Ascites represents an infrequent sequela of hypothyroidism, manifesting in fewer than 4% of affected individuals. Herein, we delineate a case characterized by profound hypothyroidism accompanied by substantial ascites, further complicated by cardiac insufficiency. A 29-year-old female, previously diagnosed with postradiation hypothyroidism subsequent to a diagnosis of Grave’s disease 11 years prior, presented with exacerbating dyspnoea, abdominal distension, and orthopnea. In January 2024, she was admitted with massive ascites, exhibiting clinical manifestations of both hypothyroidism and cardiac failure. Thyroid function tests were markedly abnormal, with a thyroid-stimulating hormone level of 77.65 mIU/L, triiodothyronine at 2.2 nmol/L, and thyroxine levels below 3.2 pmol/L. Echocardiographic evaluation revealed dilated cardiomyopathy with a significantly reduced systolic (ejection fraction of 25.9%) and diastolic function (E/A ratio of 0.87). Analysis of the ascitic fluid demonstrated a serum-ascites albumin gradient exceeding 1.1 g/L (3 g/L). Ultrasonography of the abdomen ruled out portal hypertension, while computed tomography of the abdomen confirmed extensive ascites without evidence of malignancy. Under the supervision of a specialist, the patient was administered a high dosage of levothyroxine (300 mcg), leading to a significant amelioration in both thyroid function parameters and her ascites. Subsequent thyroid function tests demonstrated a decrease in thyroid-stimulating hormone levels to 11.7 mIU/L and an increase in thyroxine levels to 15.6 pmol/L, indicating a positive response to the thyroid hormone replacement therapy. Subsequent echocardiographic assessment showed improvement in the ejection fraction to 26.9% and diastolic function (E/A ratio of 1.27). Myxedema ascites, though infrequent, is readily amenable to treatment. The serum-ascites albumin gradient exceeding 1.1 g/L may be indicative of hypothyroidism-associated ascites, although the paucity of studies renders it uncertain whether this is a characteristic feature. Further investigation into the etiology, diagnostic criteria, and management strategies for ascites in the context of hypothyroidism is warranted.