However, irregularity in the size of follicles in some animals may be normal, as the size distribution pattern of thyroid follicles is different between the peripheral and central gland regions. In rats, the larger thyroid gland follicles are mostly located at the periphery of the thyroid lobes stated that the large follicles at periphery mainly serve as a pool of old hormone, whereas the smaller, centrally distributed ones are responsible for thyroid hormone secretion [26] and [27].