May ↓ the therapeutic effects of digoxin; may need to ↑ digoxin dosage.
Concurrent use with ketamine may lead to significant hypertension and tachycardia.
↑cardiovascular effects with adrenergics (sympathomimetics).
Biotin may interfere with thyroid function test assays; discontinue biotin or biotin-containing supplements for ≥2 days before assessing TSH and/or T4 levels.
Drug-Food:
Foods or supplements containing calcium, iron, magnesium, or zinc may bind levothyroxine and prevent complete absorption.
Absorption may be delayed when used with grapefruit juice.
Powder for injection: 100 mcg/vial; 200 mcg/vial; 500 mcg/vial
Solution for injection: 20 mcg/mL; 40 mcg/mL; 100 mcg/mL
Route/Dosage⬆⬇
PO (Adults ): Hypothyroidism: 1.6 mcg/kg once daily; may ↑ by 12.525 mcg/day every 46 wk until patient clinically euthyroid based on signs/symptoms and TSH levels.
PO Geriatric Patients and Patients with Cardiac Disease): 12.525 mcg once daily; may ↑ by 12.525 mcg/day every 68 wk until patient clinically euthyroid based on signs/symptoms and TSH levels.
PO (Children >12 yr): 23 mcg/kg/day (≥150 mcg/day).
PO (Children 612 yr): 45 mcg/kg/day (100125 mcg/day).
PO (Children 15 yr): 56 mcg/kg/day (75100 mcg/day).
PO (Children 612 mo): 68 mcg/kg/day (5075 mcg/day).
PO (Infants 36 mo): 810 mcg/kg/day (2550 mcg/day).
PO (Infants 03 mo or Infants at Risk for Cardiac Failure): 1015 mcg/kg/day or 25 mcg/day; may ↑ after 46 wk to 50 mcg.
IM IV (Adults ): Hypothyroidism: 50100 mcg/day as a single dose. Myxedema coma/stupor: 300500 mcg IV; additional 100300 mcg may be given on 2nd day, followed by daily administration of smaller doses.
IM IV (Children ): Hypothyroidism: 5080% of the oral dose.
Absorption: Levothyroxine is variably (4080%) absorbed from the GI tract.
Distribution: Distributed into most body tissues.
Protein Binding: >99%.
Metabolism/Excretion: Metabolized by the liver and other tissues to active T3. Thyroid hormone undergoes enterohepatic recirculation and is excreted in the feces via the bile.
Half-Life: 67 days.
Canadian Brand Names⬆⬇
Eltroxin
Time/Action Profile⬆⬇
ROUTE
ONSET
PEAK
DURATION
PO
unknown
13 wk
13 wk
IV
68 hr
24 hr
unknown
Patient/Family Teaching⬆⬇
Explain purpose and side effects of levothyroxine to patient.
Instruct patient to take medication as directed at the same time each day. Take missed doses as soon as remembered unless almost time for next dose. If more than 23 doses are missed, notify health care professional. Do not discontinue without consulting health care professional.
Explain to patient that medication does not cure hypothyroidism; it provides a thyroid hormone supplement. Therapy is lifelong.
Advise patient to notify health care professional if headache, nervousness, diarrhea, excessive sweating, heat intolerance, chest pain, ↑ HR, palpitations, weight loss >2 lb/wk, or any unusual symptoms occur.
Caution patient to avoid taking other medications concurrently with thyroid preparations unless instructed by health care professional. Advise patient to take 4 hr apart from antacids, iron, and calcium supplements. Advise patient to stop biotin-containing supplements ≥ 2 days before assessing TSH or T4 levels.
Instruct patient to inform health care professionals of thyroid therapy.
Advise females of reproductive potential to notify health care professional if pregnancy is planned or suspected or if breastfeeding. Pregnancy may ↑ thyroid requirements. Monitor serum TSH levels and adjust levothyroxine dose accordingly during pregnancy. Since postpartum TSH levels are similar to preconception values, levothyroxine dose should return to prepregnancy dose immediately after delivery.
Emphasize importance of follow-up exams to monitor effectiveness of therapy. Thyroid function tests are performed at least yearly.
Pedi: Discuss with parents the need for routine follow-up studies to ensure correct development. Inform patient that partial hair loss may be experienced by children on thyroid therapy. This is usually temporary.