A study from the journal Thyroid found that the antibiotic ciprofloxacin — known more commonly by brand names Cipro, Proquin, Ciproxin, Ciprobay, Cirpoxine, and Ciflox — significantly decreased the absorption of the thyroid medication. [Keep in mind that some experts feel that the warning about ciprofloxacin applies also to similar quinolone antibiotics such as levofloxacin (Levaquin), lomefloxacin (Maxaquin), monifloxacin (Avelox), norfloxacin (Norox-In), and ofloxacin (Floxin).]
This means that if you are taking thyroid drugs and an antibiotic from the quinolone family, it could interfere with absorption, and cause you to become substantially more hypothyroid.
Interestingly, the antibiotic rifampin (also called rifampicin) — and known by brand names Rifadin, Rifamate and Rifater — significantly increased the absorption of the thyroid medication.
This means that if you are taking the antibiotic along with your thyroid hormone replacement medication, it could actually cause you to become overmedicated, or experience hyperthyroid symptoms.
Is there a solution? Some studies have found that the impact may be minimized if the antibiotic and thyroid medications were taken at least six hours apart. But it's not agreed upon across the board by physicians.
Your best bet is to discuss the issue with your practitioner at the time these drugs are prescribed (and before you get the prescription filled). There may be other antibiotics available for your treatment that are less likely to interfere with the effectiveness of your thyroid treatment.
Note: Remember that if you are prescribed an antibiotic, you should also take measures to help protect your digestive health.
Source: "Ciprofloxacin and Rifampin Have Opposite Effects on Levothyroxine Absorption," Goldberg, A. et. al. Thyroid, October 16, 2013