This study investigated the effect of long-term treatment upon bone density with L-Thyroxine in postmenopausal women compared with untreated postmenopausal women with climacteric symptoms. We measured spinal bone density in three groups (n = 84) of postmenopausal women: (A) those treated with TSH-suppressive doses of L-Thyroxine for a medium of 5 years after removal of thyroid cancer; (B) those on L-Thyroxine treatment for a median of 9 years after being diagnosed with chronic lymphocytic thyroiditis (CLT); and (C) those with no thyroid disease or other known pathology and without any treatment. There were no differences in dietary calcium intake and daily activity between untreated and L-Thyroxine-treated women. Measurements of bone mineral density were performed at spine level L1-L4 using a dual X-ray densitometer and serum thyroid-stimulating hormone (TSH), thyroid hormones, and bone markers (serum osteocalcin, procollagen I, urinary calcium), and PTH levels were assayed and found to be within normal ranges. Women receiving L-Thyroxine after thyroid cancer had slightly higher FT4 levels compared with women who had CLT and lower TSH levels, with serum T4 and T3 levels normal and similar in both groups. No significant differences were found in spinal bone density after L-Thyroxine treatment between Groups A and B and compared with Group C. Bone loss according to 2 SD below reference standards (age and sex matched) was found in the 12.9% of L-Thyroxine-treated patients versus 22.6% of untreated women. No correlation was found between bone loss and thyroid hormone levels and duration of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)