levothyroxine sodium (L-thyroxine, T4)

levothyroxine sodium (L-thyroxine, T₄)

(lee voe thye rox' een)

Levothroid, Levoxine, Levoxyl, Synthroid, Thyro-Tabs, Unithroid

Pregnancy Category A

Drug class

Thyroid hormone

Therapeutic actions

Increases the metabolic rate of body tissues, thereby increasing oxygen consumption; respiration and HR; rate of fat, protein, and carbohydrate metabolism; and growth and maturation.

Indications

·        Replacement therapy in hypothyroidism

·        Pituitary TSH suppression in the treatment and prevention of euthyroid goiters and in the management of thyroid cancer

·        Thyrotoxicosis in conjunction with antithyroid drugs and to prevent goitrogenesis, hypothyroidism, and thyrotoxicosis during pregnancy

·        Treatment of myxedema coma

Contraindications and cautions

·        Contraindicated with allergy to active or extraneous constituents of drug, thyrotoxicosis, and acute MI uncomplicated by hypothyroidism.

·        Use cautiously with Addison's disease (treat hypoadrenalism with corticosteroids before thyroid therapy), lactation, patients with coronary artery disease or angina.

Available forms

Tablets—25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200, 300 mcg; powder for injection—200, 500 mcg/vial

Dosages

50–60 mcg equals approximately 60 mg (1 grain) desiccated thyroid.

ADULTS

·        Hypothyroidism: Initial dose, 50 mcg PO, with increasing increments of 25 mcg PO q 6–8 wk; maintenance of up to 200 mcg/day. IV or IM injection can be substituted for the oral dosage form when oral ingestion is not possible. Usual IV dose is 50% of oral dose. Start at < 25 mcg/day in patients with long-standing hypothyroidism or known cardiac disease. Usual replacement 1.7 mcg/kg/day.

·        Myxedema coma without severe heart disease: 200–500 mcg IV as initial dose, then 100 to 200 mcg IV daily; daily maintenance of 50 to 100 mcg once a euthyroid state is established. Switch to PO once patient is able. Full effect not seen for 24 hr; dose based on improvement.

·        TSH suppression in thyroid cancer, nodules, and euthyroid goiters: Larger amounts than used for normal suppression.

·        Thyroid suppression therapy: 2.6 mcg/kg/day PO for 7–10 days.

·        Older patients may require less than 1 mcg/kg/day. For most patients older than 50 yr or under age 50 with cardiac disease, an initial dose of 12.5 to 25 mcg/day with increases of 25 mcg/day q 4–6 wk.

PEDIATRIC PATIENTS

·        Congenital hypothyroidism: Infants require replacement therapy from birth.
0–1 yr: 8–15 mcg/kg/day.
1–5 yr: 5–6 mcg/kg/day.
6–12 yr: 4–5 mcg/kg/day.
> 12 yr: 2–3 mcg/kg/day.

Pharmacokinetics

Route	Onset	Peak
Oral	Slow	1–3 wk
IV	6–8 hr	24–48 hr

Metabolism: Hepatic; T_1/2: 6–7 days

Distribution: Crosses placenta; enters breast milk

Excretion: Bile

IV facts

Preparation: Add 5 mL 0.9% sodium chloride injection, USP or bacteriostatic sodium chloride injection, USP with benzyl alcohol. Shake the vial to ensure complete mixing. Use immediately after reconstitution. Discard any unused portion.

Infusion: Inject directly, each 100 mcg over 1 min.

Incompatibilities: Do not mix with any other IV fluids.

Adverse effects

·        CNS: Tremors, headache, nervousness, insomnia

·        CV: Palpitations, tachycardia, angina, cardiac arrest

·        Dermatologic: Allergic skin reactions, partial loss of hair in first few months of therapy in children

·        GI: Diarrhea, nausea, vomiting, gagging, tablet stuck in throat, choking

Interactions

Drug-drug

·        Decreased absorption of oral thyroid preparation with cholestyramine

·        Increased risk of bleeding with warfarin—reduce dosage of anticoagulant when T₄ is begun

·        Decreased effectiveness of digitalis glycosides if taken with thyroid replacement

·        Decreased theophylline clearance when patient is in hypothyroid state; monitor levels and patient response as euthyroid state is achieved

Nursing considerations

Assessment

·        History: Allergy to active or extraneous constituents of drug, thyrotoxicosis, acute MI uncomplicated by hypothyroidism, Addison's disease, lactation

·        Physical: Skin lesions, color, T, texture; T; muscle tone, orientation, reflexes; P, auscultation, baseline ECG, BP; R, adventitious sounds; thyroid function tests

Interventions

·        Monitor response carefully at start of therapy, and adjust dosage. Full therapeutic effect may not be seen for several days.

·        Ensure that patient swallows tablet with a full glass of water.

·        Do not change brands of T₄ products, due to possible bioequivalence problems.

·        Do not add IV doses to other IV fluids.

·        Use caution in patients with CV disease.

·        Administer oral drug as a single daily dose before breakfast with a full glass of water.

·        Arrange for regular, periodic blood tests of thyroid function.

·        For children and other patients who cannot swallow tablets, crush and suspend in a small amount of water or formula, or sprinkle over soft food. Administer immediately.

·        WARNING: Most CV and CNS adverse effects indicate that the dose is too high. Stop drug for several days and reinstitute at a lower dose.

Teaching points

·        Take as a single dose before breakfast with a full glass of water.

·        This drug replaces an important hormone and will need to be taken for life. Do not discontinue without consulting your health care provider; serious problems can occur.

·        Wear a medical ID tag to alert emergency medical personnel that you are using this drug.

·        Arrange to have periodic blood tests and medical evaluations. Keep your scheduled appointments.

·        Report headache, chest pain, palpitations, fever, weight loss, sleeplessness, nervousness, irritability, unusual sweating, intolerance to heat, diarrhea.

Adverse effects in Italic are most common; those in Bold are life-threatening.