Taking calcium supplements or calcium antacids can interfere with your body's ability to absorb levothyroxine, a form of thyroid medication. If this occurs, you may exhibit symptoms of hypothyroidism, especially if your thyroid gland cannot produce its own thyroid hormone, or T4, which regulates your metabolism.
Levothyroxine is a synthetic form of thyroid hormone that has an identical structure to that of your own thyroid hormone, according to Drugs.com. Doctors commonly prescribe it orally for patients with underactive or inactive thyroid glands. Calcium carbonate antacids and supplements containing calcium carbonate are more likely to prevent your cells from absorbing levothyroxine if you take the calcium and levothyroxine at the same time.
How It Works
Calcium carbonate can form an insoluble compound with levothyroxine that prevents your cells from absorbing the medication. If your absorption of levothyroxine falls, your body typically responds by producing more thyroid-stimulating hormone, which normally induces your thyroid gland to increase its production of T4. If your thyroid gland can produce any of its own T4, the increased TSH may help it to compensate in part for the decreased absorption of levothyroxine.
Taking Your Medications
If you have gastrointestinal problems, you may be even less able to absorb levothyroxine if you take calcium carbonate. You can prevent this from happening if you allow four or more hours to pass between taking the two different agents. It may be necessary for your doctor to monitor your levels of TSH. You should also be alert to signs of hypothyroidism.
Hypothyroidism can cause symptoms like lack of energy, weight gain, constipation, depression, and brittle hair and nails, according to MayoClinic.com. Other forms of calcium supplements such as calcium acetate and calcium citrate appear to block the absorption of levothyroxine, much as the carbonate form does. The decrease in absorption ranges from 20 to 25 percent, according to a study published in the May 2011 issue of "Thyroid."
- "Pharmacy Times"; Drugs That Inhibit Levothyroxine Absorption; John Horn, et al.; November 2008
- Drugs.com: Table 2: Drug-Thyroidal Axis Interactions
- "Thyroid"; Absorption of Levothyroxine When Coadministered With Various Calcium Formulations; I. Zamfirescu, et al.; May 2011
- MyThyroid.com: Medications Used for the Treatment of Hypothyroidism