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Drug Notebook

Generic Name: levothyroxine

View All Brands
Synthroid, Eutroxsig, Evotrox, L THYROXINE ROCHE, Levothyroxine sodium causing adverse effects in therapeutic use, Levo-T, Levoxyl, Levotabs, Levotec, Levothyroxine Sodium, Eltroxin, Levothroid, Oroxine, LEVOTHYROX, Unithroid, Tirosint
A thyroid drug - used as aThyroid Suppression Test, treats Myxedema Coma and Hypothyroidism, and use... more
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(LEE voe thy ROX een)

Drug Interactions

Drugs Affecting Hepatic Microsomal Enzymes

Potential increased levothyroxine metabolism and decreased plasma levothyroxine concentrations with drugs that induce general hepatic metabolic activity resulting in increased levothyroxine dosage requirements.

Drugs That May Decrease T4 5’-Deiodinase Activity

Inhibitors of T4 5’-deiodinase decrease peripheral conversion of T4 to T3, resulting in decreased T3 concentrations. However, serum T4 concentrations usually remain within normal range but may occasionally be slightly increased.

Specific Drugs and Foods

Drug or Food Interaction Comment
Amiodarone Decreased metabolism of T4 to T3
Anticoagulants, oral (e.g., coumarins) Potentiation of anticoagulant activity Carefully monitor PT and adjust anticoagulant dosage accordingly
Antidepressants (tricyclics, tetracyclics, SSRIs)

Increased risk of cardiac arrhythmias and CNS stimulation when used with tricyclics or tetracyclics

Faster onset of action of tricyclics

Sertraline may increase levothyroxine requirements
Antidiabetic agents (biguanides, meglitinides, sulfonylureas, thiazolidediones, insulin) Levothyroxine may cause increased antidiabetic agent or insulin requirements Carefully monitor diabetic control, especially when thyroid therapy is initiated, changed, or discontinued
β-Adrenergic blocking agents (e.g., propranolol hydrochloride dosages >160 mg daily)

Decreased metabolism of T4 to T3

Impaired antihypertensive effects when hypothyroid patient is converted to euthyroid state
Bile acid sequestrants (e.g., cholestyramine, colestipol) Delayed or impaired levothyroxine absorption Administer levothyroxine at least 4 hours apart from these agents
Carbamazepine

Potential increased levothyroxine metabolism

Reduced levothyroxine serum protein binding

 May require levothyroxine dosage increase
Cardiac glycosides Decreased serum digitalis glycoside concentrations in patients with hyperthyroidism or in patients with hypothyroidism in whom a euthyroid state has been achieved; potential for reduced therapeutic effects of digitalis glycosides with levothyroxine May need to increase dosage of digitalis glycoside when hypothyroidism has been corrected
Corticosteroids (e.g., dexamethasone at dosages >4 mg daily) Decreased metabolism of T4 to T3. Short-term administration of large doses of corticosteroids may decrease serum T3 concentrations by 30% with minimal change in serum T4 levels
Ferrous sulfate Delayed or impaired levothyroxine absorption Administer levothyroxine at least 4 hours apart from this agent
Food with large amounts of fiber (e.g., cotton seed meal, infant soybean formula, soybean flour, walnuts) Delayed or impaired levothyroxine absorption
Fusosemide (at IV dosages >80 mg) Concomitant use with levothyroxine produces transient increases in serum free T4 concentrations; continued administration results in a decrease in serum T4 and normal free T4 and TSH concentrations, and therefore, patients are clinically euthyroid
GI drugs (e.g., antacids [aluminum hydroxide, magnesium hydroxide, calcium carbonate], simethicone, sucralfate) Delayed or impaired levothyroxine absorption Administer levothyroxine at least 4 hours apart from these agents
Growth hormones (e.g., somatropin) Excessive levothyroxine use with growth hormones may accelerate epiphyseal closure; however, untreated hypothyroidism may interfere with growth response to growth hormone
Heparin Concomitant use with levothyroxine produces transient increases in serum free T4 concentrations; continued administration results in a decrease in serum T4 and normal free T4 and TSH concentrations, and therefore, patients are clinically euthyroid
Hydantoins (e.g., phenytoin)

Potential increased levothyroxine metabolism

Reduced levothyroxine serum protein binding

 May require levothyroxine dosage increase
Ketamine Risk of marked hypertension and tachycardia Use with caution
NSAIAs (e.g., fenamates, phenylbutazone) Concomitant use with levothyroxine produces transient increases in serum free T4 concentrations; continued administration results in a decrease in serum T4 and normal free T4 and TSH concentrations, and therefore, patients are clinically euthyroid
Phenobarbital Potential increased levothyroxine metabolism May require levothyroxine dosage increase
Propylthiouracil Decreased metabolism of T4 to T3
Radiographic agents Reduced uptake of 123I, 131I, and99mTc
Rifampin Potential increased levothyroxine metabolism May require levothyroxine dosage increase
Salicylates (dosages >2 g daily) Inhibit binding of T4 and T3 to TBG and transthyretin; initially increases serum free T4 followed by return to normal concentrations with sustained therapeutic serum salicylate concentrations, although total T4 concentrations may decrease by as much as 30%
Sodium polystyrene sulfonate Delayed or impaired levothyroxine absorption Administer levothyroxine at least 4 hours apart from this agent
Sympathomimetic agents Potentiation of sympathomimetic or thyroid effects; increased risk of coronary insufficiency in patients with CHD
Xanthine derivatives (e.g., theophylline) Clearance of xanthine derivatives may be decreased in hypothyroid patients but returns to normal when the euthyroid state is achieved

Drugs Affecting Thyroid Function or Thyroid Function Tests

Various drugs or concomitant medical conditions (e.g., pregnancy, infectious hepatitis) may adversely affect thyroid function (e.g., alter endogenous thyroid hormone secretion, reduce TSH secretion) resulting in hypothyroidism or hyperthyroidism or interfere with laboratory tests used to assess thyroid function. Consult specialized references for information.

Some drugs may affect transport of thyroid hormones (T3, T4, levothyroxine) by affecting serum thyroxine-binding globulin (TBG) concentrations. However, free T4 concentrations may remain normal and the patient may remain euthyroid. Monitor therapy and adjust levothyroxine dosages as necessary.

Drugs Affecting Thyroxine Binding Globulin Concentration

The following drugs may increase serum TBG concentrations:

  • Estrogens, oral (including estrogen-containing oral contraceptives)
  • Fluorouracil
  • Methadone
  • Mitotane
  • Tamoxifen

    • The following drugs may decrease serum TBG concentrations:

  • Androgens
  • Asparaginase
  • Glucocorticoids
  • Niacin (sustained-release)

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    Uses Dosage and Administration Cautions Drug Interactions Pharmacokinetics Stability Actions Advice to Patients Preparations
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