There are various factors that may affect thyroid blood test results. For example, thyroid-stimulating hormone (TSH) test results may be affected by certain medications. These include antithyroid medications, lithium, potassium iodide, dopamine, amiodarone (an antiarrhythmic heart drug) and prednisone (a corticosteroid).

Thyroxine (T4) and triiodothyronine (T3) levels also may be affected by medications, leading to potentially inaccurate test results. Medications that may raise or lower T4 and T3 levels include:

• Anabolic steroids
  
  
• Androgens
  
  
• Antithyroid medication
  
  
• Birth control pills
  
  
• Estrogens
  
  
• Amiodarone
  
  
• Aspirin, meclofenamate or salsalate (nonsteroidal anti-inflammatory drugs)
  
  
• Clofibrate (a cholesterol drug)
  
  
• Lithium
  
  
• Methadone (an opioid)
  
  
• Phenytoin (an anticonvulsant)
  
  
• Propranolol (a beta blocker – see Antihypertensives)

Certain medications may affect other types of thyroid blood tests. For example, phenothiazines (antipsychotics), heroin and methadone increase levels of thyroxine binding globulin (TBG) in the blood. This may lead to inaccurate serum TBG tests. Patients should inform their physician of any medications they are taking before undergoing thyroid blood testing.

In addition to medications, certain health conditions can affect thyroid blood test results. For example, patients who have a chronic or systemic (throughout the body) illness typically have lower T4 and T3 levels. These patients are advised to wait until they are healthy before undergoing thyroid blood testing. Additionally, patients who are pregnant may have higher T4 or T3 levels.

Conditions that may affect TBG results include severe liver or kidney disease, acute intermittent porphyria (inherited metabolic disorder), human immunodeficiency virus (HIV), as well as other conditions that lower levels of albumin.