June 6, 2000 -- Most women now know that taking calcium is a cheap and easy way to protect against bone loss, but such supplementation may be tricky in those being treated for an underactive thyroid. Researchers report that calcium may interfere with the absorption of the most widely used therapy for this condition, and they raise a red flag that the two should not be taken together.
"Patients and their physicians need to be made aware that calcium can prevent the absorption of thyroxine" and this can be prevented by taking the two six to 12 hours apart, study author Jerome M. Hershman, MD, of the University of California Los Angeles School of Medicine, tells WebMD. Harshman and colleagues reported their findings in the June 7 issue of the Journal of the American Medical Association.
"Patients typically take both at breakfast, with the thyroxine being taken on an empty stomach before eating and the calcium being taken after eating," Hershman says. "Based on our findings, this probably should not be done."
An expert on thyroid therapy who was not involved with this study says taking the two drugs hours apart probably couldn't hurt, but it is not clear from this study if it is necessary. "The authors have shown that there may be an interaction here, but more study is needed to definitively prove this," Stephen I Sherman, MD, tells WebMD. "I would like to have seen different kinds of studies." Sherman is the medical director of the Thyroid Society for Education and Research and is associate professor of medicine at the University of Texas MD Anderson Cancer Center.
It has been estimated that up to one in 10 Americans have some degree of thyroid problems, although in many, the condition is never diagnosed. It is not known what percentage of people being treated with thyroxine also take calcium, but it is probably high. Underactive thyroid, or hypothyroidism, primarily occurs in women at or beyond the age of menopause -- the same group most at risk for osteoporosis or bone loss, and thus, most likely to take calcium supplements.
Hershman and colleagues examined the effect of calcium on thyroxine absorption in a group of 20 people being treated for underactive thyroid conditions. The levels of thyroxine in the women's blood were measured for several months prior to beginning calcium supplementation. Then the levels were retested during a three-month period while they were taking calcium and tested again several months after they stopped. All patients were told to take the calcium supplements daily at the same time that they took their thyroid medications.
The researchers saw a "modest, but significant" effect on thyroid function during the period patients took calcium. Four of the 20 patients had indications from blood tests that their medication wasn't getting into the blood. But these indicators returned to normal when the patients stopped taking calcium.
These findings and others, Harshman says, show that it is critical for thyroid patients to tell their physicians about all the medications they take. Prior studies have shown that other widely used therapies, such as aluminum hydroxide, found in various antacids; high-dose iron; and sucralfate, widely prescribed for gastrointestinal disorders, have a negative impact on the absorption of thyroxine.
Sherman, who conducted the sucralfate studies, agrees. He saw a "75 to 90%" rate of absorption problems in people taking thyroxine and sucralfate at the same time. "Obviously, this induced significant hypothyroidism. But because this is a drug that patients are prescribed, it is not likely a clinician won't know about its use," Sherman says. "In the case of aluminum hydroxide, high-dose iron, and now calcium, patients may not think to tell their physicians they are taking them. Physicians need to ask their patients about over-the-counter medications."
- Up to one-tenth of all Americans have some degree of thyroid problems, and an underactive thyroid primarily affects postmenopausal women, who also are at higher risk for osteoporosis.
- Many of these women take calcium supplements to protect against bone loss. This can cause the medication known as thyroxine, which is widely prescribed to treat an underactive thyroid, to have problems getting into the bloodstream. This can cause thyroxine to be less effective.
- Patients can take these two drugs six to 12 hours apart, instead of together, to prevent interference, and everyone should always inform their doctor of all medications and supplements they are taking.