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Is My Thyroid Medication Working? These Drugs Can Cause Interactions

by Dr. Sharon Orrange on February 17, 2015 at 2:46 pm

Levothyroxine is the best drug for the treatment of hypothyroidism. Almost 12% of women take lifelong thyroid replacement therapy in the form of the generic levothyroxine or the brand name Synthroid. The most common cause of hypothyroidism, an underactive thyroid gland, is autoimmune thyroiditis.

If you are taking your thyroid replacement and still just don’t feel right, pay attention to these common mistakes and drug interactions that may be affecting how well your levothyroxine is absorbed. Monitoring your thyroid function is done with a thyroid stimulating hormone (TSH) lab test to ensure the proper level of thyroid replacement. This becomes challenging when the TSH blood test is all over the place.

Obvious factors that cause fluctuations in absorption are skipped days and drug-to-drug interactions—and always remember that levothyroxine (and Synthroid) work best if taken 1 hour prior to breakfast which ensures the ideal stomach acidity for absorption.

Specific drugs, such as amiodarone, lithium, and iodine are known to affect thyroid status, but others are less obvious. Let’s look at meds that will mess with your TSH.

Taking these four may lead to you being “under-replaced” or hypothyroid on your current dose, meaning you may need more levothyroxine or Synthroid:

  • Iron. If you are taking iron supplements, watch it. Iron binds to thyroxine, disrupting absorption and decreasing the amount of thyroxine available in the circulation.
  • Calcium. Calcium carbonate is often taken as an antacid or to prevent osteoporosis in menopausal women. Free thyroid hormone levels are lowered in people taking calcium with their levothyroxine. Your levothyroxine sort of bonds to calcium carbonate in an acidic environment, which decreases the availability of thyroxine.
  • Estrogens. If you are taking estrogen oral contraceptives or hormone replacement therapy, watch it, because estrogen affects levels of free circulating thyroid hormone. It does this by increasing the amount of thyroid binding globulin, which binds to free hormone in the circulation. Patients who are taking estrogen hormone replacement therapy often have to increase their dose of levothyroxine.

These two may cause you to be “over-replaced” meaning you may have to lower your levothyroxine dose:

  • Statins. These are your cholesterol lowering drugs like atorvastatin (Lipitor), simvastatin (Zocor) and Crestor. In a recent study of 11,000 people taking levothyroxine, statin use caused a significant decrease in TSH indicating a need for a lower thyroid medication dose. We don’t know if this is a change in gastrointestinal absorption of thyroxine or has to do with liver metabolism but these are so often prescribed together—watch it.
  • Metformin. Some studies have shown a decrease in TSH in patients taking the diabetes medication metformin. Metformin may change thyroid hormone receptor affinity we just aren’t sure.

Reassuring news from a large clinical trial though: drugs that did not affect TSH in people taking levothyroxine were glucocorticoids (steroids) or the H2 receptor antagonists (Pepcid, Zantac, Tagamet, etc).

Dr O.

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