Levothyroxine Ferrous sulfate

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. 

Information is limited to these reports but it appears to be a elinieally important interaetion. Monitor the effeets of eoneurrent use and separate the doses of ferrous sulfate and levothyroxine by 2 hours or more on the assumption that reduced absorption accounts for this interaction. Monitor well. The same precautions would seem appropriate with any other iron compound. 

Ferrous sulfate causes a reduction in the effects of levothyroxine in patients with hypothyroidism. 

Fourteen patients with primary hypothyroidism had an increase in TSH levels from 1.6 to 5.4 mU/L when given ferrous sulfate 300 mg daily for 12 weeks along with their usual levothyroxine dose. The symptoms of hypothyroidism in 9 patients worsened. In another report a woman with hypothyroidism, taking levothyroxine, had a very marked rise in TSH levels when she took ferrous sulfate. Her levothyroxine dosage needed to be raised from 175 to 200 micrograms daily. ... 

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