Thyroid glands demonstrate structural alterations throughout their life; however, there is little information on age-related histology features of guinea pigs. The aim of this research was to analyze the histological features of the thyroid glands of adult guinea pigs of different ages. Thirty-six adult guinea pigs were analyzed and assigned to four age categories (9 specimens in each category), which were 6, 12, 24 and 36 months old. Masson’s trichrome and PAS staining of thyroid tissues fixed in formaldehyde solution and processed routinely were carried out. Follicle size and distribution, colloid appearance, follicular epithelium morphology, distribution of parafollicular cells, interstitium structure and blood vessel features were diverse for different age groups among the investigated tissue samples. In 6-month-old animals, mostly relatively small follicles, located in loose connective tissue, were seen. The variability of follicles sizes and colloid distribution increased with age. Large follicle, foamy-appearing colloid, low colloid areas, flat follicular epithelium, follicular epithelium degeneration and vascular congestion occurred much more often in 36-month-old animals. Such results present a descriptive study of the histological characteristics of the thyroid gland of the guinea pig depending on its age. As the level of thyroid hormones and histomorphometrical measurements have not been examined in the study, the described changes in the histological structure cannot be considered as an indirect indication of thyroid dysfunction.