What medications can interfere with thyroid function tests?

Medications That Interfere with Thyroid Function Tests

Numerous medications can significantly interfere with thyroid function tests, potentially leading to misdiagnosis or inappropriate management of thyroid disorders. These medications can affect thyroid hormone synthesis, transport, metabolism, and laboratory measurements, requiring careful interpretation of test results in patients taking these drugs.

Major Medication Categories Affecting Thyroid Tests

1. Medications Affecting T4 Absorption

• Phosphate Binders (calcium carbonate, ferrous sulfate, sevelamer, lanthanum)

  • Bind to levothyroxine and prevent absorption
  • Administer levothyroxine at least 4 hours apart from these agents 1

• Bile Acid Sequestrants (colesevelam, cholestyramine, colestipol)

  • Decrease levothyroxine absorption
  • Administer levothyroxine at least 4 hours prior to these drugs 1

• Gastric Acid Modifiers

  • Proton Pump Inhibitors
  • Sucralfate
  • Antacids (aluminum & magnesium hydroxides, simethicone)
  • These reduce gastric acidity, which is essential for levothyroxine absorption 1

2. Medications Affecting Thyroid Hormone Transport

• Medications Increasing Thyroxine-Binding Globulin (TBG)

  • Estrogen-containing medications
  • Oral contraceptives
  • Tamoxifen
  • Heroin/Methadone
  • 5-Fluorouracil
  • Mitotane 1

• Medications Decreasing TBG

  • Androgens/Anabolic Steroids
  • Glucocorticoids
  • Asparaginase
  • Slow-Release Nicotinic Acid 1

• Medications Displacing T4 from Protein Binding Sites

  • Salicylates (>2 g/day)
  • Furosemide (>80 mg IV)
  • Heparin
  • Carbamazepine
  • Hydantoins
  • Non-Steroidal Anti-inflammatory Drugs (fenamates) 1

3. Medications Affecting Hepatic Metabolism of Thyroid Hormones

• Enzyme Inducers
  • Phenobarbital
  • Rifampin
  • Carbamazepine - reduces serum protein binding of levothyroxine, potentially reducing total and free T4 by 20-40% 1, 2
  • Phenytoin 1

4. Medications Decreasing T4 to T3 Conversion

• Beta-adrenergic Antagonists (propranolol >160 mg/day)
• Glucocorticoids (dexamethasone ≥4 mg/day)
• Amiodarone - inhibits peripheral conversion of T4 to T3 1, 3

5. Medications with Direct Effects on Thyroid Function

• Iodine-Containing Medications

  • Amiodarone - contains 37% iodine by weight; can cause both hypothyroidism and hyperthyroidism 3
  • Radiographic contrast media - can interfere with thyroid scintigraphy and radioiodine treatment for up to 2 months 4
  • Iodinated glycerol 5

• Lithium

  • Causes overt hypothyroidism in 5-15% of patients
  • Can cause goiter in up to 37% of patients
  • Inhibits thyroid hormone release
  • Regular monitoring of thyroid function is essential 5, 6

• Antipsychotics

  • Phenothiazines - alter iodine capture and decrease TSH response to TRH
  • Non-phenothiazines - can induce thyroid autoantibodies and elevate TSH levels
  • Atypical antipsychotics - may decrease TRH-stimulated TSH 6

• Antidepressants

  • Tricyclic antidepressants - complex with iodine and thyroid peroxidase
  • Other antidepressants - may decrease circulating thyroid hormone levels 6

Clinical Implications and Monitoring

1. For patients on lithium, phenothiazines, and tricyclic antidepressants:

   • Close monitoring of thyroid function is recommended 6
   • Baseline thyroid function tests before starting therapy
   • Regular follow-up testing (every 6 months for lithium) 5

2. For patients on amiodarone:

   • Regular monitoring of thyroid function tests
   • Be alert for both hypothyroidism and hyperthyroidism
   • Continue monitoring even after cessation of the drug due to its long half-life 3

3. For patients on levothyroxine with potential interactions:

   • Adjust timing of medication administration (e.g., separate levothyroxine from interfering medications by at least 4 hours)
   • Monitor TSH levels when starting or stopping potentially interfering medications 1

4. For patients requiring thyroid scintigraphy or radioiodine treatment:

   • Avoid iodinated contrast media for 2 months prior to these procedures 4

Common Pitfalls

1. Failure to recognize medication effects on thyroid tests can lead to misdiagnosis and inappropriate treatment.

2. Assuming abnormal thyroid tests reflect thyroid disease when they may be medication-induced changes without clinical significance.

3. Not adjusting levothyroxine dosing when starting or stopping medications that affect thyroid hormone metabolism or absorption.

4. Overlooking the need for monitoring in high-risk patients on medications known to affect thyroid function.

By understanding these medication interactions, clinicians can better interpret thyroid function tests and make appropriate management decisions for patients on these medications.