Levothyroxine Sodium (T₄; L-thyroxine)

A to Z Drug Facts

(lee-voe-thigh-ROX-een SO-dee-uhm)

Levo-T, Levothroid, Levoxyl, Synthroid,  El-Troxin, Levo-T, PMS-Levothyroxine

Class: Thyroid hormone

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

 Indications Replacement or supplemental therapy in hypothyroidism; TSH suppression (in thyroid cancer, nodules, goiters and enlargement in chronic thyroiditis); diagnostic agent in suppression tests to differentiate suspected hyperthyroidism from euthyroidism.

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

 Route/Dosage

Individualize dosage.

Hypothyroidism

ADULTS: Initial dose: PO 0.05 mg/day, increased by 0.025 mg q 2 to 3 wk if needed.

Long-Standing Hypothyroidism

ADULTS: PO £ 0.025 mg/day, particularly if cardiovascular impairment is suspected. Reduce dosage if angina occurs. Maintenance: PO usually < 0.2 mg/day. IV/IM Half of previously established oral dosage initially. CHILDREN > 12 YR: PO > 150 mcg/day or 2 to 3 mcg/kg/day. IV/IM routes can be used for maintenance in children if child is unable to take medication orally. The initial parenteral dose should be approximately one-half the previously established oral dose. CHILDREN 6 TO 12 YR: PO 100 to 150 mcg/day or 4 to 5 mcg/kg/day. CHILDREN 1 TO 5 YR: PO 75 to 100 mcg/day or 5 to 6 mcg/kg/day. CHILDREN 6 TO 12 MO: PO 50 to 75 mcg/day or 6 to 8 mcg/kg/day. CHILDREN 0 TO 6 MO: PO 25 to 50 mcg/day or 8 to 10 mcg/kg/day.

Myxedema Coma

ADULTS: IV/Nasogastric 0.4 mg initially, followed by 0.1 to 0.2 mg/day until patient can take drug orally.

TSH Suppression

Requires larger amounts of thyroid hormone than those used for replacement therapy.

Thyroid Suppression Therapy

ADULTS: PO 2.6 mcg/kg/day for 7 to 10 days.

 Interactions

Anticoagulants, oral: May increase anticoagulant effects. Cholestyramine, cholestipol: May decrease thyroid hormone efficacy. Digitalis glycosides: May reduce effects of glycosides. Fasting: Increases absorption from GI tract. Iron salts: May decrease efficacy of levothyroxine, resulting in hypothyroidism. Theophyllines: Hypothyroidism; may cause decreased theophylline clearance; clearance may return to normal when euthyroid state is achieved.

 Lab Test Interferences Consider changes in thyroxine binding globulin concentration when interpreting thyroxine (T₄) and triiodothyronine (T₃) 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

CV: Palpitations; tachycardia; cardiac arrhythmias; angina pectoris; cardiac arrest. CNS: Tremors; headache; nervousness; insomnia. GI: Diarrhea; vomiting. OTHER: Hypersensitivity; weight loss; menstrual irregularities; sweating; heat tolerance; fever; decreased bone density (in women using long term).

 Precautions

Pregnancy: Category A. Lactation: Minimal amounts excreted in breast milk. Children: When drug is administered for congenital hypothyroidism, routine determinations of serum T₄ or TSH are strongly advised in neonates. 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. Cardiovascular disease: Use caution when integrity of cardiovascular system, particularly coronary arteries, is suspect (eg, angina, elderly). Development of chest pain or worsening cardiovascular disease requires decrease in dosage. Endocrine disorders: Therapy in patients with concomitant diabetes mellitus, diabetes insipidus or adrenal insufficiency (Addison's disease) exacerbates intensity of their symptoms. Therapy of myxedema coma requires simultaneous administration of glucocorticoids. In patients whose hypothyroidism is secondary to hypopituitarism, adrenal insufficiency, if present, should be corrected with corticosteroids. Hyperthyroid effects: Levothyroxine 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 cardiovascular risks. Small initial doses are indicated. Obesity: Drug should not be used for weight reduction; may produce serious or life-threatening toxicity in large doses, particularly when given with anorexiants.

