Hypothyroidism arises when the thyroid gland does not create enough T3 and T4 hormones. Untreated, hypothyroidism can eventually cause cardiac issues and high cholesterol, among other health issues. Laminaria japonica is one of the most important seaweeds, with various health benefits, especially in cases of hypothyroidism, due to its high level of iodine and phenolic compounds. So, this study aimed to evaluate the potential protective effect of Laminaria japonica ethanolic extract (LJEE) on carbimazole-induced hypothyroidism in male rats. Thirty male albino rats were divided into five groups (6/each); control group (G1), treated group (G2) received 1.35 mg/kg carbimazole orally, groups (3-5) received physiological saline and 100, 200, and 300 mg/kg body weight./day LJEE, respectively. An hour later, carbimazole was given orally at the same dose for 21 day. At the end of the experiment, rats were sacrificed and blood samples and thyroid glands were collected for biochemical and histopathological evaluation. Treatment of rats with carbimazole caused a significant decrease (p≤0.05) in thyroid hormones, antioxidant enzymes (SOD and CAT) and HDL levels in serum. Moreover, levels of MDA markedly increased in the treated group compared with the control group. The pretreatment of rats with LJEE at different doses not only improved thyroid hormones, antioxidants, liver enzymes (AST, ALT, ALP) and TSH levels in serum but also remarkably decreased levels of cytokines (TNF-α, I-6), lipid profiles (TC, TG, LDL, and, VLDL), serum glucose, and improved histological parameters of the thyroid gland. Therefore, LJEE is considered an effective treatment in treating hypothyroidism.