Dehydration: This is often the most common and straightforward reason for a significantly elevated heart rate upon standing, especially in individuals who have not been drinking enough fluids or have been experiencing excessive fluid loss (e.g., due to diarrhea, vomiting, or excessive sweating). Dehydration leads to a decrease in blood volume, which the body compensates for by increasing heart rate to maintain adequate circulation.

Anxiety or Panic Attack: Anxiety and panic attacks can cause significant increases in heart rate. Standing up can sometimes trigger or exacerbate these feelings, leading to a rapid heart rate.
Orthostatic Hypotension: This condition is characterized by a sudden drop in blood pressure when standing up, which can lead to an increase in heart rate as the body tries to compensate for the decreased blood pressure.
Deconditioning or Poor Physical Fitness: Individuals who are not physically fit or have been bedridden for a period may experience an elevated heart rate when standing due to their body's decreased efficiency in maintaining blood pressure and circulation.

Cardiac Arrhythmias (e.g., Supraventricular Tachycardia, SVT): Although less common, cardiac arrhythmias can cause a sudden increase in heart rate and may be triggered or become more apparent when standing. These conditions can be life-threatening if not promptly treated.
Hypovolemic Shock: This is a serious condition that occurs when there is not enough blood volume in the body, leading to inadequate blood flow and oxygen delivery to vital organs. It can be caused by severe dehydration, bleeding, or other causes of fluid loss.
Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodes of high blood pressure and rapid heart rate, sometimes triggered by standing or other positional changes.

Postural Orthostatic Tachycardia Syndrome (POTS): A condition characterized by a rapid increase in heart rate and other symptoms that occur upon standing. The exact cause is often unknown, but it can be related to autonomic nervous system dysfunction.
Hyperthyroidism: An overactive thyroid gland can cause an increase in heart rate among other symptoms. Although not typically diagnosed solely based on an elevated heart rate when standing, it is a potential underlying cause that could contribute to this symptom.
Autonomic Dysfunction: Various conditions affecting the autonomic nervous system can lead to abnormal heart rate responses to standing, including diabetes, amyloidosis, and certain neurological disorders.