Hypokalemic Periodic Paralysis: This condition is characterized by episodes of muscle weakness or paralysis, often triggered by factors such as high carbohydrate intake, insulin release, or certain medications and supplements like creatine. The rapid improvement with salt intake suggests a possible link to potassium levels, as salt can help retain potassium in the body. The patient's recent start of creatine supplements, which can affect electrolyte balance, further supports this diagnosis.

Dehydration and Electrolyte Imbalance: Given the patient's physically active lifestyle and the use of creatine, which can increase water loss, dehydration and subsequent electrolyte imbalance (including sodium and potassium disturbances) could lead to symptoms of weakness and syncope. The improvement with salt intake could be due to correction of volume status and electrolyte balance.
Adrenal Insufficiency: Although less common, adrenal insufficiency can cause hypotension, weakness, and syncope, particularly in the context of physical stress like intense gym activity. The improvement with salt (sodium) intake could be related to the correction of cortisol deficiency, which affects electrolyte balance.
Autonomic Dysfunction: Diabetes can cause autonomic neuropathy, leading to orthostatic hypotension and syncope. However, the rapid improvement with salt intake is less typical for this condition, making it less likely but still a consideration.

Cardiac Arrhythmias: Conditions such as long QT syndrome or other arrhythmias could cause syncope and might be exacerbated by electrolyte imbalances. Although the scenario does not strongly suggest a primary cardiac issue, arrhythmias can be deadly if missed.
Pheochromocytoma: This rare tumor can cause episodic hypertension, weakness, and syncope due to catecholamine surges. The improvement with salt might be coincidental, but given the potential severity, it should not be overlooked.

Bartter Syndrome or Gitelman Syndrome: These are rare genetic disorders affecting kidney function, leading to hypokalemia, alkalosis, and increased renin and aldosterone levels. They could potentially cause weakness and syncope, especially with physical exertion, and might show some improvement with salt intake due to the correction of electrolyte imbalances.
Thyroid Storm: Although the patient's presentation does not strongly suggest thyroid dysfunction, a thyroid storm can cause weakness, syncope, and electrolyte disturbances. Given its potential severity, it should be considered in the differential diagnosis.