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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 one mg of generic
levothyroxine may not have the same effect on
the body as one 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: 6 and 10 ml vials
containing 0.2 mg or 0.5mg of levothyroxine per
vial.
STORAGE: Levothyroxine
tablets usually are kept at room temperature,
15-30°C (59-86°F) in a light-resistant, tight
container. However, some manufacturers vary in
their storage recommendations. Therefore,
storage conditions for each product should be
clarified with a pharmacist.
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: Levothyroxine is
usually started at 0.05 mg/day. Starting doses
and dose changes may differ with individual
patients based upon 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.
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, e.g.,
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, e.g., 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 (e.g.,
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. | 
