assays were used to assess thyroid status.

RESULTS

The two groups showed no difference in BMD (g/cm2) of the lumbar spine (1.008 vs. 0.957, P = 0.25), femoral neck (0.745 vs. 0.735, P = 0.79) and total hip (0.878 vs. 0.837, P = 0.24). When the two groups were pooled, there was no significant difference between the patients and a reference population with femoral neck and total hip BMD expressed as a standard deviation (Z) score. However, the lumbar spine mean Z score was significantly greater than zero. For each site, there was a negative correlation of BMD with age in at least one group but, in general, BMI, FT4, FT3 and duration of T4 replacement did not correlate with BMD. T4 dose, however, had a consistent positive correlation with BMD in the spine, femoral neck and the hip (P = 0.01, 0.04 and 0.02, respectively) in group 2 but not group 1.

CONCLUSION

In this study, there is no evidence for a difference in bone mineral density in patients receiving replacement doses of thyroxine irrespective of the aetiology of their hypothyroidism. The reduced bone mineral density associated with hyperthyroidism appears to be restored, maintained and in some cases possibly improved while on long-term thyroxine replacement post-radioiodine.