Hyperthyroidism Causes Osteoporosis
Hyperthyroidism—not hypothyroidism—is the thyroid disorder that causes osteoporosis and increases fracture risk. This relationship is well-established in both overt and subclinical forms of hyperthyroidism, particularly affecting postmenopausal women 1, 2.
Mechanism of Bone Loss in Hyperthyroidism
Excess thyroid hormone accelerates bone turnover with predominant bone resorption over formation 2, 3. The thyroid hormone surplus enhances osteoclast-driven bone resorption, creating a high-bone turnover state that leads to secondary osteoporosis 4. This accelerated remodeling cycle results in net bone loss and decreased bone mineral density (BMD) 2.
Clinical Evidence for Hyperthyroidism and Fracture Risk
Overt Hyperthyroidism
- Untreated severe hyperthyroidism significantly influences bone mass and increases the probability of high-turnover osteoporosis 2
- Overt thyrotoxicosis increases fracture risk in most studies, particularly affecting hip and spine 1
- Adults with thyrotoxicosis have an established increased risk of osteoporotic fractures 5
Subclinical Hyperthyroidism
- Subclinical hyperthyroidism (low TSH with normal free thyroid hormones) decreases BMD and increases fracture risk, especially in postmenopausal women 2, 6
- Two meta-analyses reported significant declines in BMD during prolonged subclinical hyperthyroidism, with exogenous subclinical hyperthyroidism causing significant BMD loss among postmenopausal women but not premenopausal women 1
- One prospective study found increased risk of hip and spine fractures in levothyroxine-treated women older than 65 years with TSH ≤0.1 mIU/L 1
- Endogenous subclinical hyperthyroidism also adversely affects BMD and is associated with fractures 4
Iatrogenic Hyperthyroidism
- Long-term TSH-suppressive doses of levothyroxine (used for thyroid nodules or thyroid carcinoma) may decrease BMD and induce increased fracture risk 2
- These effects are particularly pronounced in postmenopausal women but less evident in premenopausal women 2
- Patients with chronically suppressed TSH should ensure adequate daily calcium (1200 mg/day) and vitamin D (1000 units/day) intake 7
Hypothyroidism and Bone Health
In contrast, hypothyroidism does not cause osteoporosis. Overt hypothyroidism actually lowers bone turnover by reducing both osteoclastic bone resorption and osteoblastic activity, which results in increased bone mineralization 2. At present, there are no clear data demonstrating any relationship between decreased BMD and hypothyroidism in adults 2. Hypothyroidism in children causes impaired skeletal maturation and mineralization, but adequate thyroid hormone replacement stimulates normal bone growth 4.
Treatment Implications
Restoration of euthyroid status reverses bone loss in most patients with overt or subclinical hyperthyroidism 4. Treatment of hyperthyroidism to normalize TSH preserves BMD, though normalization of bone turnover may be delayed up to 1 year 1. Two studies of endogenous subclinical hyperthyroidism in postmenopausal women demonstrated significant continued bone loss in untreated patients compared with bone stabilization in treated patients 1.
Special Populations at Highest Risk
Postmenopausal women with TSH suppression face the greatest osteoporosis risk 1, 2. Women over 65 years with TSH ≤0.1 mIU/L have markedly increased risk of hip and spine fractures 1, 8. For postmenopausal women receiving TSH suppression therapy for thyroid cancer, antiresorptive treatments may be indicated 4.