NEW YORK -- A drug designed to help women with the brittle-bone disease osteoporosis may block absorption of thyroid hormone, a new study suggests. This drug combination is taken by millions of women in the U.S.

Researchers from the Cleveland Clinic Foundation showed that the drug, raloxifene (Evista), could interfere with the absorption of thyroid hormone when both medications were taken at the same time, according to the study published in the Archives of Internal Medicine.

As many as 2.5 million women could be taking both medications, according to study co-author Dr. Sethu K. Reddy, chairman of the department of endocrinology, diabetes and metabolism at the clinic.

The take-home message for patients taking thyroid hormone is to have thyroid levels rechecked after they are prescribed new medications, Reddy said in an interview with Reuters Health.

"When your physician adds a new medication, then maybe in the next three months, you should get your thyroid levels checked again to make sure you don't need a dosage adjustment," Reddy said.

The new study focused on the impact of raloxifene on a 79-year-old woman who had been taking the same dose of thyroid hormone for several years. The woman was prescribed the hormone after her thyroid was surgically removed.

Within two to three months of starting on raloxifene to treat osteoporosis, the woman started having symptoms of low thyroid hormone, Reddy said. Doctors increased her dosage of thyroid hormones several times before referring the woman to Reddy's clinic.

To determine whether raloxifene was causing the problem, Reddy asked the woman to change the way she was taking the drugs. Rather than take them both at the same time, as she had been doing, the woman was asked to take the thyroid hormone first and then the raloxifene 12 hours later.

When the patient took the medications on this schedule, her thyroid levels normalized, Reddy said.

To check the result, Reddy asked the patient to take the medications at the same time again. Sure enough, the woman had trouble absorbing the thyroid hormone.

While many women might also be taking the two drugs simultaneously, not all of them might have as strong of an effect as the woman studied by Reddy and his colleagues. "She might have had a more dramatic effect," Reddy said. "In some, absorption might only be blocked by 10 or 15 percent. And that might not be enough to be noticed by the patient." Debbie Davis, a spokeswoman for Eli Lilly and Company, which markets raloxifene as Evista, told Reuters Health that the report "is a case study of a single patient and does not provide substantial evidence of raloxifene causing malabsorption of (thyroid hormone)."

Davis pointed out that the authors mention in the article that they were not aware of any other reports of Evista or other similar drugs interfering with the absorption of thyroid hormone. "Safety is of the utmost importance to Eli Lilly and Company," Davis said. "More than 30,000 postmenopausal women have participated or are participating in global clinical trials involving Evista, placing it among the most extensively researched pharmaceutical products in the history of Eli Lilly and Company and women's health."

Thyroid hormones are essential during childhood for normal physical growth and intellectual development. The hormones also help regulate a variety of metabolic processes in all age groups.

Symptoms of an under-active thyroid, or hypothyroidism, include fatigue, cold intolerance, dry skin, lethargy and weight gain. The condition can be caused by a birth defect, or it can occur as a result of inflammation of the thyroid, goiter therapy or surgical removal of the thyroid gland.