Change in adherence (e.g., missing doses)

Change in timing of medication relative to eating

Malabsorption

Autoimmune atrophic gastritis

Celiac disease

Diabetic gastropathy

Helicobacter pylori gastritis

Medication, new initiation

Androgens

Estrogens

Heroin/methadone

Medications that decrease absorption of levothyroxine (Table 5)

Medications that may alter hepatic metabolism of levothyroxine (phenobarbital, rifampin)

Medications that may decrease levothyroxine conversion to triiodothyronine (amiodarone, high-dose beta-adrenergic agonists, glucocorticoids)

Medications that may reduce serum protein binding of levothyroxine (carbamazepine [Tegretol] or phenytoin [Dilantin])

Selective serotonin reuptake inhibitors or tricyclic antidepressants

Tamoxifen

Pregnancy