The effect of long-term, non-suppressive levothyroxine treatment on quantitative ultrasonometry of bone in women

OBJECTIVE: To evaluate the impact of long-term, non-suppressive levothyroxine (L-T(4)) treatment on quantitative ultrasonometry in women. DESIGN: This was a case-control study. SUBJECTS AND METHODS: Altogether 667 women (mean age+/-s.d., 49.5+/-13.1 years) were studied. Of these, 156 (23%) had non-toxic goitre or hypothyroidism and had been taking L-T(4) (75-100 microg/day) for at least 5 years (mean+/-s.d., 12.5+/-7.5 years); the remaining 511 (77%) women were not receiving L-T(4). All women had completed a questionnaire on risk factors for thyroid dysfunction and osteoporosis, and those with diseases or treatments known to effect bone metabolism - other than thyroxine or hormone replacement therapy (HRT) - were excluded. Women underwent quantitative ultrasonometry (QUS) at the heel. Speed of sound (SOS), broadband ultrasound attenuation (BUA) and the stiffness index (SI) were compared, first, in all women taking L-T(4) and controls and, secondly, in women taking L-T(4) and controls pair-matched for age, weight, body mass index (BMI), menopausal status and HRT use. RESULTS: Even after matching for age, weight, BMI, menopausal and HRT status, women taking L-T(4) had significantly lower values for SOS and SI (P<0.05), but not for BUA. However, absolute T- and Z-scores for SI were not low in either the study or control groups. Lower values were associated, but not significantly so, with years since the menopause and duration of L-T(4) treatment. CONCLUSIONS: Long-term, non-suppressive L-T(4) treatment in women with goitre or hypothyroidism was associated with a slight reduction in QUS values, which was more pronounced in postmenopausal women. This group could be at higher risk for osteoporotic fracture.