A Rare Case of Severe Autoimmune Hypothyroidism Presenting With Massive Pericardial Effusion, Mimicking Malignancy, and Complicated by Seizures.

Severe hypothyroidism is a recognised but uncommon cause of large serous effusions, and massive pericardial involvement with haemodynamic compromise remains particularly rare. We describe a 39-year-old woman presenting with progressive dyspnoea, peripheral oedema, and abdominal distension. Imaging revealed a large circumferential pericardial effusion measuring 5.8 cm with tamponade physiology, accompanied by bilateral pleural effusions and ascites. Serum CA-125 was markedly elevated, prompting concurrent oncologic and endocrine evaluation. She underwent emergency pericardiocentesis for cardiac tamponade before the underlying cause was known, which yielded serous, non-malignant fluid with benign cytology. Cardiac, hepatic, and renal investigations were unremarkable, and subsequent endocrine testing confirmed severe primary hypothyroidism (thyroid-stimulating hormone (TSH) >150 mIU/L, undetectable free thyroxine) with positive thyroid peroxidase antibodies, consistent with autoimmune Hashimoto's thyroiditis. The patient was commenced on combined levothyroxine and liothyronine therapy, after which she developed brief self-limiting generalised seizures attributed to mild hyponatraemia during metabolic recovery. With gradual thyroid hormone replacement, her cardiovascular function stabilised and all serous effusions resolved completely. This case highlights a rare presentation of severe autoimmune hypothyroidism with massive pericardial effusion causing tamponade, multi-serosal involvement, and transient metabolic seizures, demonstrating its ability to mimic malignant or systemic disease while remaining fully reversible with timely endocrine therapy.