Levothyroxine (Synthroid, Levothroid, Levoxine)

Category:

• Hormone

Description:

• Synthetic thyroid hormone

Indications:

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

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

• Thyrotoxicosis (with antithyroid drugs)

Contraindications:

• Adrenal insufficiency, MI

• Thryrotoxicosis, hypersensitivity

Precautions:

• 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)

Dosage:

Administered orally, intramuscularly, intravenously

Adult:            

• 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

Child    

• 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 1^st few months of therapy

• Take as a single daily dose, preferably before breakfast