[Prolonged suppressive L-thyroxine therapy. Longitudinal study of the effect of LT4 on bone mineral density and bone metabolism markers in 71 patients].

Objectives: A prospective longitudinal study was conducted to investigate the influence of prolonged suppressive L-thyroxin therapy on bone density and biochemical markers of bone remodeling.
Patients and Methods: Seventy-one patients (including 28 menopaused women) taking long-term L-T4 for thyroid carcinoma were divided into 3 groups according to their TSH level: low (TSH < 0.04 mlU/l), moderate (0.04 TSH < or = 0.10 mlU/l) and high (TSH > 0.10 mlU/l). Bone density was measured in lumbar vertebrae annually for a mean 4.5 years. Bone metabolism markers were measured over a 4 year period. Bone density measurements of the femur were also obtained for 2 years in 16 menopaused women.
Results: Lumbar bone density did not decline whatever the TSH level or the duration of L-T4 treatment. Likewise for menopaused women without substitution estroprogesterone therapy. Over the 4 years, biochemical markers of bone formation, including bone alkaline phosphatases and osteocalcin, or of bone resorption, including urinary hydroxyprolin, did not vary. In addition, in menopaused women, femoral bone density was not significantly lowered over the 2 years follow-up. No lumbar or femoral osteopenia was observed in these patients taking L-thyroxin, even for those with complete TSH blockade. Biochemical markers did not demonstrate a significant acceleration of bone turnover during prolonged administration of L-T4 at suppressive levels.