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levothyroxine sodium (L-thyroxine, T4)
Pharmacologic class: Synthetic thyroxine hormone
Therapeutic class: Thyroid hormone replacement
Pregnancy risk category A
FDA Box Warning
• Drug shouldn't be used alone or with other agents to treat obesity or weight loss. In euthyroid patients, doses within range of daily hormonal requirements are ineffective for weight loss. Larger doses may cause serious or life-threatening toxicity, particularly when given with sympathomimetic amines (such as those used for anorectic effects).
Synthetic form of thyroxine that replaces endogenous thyroxine, increasing thyroid hormone levels. Thyroid hormones help regulate cell growth and differentiation and increase metabolism of lipids, protein, and carbohydrates.
Powder for injection: 200 mcg/vial in 6- and 10-ml vials, 500 mcg/vial in 6- and 10-ml vials
Tablets: 25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg, 300 mcg
Indications and dosages
➣ Hypothyroidism; treatment or prevention of euthyroid goiter
Adults: For healthy adults younger than age 50 and those over age 50 who have recently been treated or undergone short-term therapy, start at full replacement dosage of 1.7 mcg/kg P.O. daily, given 30 minutes to 1 hour before breakfast. For patients older than age 50 or younger than age 50 with heart disease, 25 to 50 mcg P.O. daily, increased q 4 to 6 weeks. In severe hypothyroidism, initial dosage is 12.5 to 25 mcg P.O. daily, adjusted by 25 mcg daily q 2 to 4 weeks. For patients who can't tolerate oral doses, adjust I.M. or I.V. dosage to roughly half of oral dosage.
➣ Congenital hypothyroidism
Children older than age 12 who have completed puberty and growth: 1.7 mcg/kg P.O. daily
Children older than age 12 who have not completed puberty and growth: Up to 150 mcg or 2 to 3 mcg/kg P.O. daily
Children ages 6 to 12: 4 to 5 mcg/kg P.O. daily
Children ages 1 to 5: 5 to 6 mcg/kg P.O. daily
Infants ages 6 to 12 months: 6 to 8 mcg/kg P.O. daily
Infants ages 3 to 6 months: 8 to 10 mcg/kg P.O. daily
Infants up to 3 months old: 10 to 15 mcg/kg P.O. daily
➣ Myxedema coma or stupor
Adults: 200 to 500 mcg I.V. as a solution containing 100 mcg/ml. Additional 100 to 300 mcg may be given on day 2 if significant improvement has not occurred. Convert to P.O. therapy when patient is clinically stable.
➣ Thyroid-stimulating hormone suppression in well-differentiated thyroid cancers and thyroid nodules
Adults: Dosage individualized based on disease and patient
• Cardiovascular disease
• Psychosis or agitation
• Elderly patients
• Hypersensitivity to drug, its components, or tartrazine
• Acute myocardial infarction
• Adrenal insufficiency
Use cautiously in:
• cardiovascular disease, severe renal insufficiency, diabetes mellitus
• elderly patients
• pregnant or breastfeeding patients.
• Be aware that all dosages are highly individualized.
• Give tablets on an empty stomach 30 minutes to 1 hour before first meal of day.
• If patient can't swallow tablets, crush them and sprinkle onto small amount of food, such as applesauce. For infants and children, dissolve tablets in small amount of water, nonsoybean formula, or breast milk and administer immediately.
• Don't give oral form within 4 hours of bile acid sequestrants or antacids.
• Reconstitute Synthroid powder for injection with 5 ml of 0.9% sodium chloride injection. Shake until clear and use immediately.
• For I.V. administration, give each 100 mcg over at least 1 minute.
• Be aware that the various levothyroxine preparations aren't bioequivalent. Patient should consistently use same brand or generic product, with dosing based on weight, age, physical condition, and symptom duration.
• When drug is given for thyroid-stimulating hormone (TSH) suppression test, TSH suppression level is not well established and radioactive iodine (131I) is given before and after treatment course.
CNS: insomnia, irritability, nervousness, headache
CV: tachycardia, angina pectoris, hypotension, hypertension, increased cardiac output, arrhythmias, cardiovascular collapse
GI: vomiting, diarrhea, abdominal cramps
GU: menstrual irregularities
Musculoskeletal: accelerated bone maturation (in children), decreased bone density (in women on long-term therapy)
Skin: alopecia (in children), diaphoresis
Other: heat intolerance, weight loss
Drug-drug. Aminoglutethimide, amiodarone, anabolic steroids, antithyroid drugs, asparaginase, barbiturates, carbamazepine, chloral hydrate, cholestyramine, clofibrate, colestipol, corticosteroids, danazol, diazepam, estrogens, ethionamide, fluorouracil, heparin (with I.V. use), insulin, lithium, methadone, mitotane, nitroprusside, oxyphenbutazone, perphenazine, phenylbutazone, phenytoin, propranolol, salicylates (large doses), sulfonylureas, thiazides: altered thyroid function test results
Antacids, bile acid sequestrants: interference with levothyroxine absorption
Anticoagulants: increased anticoagulant action
Beta-adrenergic blockers (selected): decreased beta blocker action
Cardiac glycosides: decreased cardiac glycoside blood level
Cholestyramine, colestipol: levothyroxine inefficacy
Theophyllines: decreased theophylline clearance
Drug-diagnostic tests. Thyroid function tests: decreased values
Drug-food. Foods high in iron or fiber, soybeans: decreased drug absorption
• Check vital signs and ECG routinely.
• Monitor thyroid and liver function tests.
☞ Evaluate for signs and symptoms of overdose, including those of hyperthyroidism (weight loss, cardiac symptoms, abdominal cramps).
• Monitor closely for drug efficacy.
• Check patients with Addison's disease or diabetes mellitus for worsening of these conditions.
☞ Watch for signs and symptoms of bleeding tendency, especially in patients receiving anticoagulants concurrently.
• Explain that patient may require lifelong therapy and must undergo regular blood testing.
• Tell patient or parent to report adverse effects, including signs or symptoms of hyperthyroidism or hypothyroidism.
• Caution patient to avoid driving and other hazardous activities until he knows how drug affects concentration and alertness.
• Advise patient to avoid getting overheated, as in hot environments or during vigorous exercise.
• Tell parents that child being treated may lose hair during first few months of therapy. Reassure them that this effect usually is transient.
• As appropriate, review all other significant and life-threatening adverse reactions and interactions, especially those related to the drugs, tests, and foods mentioned above.