There are a variety of reasons why you may need to take a larger-than-usual dose of thyroid hormone replacement medication, but one of the most common reason is that you are taking a prescription or over-the-counter medication that affects absorption of your thyroid medication.
For example, before you grab that Tums or Rolaids along with a thyroid drug, think again. Antacids that include calcium carbonate and/or aluminum hydroxide can significantly reduce thyroid medication absorption. (There's some evidence that antacids with magnesium may also have an effect.) They should be taken four to six hours apart from thyroid medications.
Iron also supplements impair absorption. They should be taken four to six hours apart from thyroid medications.
Some other absorption blockers:
- Sucralfate - ulcer drug - allow at least 8 hours apart
- Cholestyramine and other cholesterol lowering drugs - 4-6 hours apart
- Ciprofloxacin (Cipro) and other quinolone antibiotics - at least 4 hours apart
- Raloxifene (Evista) for osteoporosis - 12 hours apart
- Sevelamer - for kidney disease
- Antiepileptic drugs - Rifampin, efavirenz, nevirapine, rifabutin and rifapentine
- St. John's wort
- Imatinib (Gleevec) - cancer drug
- Sertraline (Zoloft)
Talk to your doctor or pharmacist about all drugs and supplements that you are taking alongside your thyroid hormone replacement medication, to make sure that your timing minimizes any interference or absorption problems.