levothyroxine sodium, Synthroid, Levoxyl, Levothroid, Unithroid, Tirosint, Levo-T (cont.)

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Converting a state of hypothyroidism (under activity) to a normal state (euthyroid state) with levothyroxine may decrease the actions of certain beta-blocking drugs, (for example, metoprolol [Lopressor] or propranolol [Inderal]). It may be necessary, therefore, to change the dose of beta-blocker. For the same reason, the dose of digoxin (Lanoxin), a drug used to manage heart failure or an irregular heart rhythm (for example, atrial fibrillation), also may need to be changed.

Converting hypothyroidism to the euthyroid state with levothyroxine may increase the blood level of theophylline (Slo-Bid), and it may be necessary to change the dose of theophylline.

Taking levothyroxine at the same time as calcium carbonate, ferrous sulphate, cholestyramine (Questran) or colestipol (Colestid) may decrease the effect of levothyroxine and lead to hypothyroidism. This occurs because the levothyroxine binds to these drugs and is not absorbed. Taking the levothyroxine one hour before or four hours after these drugs is necessary to prevent binding.

PREGNANCY: Thyroid hormone therapy during pregnancy is usually safe but should be supervised by a physician.

NURSING MOTHERS: Thyroid hormone therapy in nursing mothers is usually safe but should be supervised by a physician.

SIDE EFFECTS: Levothyroxine therapy usually is well-tolerated. If symptoms occur, they often are due to toxic levels of thyroid hormone, and the symptoms are those of hyperthyroidism. Symptoms may include all or some of the following: chest pain, increased heart rate or pulse rate, excessive sweating, heat intolerance, nervousness, headache, insomnia, diarrhea, vomiting, weight loss, or fever. Women may experience irregular menstrual cycles.

Reference: FDA Prescribing Information

Medically Reviewed by a Doctor on 3/27/2013

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