Uncommon but serious complication that occurs when hyperthyroidism is incompletely treated/untreated and then significantly worsens in the setting of acute stress such as infection, trauma, surgery.

Presents with agitation, delirium, fever, diarrhea, coma, and tachyarrhythmia (cause of death).

May see ↑ LFTs.

Treat with the 4 P's:- β-blockers (eg, Propanolol)- Propylthiouracil- Corticosteroids (eg, Prednisolone)- Potassium iodide (Lugol iodine)