Levothyroxine Carbamazepine

Cholestyramine, calcium carbonate, sucralfate, aluminum hydroxide, ferrous sulfate, soybean formula, and dietary fiber supplements may impair the absorption of levothyroxine from the GI tract. Drugs that increase nondeiodinative T4 clearance include rifampin, carbamazepine, and possibly phenytoin. Amiodarone may block the conversion of T4 to T3. 

A 41-year-old woman with a diagnosis of bipolar disorder developed severe suicidality when topiramate was added to her previous treatment (carbamazepine and levothyroxine) for mood stabilization (649). 

Clinically important, potentially hazardous interactions with alfentanil, alfuzosin, alprazolam, amiodarone, amprenavir, aprepitant, astemizole, atazanavir, bepridil, buprenorphine, bupropion, carbamazepine, chlordiazepoxide, ciclesonide, clozapine, conivaptan, cyclosporine, cyproterone, dasatinib, diazepam, dihydroergotamine, ergot alkaloids, estazolam, eszopidone, etravirine, ezetimibe, fentanyl, fesoterodine, flecainide, flurazepam, fluticasone, halazepam, ivabradine, ixabepilone, ketoconazole, lapatinib, levothyroxine, meperidine, meptazinol, methysergide, midazolam, nifedipine, nilotinib, oral contraceptives, phenytoin, pimozide, piroxicam, propafenone, propoxyphene, quazepam, quinidine, ranolazine, rifabutin, rifampin, rifapentine, rimonabant, rivaroxaban, saquinavir, sildenafil, silodosin, simvastatin, solifenacin, St John s wort, tadalafil, temsirolimus, trabectedin, triazolam, vardenafil, voriconazole, zolpidem... 

Noninterfering acetaminophen, N-acetylprocainamide, amikacin, amitriptyline, amlodi-pine, carbamazepine, cefotaxime, ceftazidime, chloramphenicol, ciprofloxacin, cisapride, clindamycin, clonidine, codeine, cyclosporine, digoxin, diphenhydramine, disopyramide, ethosuximide, fluconazole, gentamicin, gentamicin, heparin, labetalol, levothyroxine, li-docaine, lithium, methotrexate, metronidazole, minoxidil, nafcillin, nifedipine, phenobar-bital, phenobarbital, phenytoin, phenytoin, primidone, procainamide, propranolol, quini-dine, ranitidine, salicylic acid, theophylline, tobramycin, tobramycin, valproic acid, warfarin... 

An isolated report describes a patient, previously stable taking levothyroxine, who developed clinical hypothyroidism when phenytoin was given. Both carbamazepine and phenytoin can reduce endogenous serum thyroid hormone levels, but clinical hypothyroidism caused by an interaction seems to be rare. 

A number of other reports describe very significant reductions in endogenous markers of thyroid function in subjects and patients taking phenytoin or carbamazepine, " but not sodium valproate. However, there seems to be only two cases in which reversible hypothyroidism was seen, one with carbamazepine and phenytoin, and the other with carbamazepine alone. There is also a report of an arrhythmia in a patient with hypothyroidism and rheumatic heart disease given phenytoin this was attributed to the displacement of protein bound levothyroxine by phenytoin leading to an increase in free levothyroxine in the plasma. This report was later criticised by others, who suggested that the arrhythmia, if indeed there was one, was caused directly by the cardiac actions of... 

Both phenytoin and carbamazepine can increase the metabolism of endogenous thyroid hormones, thereby reducing their plasma levels. Phenytoin can also displace levothyroxine and triiodothyronine from thyroxine binding globulin."... 

Despite very clear evidence that both carbamazepine and phenytoin can cause a marked reduction in endogenous serum thyroid hormone levels, the development of clinical hypothyroidism seems to be very rare, and there seems to be only one case on record of an interaction between levothyroxine and phenytoin. There seems to be little reason for avoiding concurrent use, but the outcome should be monitored. Increase the levothyroxine dosage if necessary. Consider also Thyroid hormones + Barbiturates , below. 

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