SIDE EFFECTS
Adverse reactions, other than those indicative of hyperthyroidism
because of therapeutic ovserdosage, either initially or
during the maintenance period are rare (see OVERDOSAGE).
In rare instances, allergic skin reactions have been
reported with Cytomel (liothyronine sodium) Tablets.
DRUG INTERACTIONS
Oral Anticoagulants: Thyroid hormones appear to increase
catabolism of vitamin K-dependent clotting factors. If
oral anticoagulants are also being given, compensatory
increases in clotting factor synthesis are impaired. Patients
stabilized on oral anticoagulants who are found to require
thyroid replacement therapy should be watched very closely
when thyroid is started. If a patient is truly hypothyroid,
it is likely that a reduction in anticoagulant dosage
will be required. No special precautions appear to be
necessary when oral anticoagulant therapy is begun in
a patient already stabilized on maintenance thyroid replacement
therapy.
Insulin or Oral Hypoglycemics: Initiating
thyroid replacement therapy may cause increases in insulin
or oral hypoglycemic requirements. The effects seen are
poorly understood and depend upon a variety of factors
such as dose and type of thyroid preparations and endocrine
status of the patient. Patients receiving insulin or oral
hypoglycemics should be closely watched during initiation
of thyroid replacement therapy.
Cholestyramine: Cholestyramine binds
both T4 and T3 in the intestine, thus impairing absorption
of these thyroid hormones. In vitro studies indicate that
the binding is not easily removed. Therefore, 4 to 5 hours
should elapse between administration of cholestyramine
and thyroid hormones.
Estrogen, Oral Contraceptives: Estrogens
tend to increase serum thyroxine-binding globulin (TBg).
In a patient with a nonfunctioning thyroid gland who is
receiving thyroid replacement therapy, free levothyroxine
may be decreased when estrogens are started thus increasing
thyroid requirements. However, if the patient's thyroid
gland has sufficient function, the decreased free thyroxine
will result in a compensatory increase in thyroxine output
by the thyroid. Therefore, patients without a functioning
thyroid gland who are on thyroid replacement therapy may
need to increase their thyroid dose if estrogens or estrogen-containing
oral contraceptives are given.
Tricyclic Antidepressants: Use of thyroid
products with imipramine and other tricyclic antidepressants
may increase receptor sensitivity and enhance antidepressant
activity transient cardiac arrhythmias have been observed.
Thyroid hormone activity may also be enhanced.
Digitalis: Thyroid preparations may
potentiate the toxic effects of digitalis. Thyroid hormonal
replacement increases metabolic rate, which requires an
increase in digitalis dosage.
Ketamine: When administered to patients on a thyroid
preparation, this parenteral anesthetic may cause hypertension
and tachycardia. Use with caution and be prepared to treat
hypertension, if necessary.
Vasopressors: Thyroxine increases the
adrenergic effect of catecholamines such as epinephrine
and norepinephrine. Therefore, injection of these agents
into patients receiving thyroid preparations increases
the risk of precipitating coronary insufficiency especially
in patients with coronary artery disease. Careful observation
is required.
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