Antidepressant Discontinuation Syndrome: This condition occurs when an individual suddenly stops taking antidepressants, especially selective serotonin reuptake inhibitors (SSRIs), after a prolonged period. The symptoms can include "brain zaps" (electric shock sensations), tremors, palpitations, and other physical and psychological effects. The justification for this being the most likely diagnosis is the specific mention of "brain zaps," which is a hallmark symptom of antidepressant withdrawal.

Anxiety Disorders: Conditions such as generalized anxiety disorder or panic disorder can cause palpitations, tremors, and may be associated with sensory disturbances that could be described as "brain zaps" due to the intense anxiety and stress response.
Hypothyroidism: Although less directly linked to "brain zaps," hypothyroidism can cause tremors and palpitations. Some patients might experience neurological symptoms that could be misinterpreted as "brain zaps."
Caffeine or Nicotine Withdrawal: Withdrawal from these substances can lead to tremors and palpitations. While "brain zaps" are not typically described in the context of caffeine or nicotine withdrawal, the overall symptomatology could overlap, especially in cases of severe withdrawal.

Thyroid Storm: A life-threatening complication of untreated or undertreated hyperthyroidism, characterized by severe exacerbation of thyrotoxicosis symptoms, including palpitations, tremors, and potentially neurological symptoms. Although "brain zaps" are not a classic symptom, the severity and potential lethality of thyroid storm necessitate its consideration.
Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodes of palpitations, tremors, and other symptoms due to excessive catecholamine release. While "brain zaps" are not typically associated with pheochromocytoma, the potential for severe and life-threatening episodes makes this a critical diagnosis not to miss.
Cardiac Arrhythmias: Certain arrhythmias can cause palpitations and, in severe cases, may lead to neurological symptoms due to reduced cerebral perfusion. Tremors could be a manifestation of the body's response to these arrhythmias.

Multiple Sclerosis: This autoimmune disease can cause a wide range of neurological symptoms, including tremors and potentially "brain zaps"-like sensations due to demyelination in the central nervous system.
Vasovagal Syncope: While primarily known for causing fainting spells, in some cases, it can lead to palpitations and tremors before an episode. The link to "brain zaps" would be more speculative but could be considered in the context of pre-syncopal symptoms.
Neurodegenerative Diseases: Certain rare neurodegenerative diseases could potentially cause a combination of these symptoms, though they would typically be part of a broader and more progressive clinical picture.