Levothyroxine (Synthroid, Levothroid, Levoxine)


  • Hormone


  • Synthetic thyroid hormone


  • Hypothyroidism (including cretinism, myxedema, non-toxic goiter)

  • Pituitary TSH suppression (thyroid nodules, Hashimoto’s disease, multinodular goiter, thyroid cancer)

  • Thyrotoxicosis (with antithyroid drugs)


  • Adrenal insufficiency, MI

  • Thryrotoxicosis, hypersensitivity


  • Pregnancy category A

  • Cardiovascular disease

  • Diabetes mellitus or insipidus

Adverse Reactions (Side Effects):

  • CNS: headache, insomnia, nervousness, tremors

  • CV: angina pectoris, cardiac arrest, cardiac dysrhythmias, palpitations, tachycardia

  • GI: diarrhea,  gastric intolerance, vomiting

  • GU: menstrual irregularities

  • MISC: fever, heat intolerance, sweating, weight loss, allergic skin reactions (rare)


Administered orally, intramuscularly, intravenously


  • Hypothyroidism: 

    • PO 50mcg daily to start, increase by 25-50mcg daily at intervals of 2-4 weeks, usual dose 100-200mcg daily as a single dose; 

    • use = < 25 mcg/day in patients with long-standing hypothyroidism if cardovascular impairment present; 

    • IM/IV 50% of oral dose

  • Myxedema: 

    • IV 200-500mcg 1 time, then 100-300mcg the next day as needed; 

    • resume oral therapy as soon as clinical situation stabilized

  • TSH suppression: 

    • PO larger amounts than needed for replacement are required; 

    • optimal dose determined by laboratory findings and clinical response


  • 0-6 months: PO 8-10 mcg/kg or 25-50mcg daily

  • 6-12 months: PO 6-8 mcg/kg or 50-75mcg daily

  • 1-5 years: PO 5-6 mcg/kg or 75-100mcg daily

  • 6-12 years: PO 4-5 mcg/kg or 100-150mcg daily

  • IM/IV 75% of oral dose

Drug interactions:

  • Bile sequestrants: reduced serum thyroid hormone concentrations

  • Carbamazepine, phenytoin, rifampin: increased elimination of thryroid hormones; possible increased requirement for thryroid hormone in hypothyroid patients

  • Oral anticoagulants: thryoid hormones increase catabolism of vitamin K-dependent clotting factors; an increase or decrease in clinical thyroid status will increase or decrease the hypoprothrombinemic response to oral anticoagulants

  • Theophylline: reduced serum theophylline concentrations with initiation of thyroid therapy

Special considerations:

  • Transient, partial hair loss may be experienced by children in the 1st few months of therapy

  • Take as a single daily dose, preferably before breakfast


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