Bone density drops with thyroid therapy.

Bone density drops with thyroid therapy

Premenopausal women undergoing long-term therapy with thyroid hormones suffer a loss of hip-bone mass and may have an increased risk of hip fractures later in life, a new study indicates. The report's authors suggest thyroid hormones -- among the most commonly prescribed medications for woment -- are less innocuous than many doctors believe and dosages should be carefully monitored and tailored to individual needs.

Thyroid hormones are prescribed, often in doses higher than those considered physiologically normal, for a variety of conditions characterized by a decline in thyroid-gland activity. The therapy can restore normal hormone levels or can mimic hyperthyroidism, in which thyroid gland function is increased above normal levels, to suppress other hormones that can exacerbate cancer or goiter.

Naturally occurring hyperthyroidism has been associated with osteoporosis, a progressive weakening of the bones due to decreased bone density. But other factors -- including decreased estrogen levels in older women -- also lead to osteoporosis, and this has confounded previous studies looking at the influence of thyroid hormone therapy on bone density.

Researchers at the University of Massachusetts Medical School in Worcester studied 31 premenopausal women who received l-thyroxin (l-T.sub.4.) for at least five years and compared their bone densities with those of age- and weight-matched women not on the hormone. Both groups had similar lumbar-spine densities, but hip-bone densities in women treated with l-T.sub.4 were decreased 10 to 13 percent.

"Our finding ... Raises questions the appropriate dosage of L-T.sub.4 in these younger women, since the morbidity, mortality and expense arising from hip fractures later in life are now well documented," the researchers report in the June 3 JOURNAL OF THE AMERICAN MEDICAL ASSOCIATION. Despite the general belief that there is "realatively little risk" associated with thyroid hormone therapy, they say, high doses should be given only when absolutely necessary -- such as to suppress thyroid-stimulating hormone in women previously treated for thyroid cancer.

"The key issue is that the dose of thyroid hormone has to be appropriate for the condition being treated," Daniel T. Baran, a coauthor of the report, told SCIENCE NEWS. Improved blood tests now allow more careful monitoring of hormone levels, he adds.

Meanwhile, debate continues over the value of dietary calcium supplements for restoring bone density in women with osteoporosis. New research reported last week by Christopher E. Cann of the University of California at San Francisco suggests that women age 20 to 45 may be able to increase bone density by 3 percent over a two-year period by more than doubling dietary intake of calcium to 1,500 milligrams per day.