Acute Myocardial Infarction (MI): Hyperacute T waves are a classic early ECG sign of acute MI, particularly in the context of ST-elevation myocardial infarction (STEMI). They are often seen before the development of Q waves and are indicative of the early stages of myocardial ischemia.

Myopericarditis: Inflammation of the myocardium and pericardium can lead to ECG changes that include hyperacute T waves, mimicking those seen in acute MI. Clinical context and additional diagnostic tests are crucial for differentiation.
Early Repolarization: A benign condition that can mimic the ECG findings of hyperacute T waves, particularly in healthy young individuals. It is characterized by an elevation of the QRS-ST junction (J-point) often accompanied by a slight ST elevation and tall T waves.

Pulmonary Embolism (PE): While not typically associated with hyperacute T waves, PE can cause acute right ventricular strain, leading to ECG changes that might be confused with those of acute MI, including T wave inversions and, in some cases, hyperacute T waves in leads facing the right ventricle.
Cardiac Tamponade: Although more commonly associated with low voltage QRS and electrical alternans, cardiac tamponade can lead to acute ischemic changes due to impaired coronary perfusion, potentially resulting in hyperacute T waves.

Hypokalemia: Severe potassium deficiency can lead to various ECG abnormalities, including T wave flattening or inversion, but in rare cases, it might cause hyperacute T waves, especially if there's a significant and rapid drop in potassium levels.
Digitalis Effect: Digitalis (digoxin) toxicity can cause a variety of ECG changes, including the "digitalis effect" which might rarely present with T wave abnormalities that could be mistaken for hyperacute T waves in the appropriate clinical context.