SAT-307 - Methimazole Induced Myxedema Coma with Biochemically Masked Hypothyroidism

Methimazole Induced Myxedema Coma With Biochemically Masked Hypothyroidism
Background
Myxedema coma is a rare, life-threatening manifestation of severe hypothyroidism characterized by hypothermia, altered mental status, and cardiovascular instability. While typically associated with long standing untreated hypothyroidism, iatrogenic causes are uncommon. Antithyroid medications such as methimazole can suppress thyroid hormone synthesis and alter pituitary feedback, potentially masking biochemical severity and delaying diagnosis.
Case Presentation
A 58-year-old male with quadriplegia secondary to a C1–C2 fracture, epilepsy, recurrent urinary tract infections, and a previously identified right thyroid nodule was admitted with acute altered mental status. Methimazole had been initiated previously for an unclear indication, with no documented routine thyroid function monitoring.
On presentation, the patient was profoundly hypothermic (86.8°F), bradycardic (39 bpm), and hypertensive (162/126 mmHg), requiring intensive care unit admission and mechanical ventilation. Initial laboratory evaluation revealed a thyroid-stimulating hormone (TSH) level of 12.8 µIU/mL with a low-normal free thyroxine (FT4) of 1.02 ng/dL; triiodothyronine (T3) level was unavailable. Despite the absence of markedly abnormal thyroid indices, the constellation of hypothermia, bradycardia, altered mentation, and antithyroid drug exposure raised strong concern for myxedema coma. A myxedema coma diagnostic score of 130 supported the diagnosis.
Management and Outcome
The patient was treated empirically with stress-dose hydrocortisone, intravenous levothyroxine (200 mcg loading dose followed by 100 mcg daily), and liothyronine. He demonstrated rapid clinical improvement with normalization of temperature, resolution of bradycardia, and significant improvement in mental status.
Conclusion
This case highlights methimazole-induced myxedema coma presenting with biochemically masked hypothyroidism. Clinicians should prioritize clinical presentation over laboratory thresholds when evaluating suspected myxedema coma, particularly in patients receiving antithyroid therapy without appropriate monitoring.

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