Endocrine — hyperthyroidism, hypoglycaemia, pheochromocytoma, Cushing syndrome.
Cardiopulmonary — arrhythmia, heart failure, asthma, chronic obstructive pulmonary disease, pulmonary embolism, myocardial infarction.
Neurological — complex partial seizures, vestibular disorders.
Substances — caffeine, nicotine, stimulants, decongestants, albuterol, thyroid replacement, high-dose corticosteroids — and withdrawal from alcohol, benzodiazepines or opioids.
Medication effect — akathisia, an inner motor restlessness from antipsychotics and occasionally SSRIs, described by the person as unbearable anxiety. It is the one adverse effect that worsens every time it is treated as anxiety.