A patient taking a large dose of levothyroxine presents with exophthalmos, irritability, heat intolerance, and tachycardia. What is the MOST likely explanation?

Excessive levothyroxine (T4) intake causes iatrogenic hyperthyroidism (thyrotoxicosis). Signs include: exophthalmos (in Graves'), irritability, nervousness, heat intolerance, tachycardia, tremors, weight loss, and sweating. Exophthalmos specifically is associated with Graves' disease (autoimmune thyrotoxicosis) due to TSH receptor antibodies stimulating retro-orbital tissue. However, in an over-medicated patient, the excess thyroid hormone mimics all hyperthyroid symptoms. The excess T4 suppresses TSH.