Orthostatic Hypotension: Given the patient's history of hyperthyroidism and Graves' disease, and the use of medications such as methimazole and propranolol, orthostatic hypotension is a plausible cause for the quick lightheaded episodes. The beta-blocker propranolol can also contribute to hypotension.

Anxiety or Panic Attacks: The patient is on Wellbutrin, which can sometimes exacerbate anxiety, and the symptoms of quick lightheaded episodes could be related to anxiety or panic attacks, especially given the context of a new diagnosis of a chronic condition like Graves' disease.
Hypothyroidism (due to overtreatment): Although the patient has hyperthyroidism, overtreatment with methimazole can lead to hypothyroidism, which might cause dizziness among other symptoms.
Side Effects of Medication: The combination of medications the patient is taking (methimazole, propranolol, Wellbutrin, progesterone, and Estradiol) could be contributing to her symptoms through various mechanisms, including effects on blood pressure, heart rate, and electrolyte balance.

Cardiac Arrhythmias: Given the patient's history of TIA and the current diagnosis of Graves' disease, which can affect the heart, it's crucial not to miss cardiac arrhythmias that could cause lightheadedness or even more severe cardiac events.
Pulmonary Embolism: Although less likely, pulmonary embolism is a potentially life-threatening condition that can cause sudden episodes of lightheadedness and must be considered, especially in patients with a history of thrombotic events like TIA.
Adrenal Insufficiency: If the patient has been on steroids (not explicitly mentioned but possible given the context of Graves' disease and the use of other hormones), adrenal insufficiency could be a concern, especially if there has been a change in steroid dosage or if the patient has stopped taking steroids.

Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodes of hypertension, tachycardia, and lightheadedness, among other symptoms. Although rare, it's a diagnosis that could be considered in the context of episodic symptoms and a history of thyroid disease.
Autonomic Dysfunction: Conditions affecting the autonomic nervous system, such as dysautonomia, could lead to symptoms like orthostatic intolerance and lightheadedness. This would be a less common consideration but could be relevant if other diagnoses are ruled out.