Is My Thyroid Medication Working? These Drugs Can Cause Interactions

Dr. Sharon Orrange
Dr. Orrange is an Associate Professor of Clinical Medicine in the Division of Geriatric, Hospitalist and General Internal Medicine at the Keck School of Medicine of USC.
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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:

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

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.

Drugs featured in this story

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