Levothyroxine Sodium Injection Dosage & Rx Info

Levothyroxine Sodium Injection Generic Name & Formulations

Legal Class

General Description

Levothyroxine sodium 100mcg, 200mcg, 500mcg; pwd for IV inj after reconstitution.

Pharmacological Class

T₄ (synthetic).

How Supplied

Tabs (all)—90, 1000; Tabs (50mcg, 75mcg, 100mcg, 125mcg, 150mcg, 200mcg)—100; Single-use vials—contact supplier

Manufacturer

Various generic manufacturers

Generic Availability

YES

Mechanism of Action

Thyroid hormones exert their physiologic actions through control of DNA transcription and protein synthesis. Triiodothyronine (T3) and L-thyroxine (T4) diffuse into the cell nucleus and bind to thyroid receptor proteins attached to DNA. This hormone nuclear receptor complex activates gene transcription and synthesis of messenger RNA and cytoplasmic proteins. The physiological actions of thyroid hormones are produced predominantly by T3, the majority of which (approximately 80%) is derived from T4 by deiodination in peripheral tissues.

Levothyroxine Sodium Injection Indications

Indications

Myxedema coma.

Limitations of Use

Caution when converting from oral to IV levothyroxine.

Levothyroxine Sodium Injection Dosage and Administration

Adult

Individualize. Initially 300–500mcg IV once, then 50–100mcg IV once daily; switch to oral form and dose as soon as feasible. Elderly, underlying cardiovascular disease: may need lower doses.

Children

Not established.

Levothyroxine Sodium Injection Contraindications

Contraindications

Uncorrected adrenal insufficiency.

Levothyroxine Sodium Injection Boxed Warnings

Boxed Warning

Not for treatment of obesity or for weight loss.

Levothyroxine Sodium Injection Warnings/Precautions

Warnings/Precautions

Underlying cardiovascular disease. Monitor for cardiac arrhythmias during surgery in those with coronary artery disease. Reduce or withhold dose for 1 week if cardiac symptoms develop or worsen; restart at lower dose. Myxedema coma: use IV levothyroxine. Adults: Monitor TSH 6–8 weeks after dose change then every 6–12 months when stable. Peds: Monitor TSH and total or free-T4 at 2 and 4 weeks after initiation, 2 weeks after dose change then every 3–12 months thereafter. Correct adrenal insufficiency before initiation. Diabetes; monitor glycemic control. Increased bone resorption and decreased BMD (esp. in post-menopausal women). Use lowest effective dose. Elderly. Pregnancy: monitor TSH/free-T4 at least every trimester; adjust dose and do not discontinue (see full labeling). Nursing mothers.

Levothyroxine Sodium Injection Pharmacokinetics

Absorption

Relative bioavailability of Synthroid tablets vs an equal nominal dose of oral levothyroxine sodium solution is ~93%.

Distribution

Plasma protein bound: >99%.

Metabolism

Hepatic.

Elimination

Renal (major), fecal (~20%). Half-life: 6–7 days (T4), ≤2 days (T3).

Levothyroxine Sodium Injection Interactions

Interactions

See full labeling. Absorption reduced by some foods (eg, soy flour, cottonseed meal, walnuts, fiber, grapefruit juice), aluminum and magnesium hydroxide, simethicone, proton pump inhibitors, iron, sucralfate; monitor. Give at least 4hrs apart from calcium carbonate, ferrous sulfate, sevelamer, lanthanum, bile acid sequestrants (eg, colesevelam, cholestyramine, colestipol), ion exchange resins (eg, kayexalate). Thyroid levels may be affected by clofibrate, estrogens, heroin, methadone, 5-fluorouracil, mitotane, tamoxifen, androgens, anabolic steroids, asparaginase, glucocorticoids, nicotinic acid (slow-release), salicylates (>2g/day), β-blockers (eg, propranolol), glucocorticoids, amiodarone. Monitor thyroid parameters with concomitant carbamazepine, furosemide (>80mg IV), heparin, hydantoins, NSAIDs. Potentiates, and is potentiated by, tri- and tetracyclic antidepressants. Concomitant sympathomimetics; monitor for coronary insufficiency. Antagonized by phenobarbital, rifampin, sertraline. Antagonizes digitalis glycosides. Marked hypertension and tachycardia with ketamine. Hypothyroidism risk with concomitant tyrosine kinase inhibitors (eg, imatinib). Monitor with oral anticoagulants, antidiabetic agents, orlistat. Concomitant biotin or biotin-containing supplements may interfere with thyroid hormone immunoassays; discontinue supplements for at least 2 days prior to thyroid testing.

Levothyroxine Sodium Injection Adverse Reactions

Adverse Reactions

Arrhythmias, myocardial infarction, dyspnea, muscle spasm, headache, nervousness, irritability, insomnia, tremors, muscle weakness, increased appetite, weight loss, diarrhea, heat intolerance, menstrual irregularities, skin rash; seizures (rare); Children: pseudotumor cerebri, slipped capital femoral epiphysis.

Levothyroxine Sodium Injection Clinical Trials

See Literature

Levothyroxine Sodium Injection Note

Not Applicable

Levothyroxine Sodium Injection Patient Counseling