CONTEXT: Postmenopausal women are at increased risk for a number of health conditions, such as osteoporosis and heart disease. Nodular thyroid disease and osteoporosis share some common factors such as: elevated frequency in the general population; major prevalence in the female sex; incidence proportional to the age. There is much information in literature regarding the real impact of chronic treatment with L-thyroxin on the bone mineral density, especially in post-menopausal women. OBJECTIVE: The purpose of this study was to undertake to evaluate the effects of L-thyroxin administration for the treatment of normal functioning nodular thyroid disease on the bone mineral density in post-menopausal women after one year of continuative treatment. The much attention was paid in examining the role of some anamnestic risk factors for osteoporosis on the clinical response. METHODS: Thirty postmenopausal women of age comprised between 50 and 56 years were examined before and after 1 year of therapy with a fixed dose of L-thyroxin for the treatment of nodular thyroid disease by monitoring the following laboratory parameters: thyroid stimulating hormone (TSH), FT4, FT3, antithyroglobulin antibodies, thyroid peroxidase antibodies and serum calcium. Bone mineral density was measured by dual X-ray absorptiometry of the lumbar vertebrae. RESULTS: The results of this study showed that the patients on treatment with LT4 have a slight, but significant reduction of the bone mineral density after 1 year of treatment. Comparison between patients with unsuppressed (group I) or suppressed (group II) TSH following LT4 treatment showed that group II patients had significantly lower BMD. The following risk factors influenced, in a statistically significant manner, the BMD: Body Mass Index <19 kg/m(2); the onset of menarche after the age of 15 years; history positive for period of amenorrhoea; nulliparity; surgical menopause; lack of hormonal replacement therapy; and presence of auto-antibodies against thyroid antigens. CONCLUSION: L-thyroxin treatment in postmenopausal women reduced significantly the BMD. This treatment should be therefore prescribed with caution in this condition and particularly when the following risk factors are present: surgically driven menopause, constitutional thinness, history of nulliparity, absence of hormonal treatment, positive history of secondary amenorrhoea during the reproductive age, autoimmune thyroid disease and delayed menarche.