The British Medical Journal has reported on a several cases of unexplained hypothyroidism in thyroid patients (on levothyroxine) who were taking the popular antibiotic ciprofloxacin. Ciprofloxacin is the generic name for the fluoroquinolone antibiotic sold under the brand names Cipro, Ciproxin and Ciprobay, Cirpoxine, and Ciflox. The drug is primarily used to treat urinary tract infections, pneumonia, and sexually transmitted diseases. Ciprofloxacin was also in the news during the anthrax scare, given its use in treatment for anthrax exposure.

What the researchers found in these cases were evidence that oral ciprofloxacin interacts with levothyroxine (i.e., Synthroid, Levoxyl, Levothroid) if taken together. It's thought that the ciprofloxacin may somehow decrease the absorption of the levothyroxine.

In one case, 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?

The research suggested that thyroid tests were normalized if patients took 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 possibly request period thyroid testing to ensure that your thyroid treatment is not affected.

Note: Interestingly, there are also anecdotal reports -- including my own -- of bad reactions to other fluoroquinolone antibiotics. Read about my own experience getting what's known as "floxed," in When Drug Side Effects Get Out Of Hand: Side Effects of Quinolone Antibiotics Like Tequin (Gatifloxacin) Can Be Serious.

You may also want to read my discussion of the book Bitter Pills, along with my Q&A with the author Stephen Fried. The book details Fried's wife's debilitating reaction to quinolone antibiotics, and other drug dangers that are not made public.

Source: Cooper, John, "Ciprofloxacin interacts with thyroid replacement therapy," British Medical Journal, 2005;330:1002 (30 April), Online

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