Anticoagulants thyroid effects

When administered with cholestyramine or colestipol there is a decreased absorption of the oral thyroid preparations. These drugs should not be administered within 4 of 6 hours of the thyroid hormones. When administered with the oral anticoagulants there is an increased risk of bleeding. It may be advantageous to decrease the dosage of the anticoagulant when a thyroid preparation is prescribed. There is a decreased effectiveness of the digitalis preparation if taken with a thyroid preparation. 

Rifampin is known to induce the hepatic microsomal enzymes that metabolize various drugs such as acetaminophen, oral anticoagulants, barbiturates, benzodiazepines, beta blockers, chloramphenicol, clofibrate, oral contraceptives, corticosteroids, cyclosporine, disopyramide, estrogens, hydantoins, mexiletine, quinidine, sulfones, sulfonylureas, theophyllines, tocainide, verapamil, digoxin, enalapril, morphine, nifedipine, ondansetron, progestins, protease inhibitors, buspirone, delavirdine, doxycycline, fluoroquinolones, losartan, macrolides, sulfonylureas, tacrolimus, thyroid hormones, TCAs, zolpidem, zidovudine, and ketoconazole. The therapeutic effects of these drugs may be decreased. 

Methimazole (Tapazole) [Antithyroid Agent] Uses Hyperthy-roidism, thyrotoxicosis, pr for thyroid surgery or radiation Action Blocks T3 T4 formation Dose Adults. Initial 15-60 mg/d PO tid Maint 5-15 mg PO daily Peds. Initial 0.4-0.7 mg/kg/24 h PO tid Maint V h- U h of initial dose PO daily w/ food Caution [D, +/-] Contra Breast-feeding Disp Tabs SE GI upset, dizziness, blood dyscrasias Interactions t Effects OF digitalis glycosides, metoprolol, propranolol X effects OF anticoagulants, theophylline X effects W/ amiodarone EMS None OD May cause N/V, HA, abd pain, fever, and pale skin symptomatic and supportive... 

ANTICOAGULANTS - ORAL THYROID HORMONES Possible t anticoagulant effect Uncertain Monitor INR at least weekly until stable... 

The bile acids sequestrants, particularly cholestyramine, can decrease tlie absorption of numerous dru. For tliis reason, tlie bile acid sequestrants should be administered alone and otlier dru given at least 1 hour before or 4 hours after administration of the bile acid sequestrants. There is an increased risk of bleeding when tlie bile acid sequestrants are administered with oral anticoagulants. The dosage of the antico ulant is usually decreased. The bile acid sequestrants may bind with digoxin, tliiazide diuretics, penicillin, propranolol, tetracyclines, folic acid, and tlie thyroid hormone, resulting in decreased effects of th e dru. ... 

Cholestyramine interferes with the oral absorption of many drugs (including wrirfarin), resulting in decreased effectiveness. Aspirin and thyroid hormones enhance the action of warfarin via pharmacodynamic mechanisms. Increased anticoagulant effects with cimetidine or quini-dine result from the inhibition of metabolism of warfarin. The answer is (B). 

Some drugs can alter thyroid status as an unwanted effect, and this will also alter the response to the oral anticoagulants. For example, amiodar-one , (p.363) can cause thyrotoxicosis, which decreases warfarin requirements. Also, use ofdextrothyroxine for hypercholesterolaemia decreased the required dose of warfarin and dicoumarol, presumably because it has weak thyroid activity. 

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