Dehydration: This is a common and straightforward cause of low blood pressure (hypotension) and headaches. Dehydration can lead to a decrease in blood volume, which in turn can cause a drop in blood pressure. Headaches are also a frequent symptom due to the brain's temporary reduction in blood flow.

Anemia: Low red blood cell count or hemoglobin can lead to reduced oxygen delivery to tissues, including the brain, potentially causing headaches. Anemia can also contribute to hypotension due to decreased blood viscosity and oxygen-carrying capacity.
Orthostatic Hypotension: This condition is characterized by a significant drop in blood pressure upon standing, which can lead to headaches due to transient cerebral hypoperfusion. It's a common cause of hypotension and can be due to various factors, including dehydration, medication side effects, or autonomic dysfunction.
Vitamin Deficiencies (e.g., B12): Certain vitamin deficiencies, particularly B12, can cause neurological symptoms including headaches. While less directly linked to hypotension, the overall clinical picture can include fatigue and weakness, potentially exacerbating symptoms of low blood pressure.

Adrenal Insufficiency (despite normal cortisol): Although cortisol levels are reported as normal, adrenal insufficiency should be considered, especially if the patient has symptoms suggestive of this condition. Normal cortisol levels do not entirely rule out adrenal insufficiency, as the disease can present with variable cortisol levels, and the diagnosis might require further testing (e.g., ACTH stimulation test).
Pheochromocytoma: This rare tumor of the adrenal gland can cause episodic high blood pressure but also orthostatic hypotension due to intermittent catecholamine release and subsequent vasodilation. Headaches are a common symptom. Although less likely, missing this diagnosis could have significant consequences.
Autonomic Dysfunction: Conditions affecting the autonomic nervous system, such as autonomic failure or dysautonomia, can lead to orthostatic hypotension and a variety of symptoms including headaches. These conditions might not be immediately suspected but are crucial to diagnose due to their impact on quality of life and potential for complications.

Mitochondrial Disorders: These are a group of disorders caused by dysfunctional mitochondria, affecting energy production in cells. They can present with a wide range of symptoms, including headaches and potentially hypotension, due to the systemic nature of the disease affecting multiple organ systems.
Ehlers-Danlos Syndrome: This genetic disorder affects connective tissue and can lead to autonomic dysfunction, among other symptoms. Patients might experience orthostatic intolerance, including hypotension and headaches, due to the autonomic component of the disease.