Pharmacy Author: Omudhome Ogbru, PharmD
Medical and Pharmacy Editor: Jay W. Marks, MD

GENERIC NAME: levothyroxine sodium

BRAND NAME: Synthroid, Levoxyl, Levothroid, Unithroid

DRUG CLASS AND MECHANISM: Levothyroxine is a synthetic (man-made) version of the principle thyroid hormone, thyroxine (T4) that is made and released by the thyroid gland. Thyroid hormone increases the metabolic rate of cells of all tissues in the body. In the fetus and newborn, thyroid hormone is important for the growth and development of all tissues including bones and the brain. In adults, thyroid hormone helps to maintain brain function, food metabolism, and body temperature, among other effects.

GENERIC AVAILABLE: Yes. Generic and branded tablets of levothyroxine may differ:

• in the amount of levothyroxine they contain,
  
  
• the absorption of the levothyroxine into the body, and
  
  
• the distribution of levothyroxine throughout the body.

This means that ingestion of 1 mg of generic levothyroxine may not have the same effect on the body as 1 mg of another generic or branded levothyroxine. Practically speaking, this means that when changing between levothyroxine manufactured by different pharmaceutical companies, a change in dose may be necessary to maintain the desired effect, or to prevent toxicity.

PRESCRIPTION: Yes.

PREPARATIONS: Tablets: 0.025, 0.05, 0.075, 0.088, 0.1, 0.112, 0.125, 0.137, 0.15, 0.175, 0.2, and 0.3 mg. Powder for intravenous injection: 200 mcg and 500 mcg per vial.

STORAGE: Levothyroxine tablets and dry powder should be kept at room temperature, 15-30°C (59-86°F) in a light-resistant, tight container.

Powdered levothyroxine for intravenous injection should be used immediately once mixed with a liquid.

PRESCRIBED FOR: Levothyroxine is approved to treat hypothyroidism and to suppress thyroid hormone release in the management of cancerous thyroid nodules and growth of goiters. In addition, Synthroid, Levoxyl and Levothroid also are prescribed with anti-thyroid drugs, for example methimazole (Tapazole), to manage thyrotoxicosis (high thyroid hormone levels due to over-activity of the thyroid gland). Thyrotoxicosis may result in the growth of goiters and/or hypothyroidism.

DOSING: For adult hypothyroidism levothyroxine is started at 12.5-125 mcg/day administered orally. Starting doses and dose changes may differ with individual patients based upon age, the presence of cardiovascular disease, the development of tolerance (reduced effectiveness with continued use), side effects to the medication, and blood levels of thyroid hormone. It may take one to three weeks after initiating therapy with levothyroxine or changing the dose before effects are seen. The goal of replacement therapy is to maintain a normal blood thyroxine level.

DRUG INTERACTIONS: Initiation or discontinuation of therapy with levothyroxine in diabetic patients may create a need for an increase or decrease in the required dose of insulin and/or antidiabetic drug, [for example, glyburide (Micronase)].

Levothyroxine may increase the effect of blood thinners such as warfarin (Coumadin). Therefore, monitoring of blood clotting is necessary, and a decrease in the dose of warfarin may be necessary.

Intravenous administration of epinephrine to patients with coronary artery disease may lead to complications ranging from difficulty in breathing to a heart attack. These complications may occur more frequently among patients also taking levothyroxine. Therefore, careful observation is needed when intravenous epinephrine is given to patients receiving levothyroxine who also have coronary artery disease.

Converting a state of hypothyroidism (underactivity) 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 cholestyramine (Questran) or colestipol (Colestid), two cholesterol-lowering drugs, may decrease the effect of levothyroxine and lead to hypothyroidism. This occurs because the levothyroxine binds to the cholesterol-lowering drugs and is not absorbed. Taking the levothyroxine one hour before or four hours after cholestyramine or colestipol is necessary to prevent the 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 is usually well-tolerated. If symptoms occur, often they 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.

From the Doctors at MedicineNet.com

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