Introduction: Carbimazole is an antithyroid medication that is used for the treatment of hyperthyroidism. Carbimazole decreases serum thyroxine, thyroid-stimulating hormone, and thyrotropin-binding inhibitory immunoglobulins. However, carbimazole therapy can induce hypothyroidism and toxic effects on the thyroid gland tissue itself. Saussurea lappa root “Costus" is a traditional treatment that has anti-inflammatory, antioxidant, and immunomodulatory properties. Materials and methods: Twenty-four adult male albino rats were equally randomized into four groups and received all treatments orally by intragastric tube: group I served as control. Group II received Costus roots aqueous extract (50mg/kg body weight /2 days). Group III received carbimazole (2mg/100 gm body weight/day). Group IV received carbimazole concomitant with Costus roots aqueous extract. After 4 weeks, the animals were decapitated. Hematoxylin and eosin stain, periodic acid-Schiff reaction, and anti-calcitonin immunostaining were all performed on the prepared sections. Results: The Carbimazole group showed follicles lined by squamous epithelium, vacuolated colloid, and fused follicles. Carbimazole caused markedly decreased PAS-positive reaction due to colloid vacuolations and increased cytoplasmic immunoreactivity of C cells. In group IV, Costus could restore almost all the histopathological changes and improved C cells and decreased their cytoplasmic immunoreactivity. Conclusion: Saussurea lappa root “Costus" has a protective effect against carbimazole-induced thyroid damage.