Older women who receive too much thyroid replacement hormone or who have a history of overactive thyroid may face a greater risk of bone fractures.

However, the researchers stress, women treated with proper levels of the hormone face no heightened risk of bone breaks.

Women taking replacement thyroid hormones need to make sure their levels are being carefully monitored and kept within the normal range.

The thyroid gland churns out hormones that help regulate the body's metabolism. Normally, the gland works like a thermometer, adjusting its output based on the body's needs. But the gland can become either overactive or underactive. An underactive thyroid is typically treated with thyroid replacement hormone.

However, some evidence suggests excess thyroid hormone (through either an overactive thyroid or thyroid replacement treatment) could decrease bone mass.

Waning bone mass puts a woman at risk of developing the brittle-bone disease osteoporosis.

The investigators found that women with elevated levels of thyroid hormone were three times more likely than women with normal levels to fracture a hip, and four times more likely to sustain a spine fracture.

The majority of the women with high levels of thyroid hormone were receiving too much thyroid replacement hormone. Some however, were producing too much of the hormone in their own bodies.

Researchers suspect that elevated levels of thyroid hormone may cause high bone turnover, leading to weakened bones. ("Too much" thyroid replacement hormone was defined as a TSH level of "less than 0.1" ) Bone is constantly being broken down and rebuilt. That rate appears to be affected by excess levels of thyroid hormone.

However, women taking the proper amount of thyroid replacement were at no higher fracture risk.

An important point is if your thyroid level was in the normal range, there's been no clear evidence that thyroid hormone was bad for you.

Annals of Internal Medicine April 3, 2001;134:561-568

COMMENT:

By Mary Shomon (About.com Thyroid Expert)

Last year, an article I did on the subject, The Thyroid Treatment/Osteoporosis Controversy, presented a number of research studies that found no link between suppressive doses of thyroid hormone and increased bone loss or fracture risk, as well as other studies that, like the current one, support this theory.

Not many physicians do maintain their patients at suppressive doses such as the level described in this study. Typically, only thyroid cancer survivors -- whose TSH is suppressed to prevent cancer recurrence -- are kept at these levels.

Occasionally, a physician will treat patients with suppressive doses to reduce symptoms, but this is not common. If a physician is maintaining a patient at a suppressive dose, they will typically prescribe supplemental calcium, as well as an exercise program for building and maintaining bone. For more details, read The Thyroid Treatment/Osteoporosis Controversy.