Thyroid Function within the Upper Normal Range Is Associated with Reduced Bone Mineral Density and an Increased Risk of Nonvertebral Fractures in Healthy Euthyroid Postmenopausal Women

Abstract

CONTEXT: The relationship between thyroid function and bone mineral density (BMD) is controversial. Existing studies are conflicting and confounded by differences in study design, small patient numbers, and sparse prospective data. OBJECTIVE: We hypothesized that variation across the normal range of thyroid status in healthy postmenopausal women is associated with differences in BMD and fracture susceptibility. DESIGN: The Osteoporosis and Ultrasound Study (OPUS) is a 6-yr prospective study of fracture-related factors. SETTING: We studied a population-based cohort from five European cities. PARTICIPANTS: A total of 2374 postmenopausal women participated. Subjects with thyroid disease and nonthyroidal illness and those receiving drugs affecting thyroid status or bone metabolism were excluded, leaving a study population of 1278 healthy euthyroid postmenopausal women. INTERVENTIONS: There were no interventions. MAIN OUTCOME MEASURES: We measured free T(4) (fT4) (picomoles/liter), free T(3) (fT3) (picomoles/liter), TSH (milliunits/liter), bone turnover markers, BMD, and vertebral, hip, and nonvertebral fractures. RESULTS: Higher fT4 (beta = -0.091; P = 0.004) and fT3 (beta = -0.087; P = 0.005) were associated with lower BMD at the hip, and higher fT4 was associated with increasing bone loss at the hip (beta = -0.09; P = 0.015). After adjustment for age, body mass index, and BMD, the risk of nonvertebral fracture was increased by 20% (P = 0.002) and 33% (P = 0.006) in women with higher fT4 or fT3, respectively, whereas higher TSH was protective and the risk was reduced by 35% (P = 0.028). There were independent associations between fT3 and pulse rate (beta = 0.080; P = 0.006), increased grip strength (beta = 0.171; P<0.001), and better balance (beta = 0.099; P < 0.001), indicating that the relationship between thyroid status and fracture risk is complex. CONCLUSIONS: Physiological variation in normal thyroid status is related to BMD and nonvertebral fracture.