Thyroid Hormone Use and Bone Mineral Density in Elderly Women: Effects of Estrogen

Abstract

OBJECTIVE: To determine the effect of long-term use of thyroid hormone on bone mineral density (BMD) in elderly women and the potential mitigating effects of estrogen replacement therapy. DESIGN: Cross-sectional, community-based study. SETTING: Rancho Bernardo, Calif. PARTICIPANTS: A total of 991 white women aged 50 to 98 years who participated in a study of osteoporosis. MAIN OUTCOME MEASURES: Bone mineral density at the ultradistal radius and midshaft radius using single-photon absorptiometry and at the hip and lumbar spine using dual-energy x-ray absorptiometry. RESULTS: A total of 196 women taking thyroid hormone for a mean duration of 20.4 years were compared with 795 women who were not using thyroid hormone. Women taking daily thyroxine-equivalent doses of 200 micrograms or more had significantly lower BMD levels at the midshaft radius and hip compared with those taking less than 200 micrograms. Daily doses of 1.6 micrograms/kg and greater were associated with lower bone mass at all four sites compared with nonuse, whereas doses less than 1.6 micrograms/kg were not associated with lower BMD levels. These associations were independent of age, body mass index, smoking status, and use of thiazides, corticosteroids, and estrogen. Women taking both estrogen and a thyroid hormone dose of 1.6 micrograms/kg or greater had significantly higher BMD levels at all four sites than women taking the same thyroid hormone dose alone. Women taking both thyroid hormone and estrogen had BMD levels comparable with those observed in women taking only estrogen. CONCLUSIONS: Long-term thyroid hormone use at thyroxine-equivalent doses of 1.6 micrograms/kg or greater was associated with significant osteopenia at the ultradistal radius, midshaft radius, hip, and lumbar spine. Estrogen use appears to negate thyroid hormone-associated loss of bone density in postmenopausal women.