Background: The heart is one of the organs which is affected by the thyroid hormones and hence the alterations in the thyroid status influences its function and structure. Objectives: This study aimed to reassess hypothyroidism-induced histopathological cardiac changes and to evaluate the proposed protective role of thymoquinone (TQ) against these changes. In addition, the mechanism of TQ-induced protection is studied regarding the oxidative stress and nitric oxide (NO) pathway. Materials and Methods: A model of propylthiouracil (PTU)-induced hypothyroidism in Wister rats is used in this study. Four groups of rats were used; control, TQ, PTU (hypothyroidism) and PTU+TQ groups. Thyroid hormones, cardiac enzymes, NO and antioxidants were assessed in the blood. Hearts were histopathologically and immunohistochemically examined. Results: A significant increase in plasma cardiac enzymes activity was recorded in hypothyroid rats, which is accompanied by significant histopathological changes in the left ventricle. Treatment of rats with TQ significantly protected the heart muscle against hypothyroidism- induced histopathological and immunohistochemical changes. It also significantly decreased plasma cardiac enzymes activity. TQ caused a reduction in malondialdehyde (MDA) formation, and increased, reduced glutathione (GSH), NO and superoxide dismutase (SOD) production. It also, increased the expression of constitutive nitric oxide synthase (eNOS) activity. Conclusion: hypothyroidism may induce cardiac pathological changes, which is prevented by TQ as it restores thyroid hormones, increases NO formation and eNOS expression, and decreases reactive oxygen species (ROS) production.