We observed spontaneous dwarfism among Wistar Hannover GALAS rats, caused by primary hypothyroidism. The dwarf (D) rats showed markedly high serum thyroid-stimulating hormone (TSH) values. In this study, we investigated the incidence and histopathological characteristics of the proliferative lesions of the thyroid follicular cells in our D rats. Gross enlargement of thyroid and histological vacuolar change of the thyroid follicular cells with age-related progression suggesting diffuse hypertrophy/hyperplasia were seen in all the D rats. In addition, focal proliferative lesions—namely hyperplasias, activated hyperplasias, adenomas, and adenocarcinomas—were seen in D rats aged over 50 weeks. Taken together, these results suggested that the sustained stimulation of elevated levels of serum TSH caused and enhanced diffuse hypertrophy of thyroid follicular cells and subsequent development of focal proliferative lesions.