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Active ingredient: Levothyroxine - Brands, Medical Use, Clinical Data

Brands, Medical Use, Clinical Data

Drug Category

  • Antithyroid Agents
  • Anticoagulants

Dosage Forms

  • Liquid
  • Powder for intravenous injection (6 and 10 mL vials containing 0.2 mg or 0.5mg of levothyroxine per vial)
  • Tablet (0.025, 0.05, 0.075, 0.088, 0.1, 0.112, 0.125, 0.137, 0.15, 0.175, 0.2, and 0.3 mg)

Brands / Synonyms

(-)-Thyroxine; 3,3',5,5'-Tetraiodo-L-thyronine; D-Thyroxine; Eltroxin; Euthyrox; L-Thryoxin; L-Thyroxine; Laevothyroxinum; Laevoxin; Letter; Levaxin; Levo-t; Levolet; Levothroid; Levothyrox; Levothyroxin; Levothyroxine Sodium; Levothyroxine sodium anhydrous; Levoxine; Levoxyl; Novothyrox; Oroxine; Synthroid; Synthroid Sodium; Tetraiodothyronine; THX; Thyratabs; Thyrax; Thyreoideum; Thyro-tabs; Thyrolar; Thyroxevan; Thyroxin; Thyroxinal; Thyroxine iodine; Tirosint; Unithroid


For use alone or in combination with antithyroid agents to treat hypothyroidism, goiter, chronic lymphocytic thyroiditis, myxedema coma, and stupor.


Levothyroxine (T4) is a synthetically prepared levo isomer of thyroxine, the major hormone secreted from the thyroid gland. Thyroxine is released from thyroglobulin by proteolysis and secreted into the blood. Thyroxine is peripherally deiodinated to form triiodothyronine which exerts a broad spectrum of stimulatory effects on cell metabolism. Thyroid hormone increases the metabolic rate of cells of all tissues in the body. In the fetus and newborn, thyroid hormone is important for the growth and development of all tissues including bones and the brain. In adults, thyroid hormone helps to maintain brain function, food metabolism, and body temperature, among other effects. The symptoms of thyroid deficiency relieved by levothyroxine include slow speech, lack of energy, weight gain, hair loss, dry thick skin and unusual sensitivity to cold.

Mechanism of Action

Levothyroxine acts like the endogenous thyroid hormone thyroxine (T4, a tetra-iodinated tyrosine derivative). In the liver and kidney, T4 is converted to T3, the active metabolite. In order to increase solubility, the thyroid hormones attach to thyroid hormone binding proteins, thyroxin-binding globulin, and thyroxin-binding prealbumin (transthyretin). Transport and binding to thyroid hormone receptors in the cytoplasm and nucleus then takes place. Thus by acting as a replacement for natural thyroxine, symptoms of thyroxine deficiency are relieved.


Bioavailability varies from 48% to 80%. Human studies have confirmed the importance of an intact jejunum and ileum for levothyroxine absorption and have shown some absorption from the duodenum.


LD50=20 mg/kg (orally in rat). Hypermetabolic state indistinguishable from thyrotoxicosis of endogenous origin. Symptoms of thyrotoxicosis include weight loss, increased appetite, palpitations, nervousness, diarrhea, abdominal cramps, sweating, tachycardia, increased pulse and blood pressure, cardiac arrhythmias, tremors, insomnia, heat intolerance, fever, and menstrual irregularities.

Biotrnasformation / Drug Metabolism

Hepatic (primarily via cytochrome P450 isozyme 3A4).


Levothyroxine sodium is contraindicated in patients with untreated thyrotoxicosis of any etiology or an apparent hypersensitivity to thyroid hormones or any of the inactive product constituents. (The 50 mcg tablet is formulated without color additives for patients who are sensitive to dyes.) There is no well-documented evidence of true allergic or idiosyncratic reactions to thyroid hormone. Levothyroxine sodium is also contraindicated in the patients with uncorrected adrenal insufficiency, as thyroid hormones increase tissue demands, for adrenocortical hormones and may thereby precipitate acute adrenal crisis.

