LEVOTHYROXINE SODIUM 
Pronounciation
Trade Name(s)
- T4, L-Thyroxine
When oral ingestion is not practical, IV dose should be one-half of any previously established oral dose; see Indications and Uses, Limitation of Use. Adjust in small increments as indicated. Initiate oral treatment as soon as possible.
Hypothyroidism (when oral therapy is not possible):
Usual IV starting dose would be 6.25 to 12.5 mcg/day. Increase in increments of 12.5 mcg every 2 to 4 weeks. Base dosing on clinical response and serum thyroid and TSH levels. Average maintenance dose is 50 to 100 mcg/day PO.
300 to 500 mcg as an initial dose. Follow with once-daily IV maintenance doses between 50 and 100 mcg.
Given orally (may be crushed in food or liquid). See Precautions. Any IV dose should be 50% to 75% of the established oral dose. See pediatric literature for dosing guidelines.
Age, general condition, cardiac risk factors, and clinical severity of myxedema symptoms should be considered when determining the starting and maintenance dosages.
■ Reduce dose in elderly, functional, or ECG evidence of cardiovascular disease (including angina), long-standing thyroid disease, other endocrinopathies, and severe hypothyroidism.
■ See Drug/Lab Interactions.
Available in different strengths; read label carefully. Each vial of lyophilized powder is diluted with 5 mL of NS for injection (without preservatives). Shake well to dissolve completely. Reconstituted concentrations will be 20 mcg/mL for the 100-mcg vial and 100 mcg/mL for the 500-mcg vial. Do not add to IV solutions. May be given through Y-tube of infusion set.
Store dry product at CRT and protect from light. Reconstituted solution is preservative-free and stable for 4 hours; any remaining solution is discarded.
A synthetic thyroid hormone. Effective replacement for decreased or absent thyroid hormone. Thyroid hormone synthesis and secretion is regulated by the hypothalamic-pituitary-thyroid axis. Thyroid hormones regulate multiple metabolic processes and play an essential role in normal growth and development. Actions are produced predominantly by T3 (triiodothyronine). Approximately 80% of T3 is derived from T4 (levothyroxine) by deiodination in peripheral tissues. The metabolic actions of thyroid hormones include augmentation of cellular respiration and thermogenesis, as well as metabolism of proteins, carbohydrates, and lipids. Circulating thyroid hormones are greater than 99% bound to plasma proteins (thyroxine-binding globulin [TBG], thyroxine-binding prealbumin [TBPA], and albumin [TBA]). The higher affinity of both TBG and TBPA for T4 partially explains the higher serum levels, slower metabolic clearance, and longer half-life of T4 (6 to 8 days) compared with T3 (less than 2 days). The major pathway of thyroid hormone metabolism is through sequential deiodination. The liver is the major site of degradation for both T4 and T3. Primary route of elimination is through the kidneys. Small amounts are eliminated into the bile and feces.
Treatment of myxedema coma.
■ Specific replacement therapy for reduced or absent thyroid function due to any cause (usually given orally).
The relative bioavailability between oral and IV levothyroxine sodium for injection has not been established but has been estimated to be between 48% and 74%. Caution should be used when switching patients from oral products to the parenteral product because adequate dosing conversions have not been studied.
Excessive bolus doses (greater than 500 mcg) are associated with cardiac complications, particularly in the elderly and in patients with underlying cardiac disease; see Dose Adjustments.
■ Patients with undiagnosed endocrine disorders (e.g., adrenal insufficiency, hypopituitarism, diabetes insipidus) may experience new or worsening symptoms of these endocrinopathies. Patients should be treated with replacement glucocorticoids before initiation of treatment with levothyroxine until adrenal function has been adequately assessed. Failure to do so may precipitate an acute adrenal crisis when thyroid hormone therapy is initiated as a result of increased metabolic clearance of glucocorticoids by thyroid hormone. Patients with myxedema coma should be monitored for previously undiagnosed diabetes insipidus.
■ Not indicated for treatment of obesity or for weight loss. Doses within the range of daily hormonal requirements are ineffective for weight reduction. Larger doses may produce serious or even life-threatening manifestations of toxicity, particularly when given in conjunction with sympathomimetic amines, such as those used for the anorectic effects.
Observe patient closely and monitor vital signs.
■ Monitor thyroid function tests (e.g., free T4 index, TSH).
■ TSH and thyroid hormone levels should be measured a few weeks after switching from IV to oral therapy. Adjust dose according to results and clinical status.
■ Monitor TSH every 6 to 8 weeks until normalized, every 8 to 12 weeks after dose changes, and every 6 to 12 months throughout therapy.
Category A: may be used during pregnancy.
■ Presumed safe in breast-feeding; use caution and observe infant.
■ Myxedema coma is a disease of the elderly. An approved oral dosage form should be used in pediatric patients for maintaining a euthyroid state in noncomplicated hypothyroidism; see Pediatric Dose.
See Dose Adjustments; more sensitive to effects. Atrial fibrillation is a common side effect associated with levothyroxine treatment in the elderly.
Addition of levothyroxine to antidiabetic or insulin therapy may result in increased antidiabetic or insulin requirements. Monitor glucose values closely, especially when thyroid therapy is initiated, changed, or discontinued.
■ Levothyroxine increases response to oral anticoagulant therapy (e.g., warfarin). Reduction in dose of oral anticoagulant may be required. Monitor PT/INR.
■ May decrease therapeutic effects of digoxin, necessitating an increase in the digoxin dose. Monitor serum digoxin levels.
■ Concurrent use of tricyclic (e.g., amitriptyline) or tetracyclic (e.g., maprotiline) antidepressants and levothyroxine may increase the therapeutic and toxic effects of both drugs, possibly due to increased receptor sensitivity to catecholamines.
■ Administration of sertraline in patients stabilized on levothyroxine may result in increased levothyroxine requirements.
■ Concurrent use with ketamine may produce marked hypertension and tachycardia. Use caution.
■ Concurrent use with sympathomimetics (e.g., epinephrine, norepinephrine) may increase the effects of sympathomimetics or thyroid hormone. May increase risk of coronary insufficiency.
■ Carbamazepine, fosphenytoin, phenytoin, and rifampin may decrease the serum concentration of thyroid hormones. Monitor levels closely.
■ Levothyroxine can increase the metabolism of theophylline, necessitating an increase in theophylline dose. Monitor theophylline levels.
■ Changes in TBG concentration must be considered when interpreting T3 and T4 values, which necessitates measurement and evaluation of unbound (free) hormone and/or determination of the free levothyroxine index. Several medications and disease states can alter TBG concentrations. See manufacturer’s prescribing information.
Abdominal cramps, angina, arrhythmias, chest pain, diarrhea, heart palpitations, heat intolerance, increased pulse and BP, insomnia, menstrual irregularities, muscle cramps, nervousness, sweating, tachycardia, tremors, vomiting, and weight loss.