Side Effects of Cytomel (liothyronine sodium)

Cytomel (liothyronine sodium) is a synthetic (man-made) version of one of the two hormones made by the thyroid gland, triiodothyronine, used to treat individuals who are hypothyroid (do not produce enough thyroid hormones) and prevent various types of euthyroid goiters. 

Thyroid hormones increase the metabolism (activity) of all cells in the body. In the fetus, newborn infant and child, thyroid hormones promote growth and development of tissues. In adults, thyroid hormones help to maintain the function of the brain, the use of food by the body, and body temperature. 

Common side effects of Cytomel include

• nausea and
• irregular menstrual cycles. 

Serious side effects of Cytomel include

• toxic (too high) levels of thyroid hormone (hyperthyroidism). Symptoms of hyperthyroidism include
  • chest pain,
  • increased heart rate,
  • excessive sweating,
  • heat intolerance,
  • nervousness,
  • headache,
  • tremor,
  • insomnia,
  • diarrhea,
  • vomiting,
  • weight loss,
  • fever, and, rarely,
  • cardiac arrest.

Drug interactions of Cytomel include cholestyramine, which binds Cytomel and other thyroid in hormones in the intestine, reducing the absorption and levels of Cytomel. Thyroid hormones should not be administered for 4 to 5 hours after administering cholestyramine. 

The administration of Cytomel increases the action of the blood-thinning drug, warfarin.

• By increasing the body's metabolism (elimination) of clotting factors, thyroid hormones reduce the body's ability to clot and enhance the effects of warfarin.
• Since this can lead to excessive bleeding, the dose of warfarin may need to be reduced. 

Cytomel and other thyroid hormones may change the action of many drugs.

• Individuals who are hypothyroid have slower theophylline elimination.
• The elimination of theophylline and similar drugs increases as the dose of thyroid hormone increases.
• Therefore, when the concentration of thyroid hormone is returned to normal with Cytomel, the elimination of theophylline is increased.
• This reduces the concentration of theophylline in the body and can reduce the effectiveness of theophylline. 

Thyroid hormones do not easily cross the placenta and, therefore, are safe for pregnant women to take. 

Very small amounts of thyroid hormone may be excreted in breast milk. This is not a serious problem for breastfed infants. Consult your doctor before breastfeeding. 

Liothyronine therapy generally is well-tolerated. If symptoms occur, they usually occur because there are toxic (too high) levels of thyroid hormone (hyperthyroidism).

Symptoms of hyperthyroidism include:

• chest pain,
• increased heart rate,
• excessive sweating,
• heat intolerance,
• nervousness,
• headache,
• tremor,
• insomnia,
• diarrhea,
• vomiting,
• weight loss,
• fever, and,
• rarely, cardiac arrest.

Women also may experience irregular menstrual cycles.

Since thyroid hormone affects heart rate and metabolism, the impact of treatment of thyroid hormone on the control of diseases such as

• atrial fibrillation,
• diabetes and
• high cholesterol levels always should be considered.

Adverse reactions, other than those indicative of hyperthyroidism because of therapeutic overdosage, either initially or during the maintenance period are rare. In rare instances, allergic skin reactions have been reported with Cytomel (liothyronine sodium) Tablets.

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.

Drug And Laboratory Test Interactions

The following drugs or moieties are known to interfere with laboratory tests performed in patients on thyroid hormone therapy:

• androgens,
• corticosteroids,
• estrogens,
• oral contraceptives containing estrogens,
• iodine-containing preparations and
• the numerous preparations containing salicylates.

Changes in TBg concentration should be taken into consideration in the interpretation of T4 and T3 values. In such cases, the unbound (free) hormone should be measured.

• Pregnancy, estrogens and estrogen-containing oral contraceptives increase TBg concentrations.
• TBg may also be increased during infectious hepatitis.
• Decreases in TBg concentrations are observed in nephrosis, acromegaly and after androgen or corticosteroid therapy.
• Familial hyper- or hypo-thyroxine-binding-globulinemias have been described.
• The incidence of TBg deficiency approximates 1 in 9000.
• The binding of thyroxine by thyroxine-binding prealbumin (TBPA) is inhibited by salicylates.

Medicinal or dietary iodine interferes with all in vivo tests of radioiodine uptake, producing low uptakes which may not be reflective of a true decrease in hormone synthesis.

The persistence of clinical and laboratory evidence of hypothyroidism in spite of adequate dosage replacement indicates either poor patient compliance, poor absorption, excessive fecal loss, or inactivity of the preparation. Intracellular resistance to thyroid hormone is quite rare.