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Drugs reference index «Levothyroxine Sodium (T4; L-thyroxine)»

Levothyroxine Sodium

( T 4 ; L-thyroxine ) Pronunciation: (lee-voe-thigh-ROX-een SO-dee-uhm)Class: Thyroid hormone

Trade Names:Levothroid- Tablets 0.025 mg- Tablets 0.05 mg- Tablets 0.075 mg- Tablets 0.088 mg- Tablets 0.1 mg- Tablets 0.112 mg- Tablets 0.125 mg- Tablets 0.15 mg- Tablets 0.175 mg- Tablets 0.2 mg- Tablets 0.3 mg

Trade Names:Levoxyl- Tablets 0.025 mg- Tablets 0.05 mg- Tablets 0.075 mg- Tablets 0.088 mg- Tablets 0.1 mg- Tablets 0.112 mg- Tablets 0.125 mg- Tablets 0.137 mg- Tablets 0.15 mg- Tablets 0.175 mg- Tablets 0.2 mg- Tablets 0.3 mg

Trade Names:Synthroid- Tablets 0.025 mg- Tablets 0.05 mg- Tablets 0.075 mg- Tablets 0.088 mg- Tablets 0.1 mg- Tablets 0.112 mg- Tablets 0.125 mg- Tablets 0.15 mg- Tablets 0.175 mg- Tablets 0.2 mg- Tablets 0.3 mg

Trade Names:Unithroid- Tablets 0.025 mg- Tablets 0.05 mg- Tablets 0.075 mg- Tablets 0.088 mg- Tablets 0.1 mg- Tablets 0.112 mg- Tablets 0.125 mg- Tablets 0.15 mg- Tablets 0.175 mg- Tablets 0.2 mg- Tablets 0.

Eltroxin (Canada)Euthyrox (Canada)


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



Bioavailability is 48% to 79%. Fasting increases absorption. Effective by parenteral route.


More than 99% protein bound.


The t ½ is 6 to 7 days for T 4 .

Indications and Usage

Replacement or supplemental therapy in hypothyroidism; thyroid-stimulating hormone (TSH) suppression (in thyroid cancer, nodules, goiters, and enlargement in chronic thyroiditis).


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.

Infants and Children

In infants with congenital or acquired hypothyroidism, institute therapy with full doses as soon as diagnosis is made. In children with chronic or severe hypothyroidism, an initial oral 25 mcg/day dose is recommended with increments of 25 mcg every 2 to 4 wk until desired effect is achieved. The following guidelines are recommended:

Children older than 12 yr of age (growth/puberty complete)

PO 1.7 mcg/kg/day.

Children older than 12 yr of age (growth/puberty incomplete)

PO 2 to 3 mcg/kg/day.

Children 6 to 12 yr of age

PO 4 to 5 mcg/kg/day.

Children 1 to 5 yr of age

PO 5 to 6 mcg/kg/day.

Children 6 to 12 mo

PO 6 to 8 mcg/kg/day.

Children 3 to 6 mo of age

PO 8 to 10 mcg/kg/day.

Children 0 to 3 mo of age

PO 10 to 15 mcg/kg/day. Consider a lower starting dose (eg, 25 mcg/day) in infants at risk for cardiac failure, increasing the dose in 4- to 6-wk intervals based on clinical and laboratory response.

Hypothyroidism in Adults and Children in Whom Growth and Puberty are CompleteAdults and Children

PO Average full replacement dose is approximately 1.7 mcg/kg/day (eg, 100 to 125 mcg/day for 70 kg adult). Older patients may require less than 1 mcg/kg/day. Doses greater than 200 mcg/day are seldom required. For most patients older than 50 yr or patients younger than 50 yr with underlying cardiac disease, an initial starting dose of 25 to 50 mcg/day is recommended, with gradual increments in dose at 6- to 8-wk intervals, as needed. The recommended starting dose in elderly patients with cardiac disease is 12.5 to 25 mcg/day, with gradual dose increments at 4- to 6-wk intervals.

Severe HypothyroidismAdults

PO Recommended starting dose is 12.5 to 25 mcg/day with increases of 25 mcg/day every 2 to 4 wk, accompanied by clinical and laboratory assessment, until TSH level in normalized. IV / IM May be substituted for oral form when oral ingestion is precluded for long periods of time. Initial parenteral dosage should be approximately 50% the previously established oral dosage. A daily maintenance dose of 50 to 100 mcg parenterally should maintain the euthyroid stat once established. Monitor the patient and adjust the dosage as needed.

Subclinical HypothyroidismAdults

PO If treated, a lower dose (eg, 1 mcg/kg/day) than that used for full replacement may be adequate to normalize serum TSH level.

Myxedema ComaAdults

IV In myxedema coma or stupor, without concomitant severe heart disease, 200 to 500 mcg may be administered as a solution containing 100 mcg/mL. Full therapeutic effect may not be evident until the following day. An additional 100 to 300 mcg or more may be given on the second day if evidence of significant and progressive improvements has not occurred.

TSH Suppression in Well-Differentiated Thyroid Cancer and Thyroid NodulesAdults

PO TSH suppression to less than 0.1 milliunit/L usually requires a levothyroxine dose greater than 2 mcg/kg/day; however, in patients with high-risk tumors, the target TSH suppression level may be less than 0.01 milliunit/L. In treatment of benign nodules and nontoxic multinodular goiter, TSH generally is suppressed to a higher target (eg, 0.1 to 0.5 milliunit/L or 1 milliunit/L) than that used for treatment of thyroid cancer.

General Advice

  • Administer prescribed dose once daily. When given on empty stomach, absorption is increased. However, to maintain steady blood levels, be consistent in giving drug with or without food.
  • Do not give sooner than 4 h after administration of cholestyramine or colestipol. Cholestyramine or colestipol reduces effectiveness of levothyroxine.
  • 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 also may be sprinkled over small amount of food (eg, cooked cereal, applesauce).
  • Reconstitute powder for injection by adding 5 mL of sodium chloride 0.9% injection. Shake vial to ensure complete dissolution.
  • Use IV solution immediately after reconstitution. Discard any unused portion.
  • Store in tightly closed, light-resistant container at controlled room temperature (59° to 86°F).

Drug Interactions

Anticoagulants, oral

May increase anticoagulant effects.

Cholestyramine, colestipol

May decrease thyroid hormone efficacy.

Digitalis glycosides

May reduce effects of glycosides.


Increases absorption from GI tract.

Iron salts

May decrease efficacy of levothyroxine, resulting in hypothyroidism.


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


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


Tremors; headache; nervousness; insomnia.


Diarrhea; vomiting.


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



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.


Category A .


Minimal amounts excreted in breast milk.


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.

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 symptoms. Therapy of myxedema coma requires simultaneous administration of glucocorticoids. In patients whose hypothyroidism is secondary to hypopituitarism, correct adrenal insufficiency, if present, with corticosteroids.

Hyperthyroid effects

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


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.



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

  • 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 health care provider.
  • 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 health care provider.
  • Teach patient how to monitor for signs and symptoms of thyroid deficit or excess. Instruct patient to notify health care provider of following persistent signs and symptoms: headache, nervousness, diarrhea, excessive sweating, heat intolerance, chest pain, increased pulse rate, palpitations.
  • Teach patient to keep a record of signs and symptoms for health care provider review.
  • Advise patient to wear medical identification bracelet or necklace and to carry medical identification card in wallet.
  • Caution patient not to take levothyroxine for weight control.
  • Explain that partial hair loss may be experienced by children in first few months of therapy but that this adverse reaction is transient.

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