Anxiety or Panic Disorder: Given the symptoms of shakiness, jitteriness, and lightheadedness, and with all labs being normal except for a slightly low vitamin D level, anxiety or panic disorder is a plausible explanation. These conditions can manifest physically in ways that mimic other medical issues, and the psychological aspect should be considered, especially in adolescents.

Hyperventilation Syndrome: This condition can cause lightheadedness, shakiness, and jitteriness due to rapid breathing, leading to respiratory alkalosis. It's often associated with anxiety but can be a standalone issue.
Dehydration or Electrolyte Imbalance: Although the CBC and CMP are normal, mild dehydration or an electrolyte imbalance not severe enough to be detected by these tests could still cause symptoms. The slightly low vitamin D level might also contribute to muscle weakness or discomfort, potentially exacerbating feelings of shakiness.
Caffeine or Substance-Induced Anxiety: Consumption of caffeine, nicotine, or other substances can lead to jitteriness, shakiness, and feelings of lightheadedness, especially in sensitive individuals or at high doses.

Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodes of hypertension, tachycardia, sweating, and tremors. Although unlikely, missing this diagnosis could be fatal, so it's crucial to consider it, especially if the patient reports episodic symptoms.
Cardiac Arrhythmias: Conditions like supraventricular tachycardia (SVT) can cause palpitations, lightheadedness, and shakiness. An ECG would be necessary to diagnose arrhythmias, which could be life-threatening if not addressed.
Orthostatic Hypotension: A significant drop in blood pressure upon standing, which can cause lightheadedness and shakiness. This could be due to dehydration, blood loss, or certain medications.

Mitochondrial Myopathies: A group of disorders that affect the mitochondria, leading to muscle weakness and other systemic symptoms. While rare, they could potentially cause the described symptoms, especially if there's a family history.
Wilson's Disease: A genetic disorder leading to copper accumulation in the body, which can cause neurological symptoms, including tremors. It's rare but should be considered in young patients with unexplained neurological symptoms.
Adrenal Insufficiency: Although the cortisol level is not mentioned, adrenal insufficiency (Addison's disease) can cause a range of symptoms, including lightheadedness and shakiness, due to insufficient production of adrenal hormones.