DrugindexOnline^{2nd edition}

( T ₃ ; triiodothyronine ) Pronouncation: (lie-oh-THIGH-row-neen SO-deeuhm)
Class: Thyroid hormone

Trade Names:
Cytomel
- Tablets 5 mcg
- Tablets 25 mcg
- Tablets 50 mcg

Trade Names:
Triostat
- Injection 10 mcg/mL

Mechanism of Action

Pharmacology

Increases metabolic rate of body tissues; is needed for normal growth and maturation.

Pharmacokinetics

Absorption

95% absorbed.

Distribution

Protein binding is more than 99%.

Elimination

Biological t _½ is 2.5 days.

Onset

Within a few h.

Peak

Within 2 to 3 days.

Indications and Usage

Replacement or supplemental therapy in hypothyroidism; TSH suppression for treatment or prevention of euthyroid goiters (eg, thyroid nodules, multinodular goiters, enlargement in chronic thyroiditis); diagnostic agent in suppression tests to differentiate suspected hyperthyroidism from euthyroidism; treatment of myxedema coma/precoma (IV).

Contraindications

Acute MI and thyrotoxicosis uncomplicated by hypothyroidism; coexistence of hypothyroidism and hypoadrenalism (Addison disease), unless treatment of hypoadrenalism with adrenocortical steroids precedes initiation of thyroid therapy.

Dosage and Administration

Individualize dosage.

Hypothyroidism
Adults

PO 25 mcg/day initially, increase by up to 25 mcg every 1 to 2 wk if needed.

Children

PO 5 mcg/day initially, increase by 5 mcg/day at 2 wk intervals, if needed.

Congenital Hypothyroidism
Children

PO 5 mcg/day initially; increase by 5 mcg/day every 3 to 4 days until desired response achieved. Infants a few mo of age may require only 20 mcg/day for maintenance; at 1 yr, 50 mcg/day may be required; and, above 3 yr, full adult dosage may be required.

Simple (Nontoxic) Goiter
Adults

PO 5 mcg/day initially, increase by 5 to 10 mcg every 1 to 2 wk. When 25 mcg/day is reached, increase by 12.5 to 25 mcg every 1 to 2 wk if needed.

Children

PO 5 mcg/day initially, increase by 5 mcg/day at 2-wk intervals, if needed.

Myxedema
Adults

PO 5 mcg/day initially, increase by 5 to 10 mcg every 1 to 2 wk. When 25 mcg/day is reached, increase by 12.5 to 25 mcg every 1 to 2 wk if needed.

Children

PO 5 mcg/day initially, increase by 5 mcg/day at 2-wk intervals, if needed.

Myxedema Coma/Precoma
Adults

IV 25 to 50 mcg initially. In patients with known or suspected CV disease, an initial dose of 10 to 20 mcg is suggested; however, base doses on continuous monitoring of the condition and response to therapy.

TSH Suppression Test
Adults

PO 75 to 100 mcg/day for 7 days.

Storage/Stability

Store tablets in tightly closed container at controlled room temperature (59° to 86°F); store injection in refrigerator (36° to 46°F).

Drug Interactions

Anticoagulants, oral

May increase anticoagulant effects.

Beta blockers

May reduce effects of beta blockers.

Cholestyramine, colestipol

May decrease thyroid hormone efficacy.

Digitalis glycosides

May reduce effects of glycosides.

Theophyllines

Hypothyroidism; may cause decreased theophylline Cl; Cl may return to normal when euthyroid state is achieved.

Laboratory Test Interactions

Consider changes in thyroxine-binding globulin concentration when interpreting thyroxine (T ₄ ) and triiodothyronine (T ₃ ) values; medicinal or dietary iodine interferes with all in vivo tests of radioiodine uptake, producing low uptakes that may not reflect true decrease in hormone synthesis.

Adverse Reactions

Cardiovascular

Palpitations; tachycardia; cardiac arrhythmias; angina pectoris; cardiac arrest.

CNS

Tremors; headache; nervousness; insomnia.

GI

Diarrhea; vomiting.

Miscellaneous

Hypersensitivity; weight loss; menstrual irregularities; sweating; heat intolerance; fever; decreased bone density (in women using drug long term).

Precautions

Warnings Not for use in obesity treatment. Ineffective for weight reduction indications and may produce life-threatening or serious consequences when used in large doses or in combination with other anorectics.

Pregnancy

Category A .

Lactation

Minimal amounts excreted in breast milk.

Children

When drug is administered for congenital hypothyroidism, routine determinations of serum T ₄ or TSH are strongly advised in newborns. In infants, excessive doses of thyroid hormone preparations may produce craniosynostosis. Children may experience transient partial hair loss in first few months of thyroid therapy.

Elderly

Therapy should be started with 5 mcg every day and increased by 5 mcg increments at recommended intervals.

CV disease

Use caution when integrity of CV system, particularly coronary arteries, is suspect (eg, angina, elderly). Development of chest pain or worsening CV disease requires decrease in dosage.

Endocrine disorders

Therapy in patients with concomitant diabetes mellitus, diabetes insipidus, or adrenal insufficiency (Addison disease) exacerbates intensity of their symptoms. Therapy of myxedema coma requires simultaneous administration of glucocorticoids. Use corticosteroids to correct adrenal insufficiency in patients whose hypothyroidism is secondary to hypopituitarism.

Hyperthyroid effects

Liothyronine may rarely precipitate hyperthyroid state or may aggravate existing hyperthyroidism.

Infertility

Drug is unjustified for treatment of male or female infertility unless condition is accompanied by hypothyroidism.

Morphologic hypogonadism and nephrosis

Rule out before therapy.

Myxedema coma

Patients are particularly sensitive to thyroid preparations. Sudden administration of large doses is not without CV risks. Small initial doses are indicated.

Overdosage

Symptoms

Symptoms of hyperthyroidism: Headache, irritability, nervousness, sweating, tachycardia, increased bowel motility, menstrual irregularities, palpitations, vomiting, psychosis, seizure, fever, angina pectoris, CHF, shock, arrhythmias, thyroid storm.

Patient Information

• Instruct patient to take liothyronine as directed. Do not change or discontinue dosage without consulting health care provider. Explain that liothyronine does not cure hypothyroidism and that therapy will continue for rest of life.
• Instruct patient with diabetes mellitus to closely monitor urinary glucose levels. The daily dosage of antidiabetic medication may need readjustment as thyroid hormone replacement is achieved or if thyroid medication is stopped.
• Explain that partial hair loss may be experienced by children in first few months of therapy, but that this side effect is transient.
• Advise patient to wear medical identification (eg, card, bracelet).
• Inform patient that liothyronine's effects are more rapid than levothyroxine, which requires several days before onset of action.
• Teach patient to take pulse and inform the health care provider if signs of tachycardia or dysrhythmias occur.
• Instruct patient to call the health care provider immediately if any adverse symptoms such as chest pain, palpitations, headaches, irritability, increased nervousness, diaphoresis, tachycardia, dysrhythmias, or heat intolerance occur.
• Caution patient not to take liothyronine for weight control.