Summaryobjective  To investigate whether previous hyperthyroidism is a cause of permanent secondary osteoporosis.design and patients  In this cross‐sectional study, 164 women with untreated or previously treated overt and symptomatic hyperthyroidism were examined 0–31 years after the initial episode of hyperthyroidism and its treatment, and were compared with a control group of 79 age‐matched women without previous history of hyperthyroidism. Subjects with current or previous metabolic bone disease, any antiresorptive treatment for osteoporosis or treatments and habits known to affect bone metabolism were excluded.measurements  The age of the first manifestation of the disease, the age at the measurement of bone mineral density (BMD) at the spine and femoral neck and the interval between diagnosis and treatment of hyperthyroidism and BMD measurement were recorded and the Z‐scores and T‐scores of BMD were analysed.results  Untreated hyperthyroidism and hyperthyroidism up to 3 years after its diagnosis and treatment were associated with decreased BMD. Three or more years after the first episode of the disease the mean Z‐score at both skeletal sites was near zero and not different from the controls. The age at which hyperthyroidism was manifested for the first time had no effect on the final outcome. Women affected at a young age (13–30 years) had a more pronounced loss of BMD when examined untreated or early (< 3 years) after diagnosis, but a BMD significantly above zero if examined later (> 3 years). Older women (aged 51–70 years) showed a similar pattern, although the differences were not significant. Middle‐aged subjects (31–50 years) had the smallest loss of BMD during the first 3 years. Analysis of T‐scores of former hyperthyroid women aged ≥ 51 years showed no significantly different relative risk (RR) for osteoporosis in comparison with the controls. However, the study was not powered enough to give meaningful RR results.conclusions  Overt symptomatic hyperthyroidism is associated with decreased BMD during the first 3 years after diagnosis and treatment of the disease. After this interval, former hyperthyroid women have a Z‐score near zero and not different from women without a history of the disease, apparently because of recovery of the bone density lost early during the course of the disease. Symptomatic hyperthyroidism does not seem to be a cause of long‐lasting osteoporosis, and the age of the patient during the first episode is irrelevant.