Too Much Thyroid Hormone Increases Risk for Bone Fractures

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The summary below is from the full report titled "Risk for Fracture in Women with Low Serum Levels of Thyroid-Stimulating Hormone." It is in the 3 April 2001 issue of Annals of Internal Medicine (volume 134, pages 561-568). The authors are DC Bauer, B Ettinger, MC Nevitt, and KL Stone, for the Study of Osteoporotic Fractures Research Group.

What is the problem and what is known about it so far?

As people get older, their bones lose calcium and become less dense. When this bone thinning becomes severe, it is called osteoporosis. Persons with osteoporosis are at high risk for broken bones (fractures). Many older women have osteoporosis. Some older women also have problems with the thyroid gland, a gland located in the neck that makes thyroid hormone. Thyroid hormone is important in regulating the body's metabolism, particularly its use of energy, fats, and protein. Several past studies have examined whether high levels of thyroid hormone lead to osteoporosis. Results of these studies have been confusing, and few have looked at whether women with high levels of thyroid hormone actually have more fractures than women with normal thyroid hormone levels.

Why did the researchers do this particular study?

To see whether women with high thyroid hormone levels are more likely than women with normal thyroid hormone levels to have bone fractures.

Who was studied?

The study included 686 white women older than 65 years of age who were recruited from medical centers in four U.S. cities.

How was the study done?

The researchers took spine x-rays and measured thyroid-stimulating hormone (TSH) levels in all women as they entered the study. Thyroid-stimulating hormone is an indirect way of measuring actual thyroid hormone; TSH levels are low when thyroid hormone levels are high. The researchers then contacted the women by mail every 4 months for 4 years to ask whether they had had a bone fracture since the last mailing. Because many spine fractures go undiagnosed, the researchers confirmed each woman's report of a fracture by getting copies of x-rays or written x-ray reports. After 4 years, the women had another set of spine x-rays. The researchers then compared the frequency of fractures in women with and without high thyroid hormone levels (as suggested by low TSH levels).

What did the researchers find?

During the 4 years of the study, 48% of the women had a hip fracture and 56% had a spine fracture. Women whose TSH level indicated a high thyroid hormone level were three times more likely to have a hip fracture and four times more likely to have a spine fracture than were women with normal levels of TSH.

What were the limitations of the study?

Thyroid function was measured indirectly by looking at TSH levels. Measurement of thyroid hormone might provide more information about risk for fracture. Also, the study included only white women.

What are the implications of the study?

Too much thyroid hormone may put older women at risk for bone fractures. Women with abnormal thyroid function may benefit from close monitoring to ensure that their thyroid hormone levels are kept in the normal range.