 Administration/Storage

• Administer oral form once each day before breakfast. When given on empty stomach, absorption is increased. To maintain steady blood levels, be consistent in giving drug either with or without food.
• Do not give sooner than 4 hr after administration of cholestyramine or colestipol. Cholestyramine or colestipol reduces effectiveness of levothyroxine.
• Do not switch from one brand to another without comparison studies of bioavailability.
• For infants and children who cannot swallow intact tablets, crush proper dose tablet and suspend freshly crushed tablet in small amount of formula or water; give by spoon or dropper. Do not store suspension for any period of time. Crushed tablet may also be sprinkled over small amount of food (eg, cooked cereal, applesauce).
• Parenteral therapy may be used when oral form of medication cannot be tolerated.
• IV therapy is preferred in emergency treatment of myxedema coma. Can be administered via nasogastric tube. Give no faster than 100 mcg/min. Too rapid administration causes adverse cardiovascular reactions. Monitor closely during administration.
• Reconstitute injectable solution by adding 5 mL of 0.9% Sodium Chloride or Bacteriostatic Sodium Chloride (benzyl alcohol). Shake vial to ensure complete dissolution.
• Administer via Y-tubing or 3-way stopcock. Do not add to IV infusion.
• Use IV solution immediately after reconstitution. Discard any unused portion.
• Store in tightly closed, light-resistant container at room temperature.

 Assessment/Interventions

• Obtain patient history, including drug history and any known allergies.
• Prior to initial administration, obtain baseline data of TSH and T₄ levels.
• Successful therapy achieves euthyroid state. Expect responses to initial therapy to be diuresis, weight loss, increased sense of well-being, increased appetite and activity, increased energy and return to normal texture of hair and skin.
• Monitor for signs and symptoms of thyroid deficit or excess.
• Do not give if pulse > 100 bpm.
• In infants, monitor for normal growth, development and intellectual functioning; monthly measurement of height is a good index.
• Compare laboratory tests of TSH and T₄ with baseline data; serum levels should return to normal. In some children TSH levels may remain abnormal.
• When given with oral anticoagulants, expect oral anticoagulant dosage to be reduced. Monitor for bleeding more closely.
• Give drug cautiously with catecholamines (eg, epinephrine, dopamine). When drug is given with catecholamines, monitor for cardiac arrhythmias.
• If patient is taking insulin, oral hypoglycemics or digitalis, monitor effectiveness of these agents. Dosages of these agents may need to be altered. Monitor serum glucose levels, ECG and pulse.
• Monitor thyroid function test results closely in patients over the age of 65 years; less levothyroxine is usually needed by elderly.

 Patient/Family Education

• Explain to patient that medication will probably need to be taken for life. Instruct patient not to discontinue taking medication or change dosage without consulting physician.
• Instruct patient to take levothyroxine at same time each day, preferably in morning before breakfast.
• Instruct patient not to switch from one brand of medication to another unless advised to do so by physician.
• Teach patient how to monitor for signs and symptoms of thyroid deficit or excess. Instruct patient to notify physician of following persistent signs and symptoms: Headache, nervousness, diarrhea, excessive sweating, heat intolerance, chest pain, increased pulse rate and palpitations.
• Teach patient to keep a record of signs and symptoms for physician review.
• Advise patient of importance of keeping appointments with physician for regular checkups.
• Caution patient not to take otc or other prescribed medications without consulting physician.
• Advise patient to wear Medi-Alert bracelet or necklace and to carry Medi-Alert card in wallet.
• Caution patient not to take levothyroxine for weight control.
• Explain that partial hair loss may be experienced by child in first few months of therapy but that this side effect is transient.

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Copyright © 2003 Facts and Comparisons
David S. Tatro
A to Z Drug Facts