Calcium Supplements May Interfere With Thyroid Treatment
WebMD News Archive
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
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
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.