L-thyroxine prevents the bone-conserving effect of HRT in postmenopausal women with subclinical hypothyroidism

Hypothyroidism, which is a common disorder among postmenopausal women, may be associated with higher than average bone mineral content. Contrarily, treatment with L-thyroxine may cause a significant bone loss. The aim of our study was to evaluate the effects of hormone-replacement therapy (HRT) on bone density in women with subclinical hypothyroidism treated with L-thyroxine. A total 73 postmenopausal women with thyroid-stimulating hormone (TSH) levels > 5 mU/l and normal free thyroxine values, who never used HRT or L-thyroxine, were divided into three groups according to the treatment given during a 3-year follow-up period: 34 women received only HRT; 20 women received HRT and L-thyroxine, and the remaining 19 women received neither medications. A euthyroid control group included 41 postmenopausal women with TSH levels between 0.5 and 1.5 mU/l, who were using HRT since the initial visit. Lumbar spine bone density measurements were performed at baseline and study termination. Taken as a whole, the hypothyroid women had a non-significant higher baseline bone mineral density (BMD) as compared to the euthyroid controls (1.068 ± 0.19 g/cm² vs. 1.024 ± 0.15). After 3 years, both the euthyroid and hypothyroid women on HRT only had an increase in BMD (0.032 ± 0.04 g/cm² and 0.028 ± 0.05 g/cm², respectively; p < 0.001 for both, compared to baseline). Hypothyroid women using no medication had a decrease of 0.034 ± 0.07 g/cm² in BMD, and those receiving both HRT and L-thyroxine lost the most: 0.04 ± 0.08 g/cm² (p < 0.05 for both, compared to baseline). The addition of L-thyroxine thus prevented the beneficial effect of HRT on BMD. Thyroid hormone replacement is recommended only when overt symptoms of hormone deficiency occur. In such cases, a single bone-conserving treatment with HRT may not suffice.