↓ Levothyroxine absorption:
Bile acid-binding agents (eg, cholestyramine)
Iron, calcium, aluminum hydroxide
Proton pump inhibitors, sucralfate
↑ TBG concentration:
Estrogen replacement, oral contraceptives
Tamoxifen
↓ TBG concentration:
Androgens, glucocorticoids
Anabolic steroids
↑ Thyroid hormone metabolism:
Rifampin
Phenytoin
Carbamazepine