This patient has thyroid storm, most likely precipitated by the iodine contained in the dye load from the cardiac catheterization in the context of undertreated Graves disease following her discontinuation of methimazole. Graves disease is the most common underlying condition associated with thyroid storm, and onset may be triggered by a number of factors in addition to iodine, including infection, surgery, myocardial infarction, trauma, or parturition. This patient has typical features of thyroid storm: fever, tachycardia, heart failure, gastrointestinal dysfunction, and mental status changes. Scoring systems exist that can be used to provide a more objective measurement of the severity of the thyrotoxicosis. Treatment of thyroid storm is directed toward supportive care, reduction of thyroid hormone production, decreasing peripheral conversion of thyroxine (T₄) to triiodothyronine (T₃), addressing adrenergic symptoms and thermoregulatory changes, and treating any identified precipitating factors. Thionamides and β-blockers are the mainstay of treatment to reduce thyroid hormone production and control adrenergic symptoms.

This patient's clinical presentation is not consistent with euthyroid sick syndrome. Although this patient's low level of serum thyroid-stimulating hormone (TSH) is consistent with euthyroid sick syndrome, her elevated serum thyroid hormone level is not. In euthyroid sick syndrome, serum thyroid hormone levels are typically low, creating a clinical picture similar to that seen in central hypothyroidism.

Pheochromocytoma typically presents with hypertension and tachycardia but thyromegaly is not a clinical feature of this disorder. Additionally, confusion and altered mental status are not presenting signs of the disease.

Subacute thyroiditis is typically precipitated by a recent viral illness. An iodine load, such as this patient had, is an unlikely cause. Although patients with subacute thyroiditis may present with significant thyrotoxicosis, the thyroid is typically painful to palpation, a clinical finding that was not present in this patient.