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.
Last Editorial Review: 11/16/2007
Report Problems to the Food and Drug Administration
You are encouraged to report negative side effects of prescription drugs to the FDA. Visit the FDA MedWatch website or call 1-800-FDA-1088.
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