Fluoroquinolone Antibiotics and Thyroid Medication Interactions

Cipro Antibiotic Treament for Anthrax
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Antibiotics are life-saving medications, and many thyroid patients will get bacterial infections along the way whether a sinus infection, urinary tract infection, or other common infection and be prescribed an antibiotic. This article looks at the key things to know about the interaction between fluoroquinolone antibiotics like Cipro and the potential for interaction with your thyroid medication and treatment.

There are over one hundred antibiotics, but most come from only a few specific types. Most antibiotics have two names — the brand name (also known as a trade name) and a generic name which is based on the antibiotic’s chemical class/structure.

The main classes of antibiotics are listed here. For reference, the brand names are capitalized, and the generic names are in lower case.

  • Aminoglycosides – Garamycin (gentamicin) and Tobrex (tobramycin)
  • Cephalosporins – Keflex (cephalexin)
  • Macrolides – E-Mycin (erythromycin), Biaxin (clarithromycin), Zithromax (azithromycin)
  • Penicillins – amoxicillin and penicillin
  • Sulfonamides – Bactrim (co-trimoxazole) and Proloprim (trimethoprim)
  • Tetracyclines – Sumycin (tetracycline) and Vibramycin (doxycycline)
  • Fluoroquinolones – Cipro (ciprofloxacin), Levaquin (levofloxacin), Floxin (ofloxacin)

Antibiotics and Your Thyroid Medication

Fluoroquinolone antibiotics are often prescribed for urinary tract infections, pneumonia, and sexually transmitted diseases.

Cipro (ciprofloxacin) was also in the news in the past during the anthrax scare, given its use in treatment for anthrax exposure.

What Thyroid Patients Need to Know

The common fluoroquinolone antibiotic ciprofloxacin — brand names include Cipro, Proquin, Ciproxin, Ciprobay, Cirpoxine, and Ciflox — have been shown to significantly decrease absorption of levothyroxine, the common thyroid hormone replacement medication (known by brands names such as Synthroid, Levoxyl, and Tirosint.) 

While the research is less clear, some experts believe that other antibiotics — including Levaquin (levofloxacin), Maxiquin (lomefloxacin), Avelox (moxifloxacin), Norox-In (norfloxacin) and Floxin (ofloxacin) — can also cause decreased absorption of levothyroxine.

What happens with these antibiotics is that they can have several effects:

  • They may be decreasing your absorption of the levothyroxine you are taking, resulting in lower circulating blood levels of thyroid medication in your bloodstream
  • They may increase the rate at which your body metabolizes and gets rid of the levothyroxine, again resulting in lower circulating blood levels of thyroid medication in your bloodstream
  • They may change the way levothyroxine binds to proteins in your bloodstream and is absorbed, affecting your thyroid levels as well.

The British Medical Journal Findings

The British Medical Journal reported on several cases of unexplained hypothyroidism in thyroid patients (on levothyroxine) who were taking ciprofloxacin.

Researchers have found that oral ciprofloxacin interacts with levothyroxine (i.e., Synthroid, Levoxyl) if taken together.

In one case studied, a woman taking 125 mcg a day of levothyroxine took ciprofloxacin (750 mg twice a day) and her TSH level rose to 44 after four weeks. Even when her dosage was raised to 200 mcg a day, her TSH didn't respond. Only when the ciprofloxacin was stopped did the TSH return to normal.

In another case, a woman who was stabilized on 150 mcg a day of levothyroxine saw her TSH go from 1.6 to 19 after 3 weeks of treatment with ciprofloxacin at 500 mg twice a day.

What Can You Do?

If you're a thyroid patient who is taking ciprofloxacin, what should you do?

First, it's important to know that while this warning may be included in your drug information insert, your physician or pharmacist may not mention consider or mention the possibility of interaction with your thyroid medication when prescribing or dispensing these antibiotics. You may also go to a walk-in or “Minute Clinic” type facility, and be prescribed these antibiotics, without a thorough evaluation of the potential for interaction with your existing medications.

Second, when a physician is prescribing these drugs, you should mention that you are taking levothyroxine, and ask about an antibiotic that is not in the fluoroquinolone family. In most cases, one of the other antibiotics can be prescribed.

Third, if you must take these particular antibiotics, the research has suggested that thyroid tests were normalized if patients take the levothyroxine and ciprofloxacin at least six hours apart. So, if you are on levothyroxine, it makes sense to allow at least six hours apart from taking your ciprofloxacin. And if you have to take the antibiotic for a lengthy period, you should discuss the potential impact on your thyroid with your doctor, and request period thyroid testing to ensure that your thyroid treatment is not affected.

Additional Concerns

Interestingly, there are also anecdotal reports of bad reactions to other fluoroquinolone antibiotics. The FDA added a black box warning about fluoroquinolones due the increased risk of tendon ruptures when taking the drug.

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Article Sources
  • Cooper, John, "Ciprofloxacin interacts with thyroid replacement therapy," British Medical Journal, 2005;330:1002 (30 April).