Orthostatic Hypotension: This condition is characterized by a significant drop in blood pressure when standing up from sitting or lying down, leading to reduced blood flow to the brain and causing lightheadedness. It is a common cause of this symptom, especially in older adults or those with dehydration, medication side effects, or autonomic nervous system dysfunction.

Dehydration: Inadequate fluid intake or excessive fluid loss can lead to dehydration, which reduces blood volume and can cause lightheadedness when standing.
Anemia: A decrease in red blood cells or hemoglobin can reduce oxygen delivery to tissues, including the brain, leading to lightheadedness, especially when changing positions.
Medication Side Effects: Certain medications, such as diuretics, beta-blockers, and antidepressants, can cause orthostatic hypotension as a side effect.

Cardiac Conditions: Conditions like heart failure, arrhythmias, or cardiac tamponade can lead to reduced cardiac output and lightheadedness upon standing. Missing these diagnoses can be life-threatening.
Neurological Disorders: Autonomic nervous system disorders, such as multiple system atrophy or pure autonomic failure, can cause orthostatic hypotension and are critical not to miss due to their significant impact on quality of life and potential for other serious complications.
Severe Blood Loss or Hypovolemia: Significant blood or fluid loss can lead to hypovolemic shock, which is life-threatening and requires immediate attention.

Postural Orthostatic Tachycardia Syndrome (POTS): A condition characterized by a rapid increase in heart rate and other symptoms upon standing, which can include lightheadedness.
Adrenal Insufficiency: Rarely, adrenal insufficiency can cause orthostatic hypotension due to the lack of cortisol and aldosterone production.
Vasovagal Syncope: Although more commonly associated with fainting, vasovagal syncope can sometimes present with lightheadedness upon standing due to a reflex of the involuntary nervous system.