The main causes of hypothyroidism are Hashimoto’s thyroiditis, iodine-131, and thyroid surgery. 2. Secondary hypothyroidism: It is usually secondary to pituitary gland, which is characterized by lower TSH, lower free T3 and T4, as well as hypopituitary gland function, and secondary hypothyroidism should be treated with adrenal corticosteroid replacement therapy in addition to eugenol.