Objective: The results of studies examining the influence of T4 therapy upon bone mineral density (BMD) are conflicting. This conflict may, in part, reflect inclusion of patients with varying thyroid disorders. We have therefore examined the influence of preceding thyroid history and T4 therapy on BMD.
Design: Case-control studies of patients on long-term T4 therapy who have or have not previously received radioiodine treatment for thyrotoxicosis, as well as previously thyrotoxic patients who have not required T4 replacement.
Patients: Twenty-seven premenopausal and 60 postmenopausal females with a past history of thyrotoxicosis and subsequent T4 treated hypothyroidism (group 1), 39 post-menopausal females with a past history of radioiodine treated thyrotoxicosis not receiving T4 (group 2) and 22 post-menopausal females with primary hypothyroidism on T4 (group 3). Female controls individually matched to patients by age and menopausal status.
Measurements: BMD measured by dual-energy X-ray absorptiometry. Serum biochemistry and tests of thyroid function.
Results: No significant differences were found in femoral or lumbar spine BMD measurements between premenopausal patients and controls in group 1 or between group 2 patients and controls. Measurements of BMD at all sites were lower in post-menopausal patients in groups 1 and 2 than in controls; when allowance was made for differences in BMD due to body mass index by analysis of variance, significant reductions in femoral trochanter BMD (3.9%, P < 0.05) and lumbar spine (5.6-8.5%, P < 0.01) BMD results were found in post-menopausal females in group 1 and reductions in femoral trochanter (3.9%, P < 0.01), Ward's triangle (5.6%, P < 0.05) and lumbar spine (8.5%, P < 0.01) BMD results in group 2. Separate analysis of BMD results of those with normal or reduced serum TSH did not affect outcome. BMD measurements were not significantly correlated with duration of T4 therapy, T4 dose, or serum free T4 or TSH in any patient group.
Conclusions: Thyroxine therapy alone does not represent a significant risk factor for loss of bone mineral density but there is a risk of bone loss in post-menopausal (but not premenopausal) females with a previous history of thyrotoxicosis treated with radioiodine.