Studies characterising the thyroid gland structure of koalas are limited and have not previously been undertaken in South Australian populations. Hence, this study aimed to describe the thyroid gland morphology of koalas from the Mount Lofty region, South Australia. Results showed that thyroid gland morphology was highly variable between individual koalas (n = 36), ranging from that considered typical for healthy mammals, in which small colloid-filled follicles were lined by cuboidal epithelium, to that consistent with colloidal goitre, in which macrofollicles distended with colloid were lined by flattened epithelium. Juvenile koalas more frequently showed typical thyroid gland morphology than adults, with significantly higher thyroid follicle density (P < 0.05) and a higher proportion of follicles lined by cuboidal epithelium compared with flattened epithelium (P < 0.05). Thyroid glands of most adult koalas were characterised by colloidal macrofollicles (P < 0.01), and classified as colloidal goitre. There were no significant differences in thyroid gland morphology based on health status or sex of koalas. These findings suggest that an age-dependent colloidal goitre occurs in adult koalas, which is unlikely to have ill effects and may be associated with the low metabolic rate of this species, or exposure to a dietary goitrogen.