Drug Interactions

The magnitude and relative importance of the effects noted below are likely to be patient specific and may vary by such factors as age, gender, race, intercurrent illnesses, dose of either agent, additional concomitant medications, and timing of drug administration. Any agent that alters thyroid hormone synthesis, secretion, distribution, effect on target tissues, metabolism, or elimination may alter the optimal therapeutic dose of levothyroxine sodium.

Levothyroxine Sodium Absorption: The following agents may bind and decrease absorption of levothyroxine sodium from the gastrointestinal tract: aluminum hydoxide, cholestyramine resin, colestipol hydrochloride, ferrous sulfate, sodium polystyrene sulfonate, soybean flour (e.g., infant formula), sucralfate.

Binding to Serum Proteins: The following agents may either inhibit levothyroxine sodium binding to serum proteins or alter the concentrations of serum binding proteins: androgens and related anabolic hormones, asparaginase, clofibrate, estrogens and estrogen-containing compounds, 5-fluorouracil, furosemide, glucocorticoids, meclofenamic acid, mefenamic acid, methadone, perphenazine, phenylbutazone, phenytoin, salicylates, tamoxifen.

Thyroid Physiology: The following agents may alter thyroid hormone or TSH levels, generally by effects on thyroid hormone synthesis, secretion, distribution, metabolism, hormone action, or elimination, or altered TSH secretion: aminoglutethimide, p-aminosalicylic acid, amiodarone, androgens and related anabolic hormones, complex anions (thiocyanate, perchlorate, pertechnetate), antithyroid drugs, b-adrenergic blocking agents, carbamazepine, chloral hydrate, diazepam, dopamine and dopamine agonists, ethionamide, glucocorticoids, heparin, hepatic enzyme inducers, insulin, iodinated cholestographic agents, iodine- containing compounds, levodopa, lovastatin, lithium, 6-mercaptopurine, metoclopramide, mitotane, nitroprusside, phenobarbital, phenytoin, resorcinol, rifampin, somatostatin analogs, sulfonamides, sulfonylureas, thiazide diuretics.

Adrenocorticoids: Metabolic clearance of adrenocorticoids is decreased in hypothyroid patients and increased in hyperthyroid patients, and may therefore change with changing thyroid status.

Amiodarone: Amiodarone therapy alone can cause hypothyroidism or hyperthyroidism.

Anticoagulants (Oral): The hypoprothrombinemic effect of anticoagulants may be potentiated, apparently by increased catabloism of vitamin K-dependent clotting factors.

Antidiabetic Agents (Insulin, Sulfonylureas): Requirements for insulin or oral antidiabetic agents may be reduced in hypothyroid patients with diabetes mellitus and may subsequently increase with the initiation of thyroid hormone replacement therapy.

b-Adrenergic Blocking Agents: Actions of some of beta-blocking agents may be impaired when hypothyroid patients become euthyroid.

Cytokines (interferon, interleukin): Cytokines have been reported to induce both hyperthyroidism and hypothyroidism.

Digitalis Glycosides: Therapeutic effects of digitalis glycosides may be reduced. Serum digitalis levels may be decreased in hyperthyroidism or when a hypothyroid patient becomes euthyroid.

Ketamine: Marked hypertension and tachycardia have been reported in association with concomitant administration of levothyroxine sodium and ketamine.

Maprotiline: Risk of cardiac arrhythmias may increase.

Sodium Iodide (123I and 131I), Sodium Pertechnetate Tc99m: Uptake of radiolabeled ions may be decreased.

Somatrem/Somatropin: Excessive concurrent use of thyroid hormone may accelerate epiphyseal closure. Untreated hypothyroidism may interfere with the growth response to somatrem or somatropin.

Theophylline: Theophylline clearance may decrease in hypothyroid patients and return toward normal when a euthyroid state is achieved.

Tricyclic Antidepressants: Concurrent use may increase the therapeutic and toxic effects of both drugs, possibly due to increased catecholamine sensitivity. Onset of action of tricyclics may be accelerated.

Sympathomimetic Agents: Possible increased risk of coronary insufficiency in patients with coronary artery disease.

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