Action

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.

Availability

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

Dosage adjustment

• Cardiovascular disease

• Psychosis or agitation

• Elderly patients

Contraindications

• Hypersensitivity to drug, its components, or tartrazine

• Acute myocardial infarction

• Thyrotoxicosis

• Adrenal insufficiency

Precautions

Use cautiously in:

• cardiovascular disease, severe renal insufficiency, diabetes mellitus

• elderly patients

• pregnant or breastfeeding patients.

Administration

• 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.

Adverse reactions

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

Metabolic: hyperthyroidism

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

Interactions

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

Patient monitoring

• Check vital signs and ECG routinely.

• Monitor thyroid and liver function tests.

See 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.

See Watch for signs and symptoms of bleeding tendency, especially in patients receiving anticoagulants concurrently.

Patient teaching

• 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.