Two zoo-maintained short-beaked echidnas (Tachyglossus aculeatus) had long histories of intermittent anorexia and lethargy. Case 1 presented with a recurrence of these signs after transfer to another facility and died shortly after arrival. A focal area of hyperattenuation within the paratracheal tissue of the cranial mediastinum was noted antemortem on CT. Postmortem, this corresponded with severe thyroid follicular hyperplasia with lymphoplasmacytic thyroiditis. Additional findings included a systemic fungal infection without an inflammatory response, suggesting underlying factors such as torpor or immunosuppression. In Case 2, an intrathoracic mass was identified during a preshipment examination. CT confirmed a contrast-enhanced mass compressing the cranial vena cava and right atrium, and the animal was euthanized. The mass was diagnosed histologically as thyroid adenocarcinoma. These cases report thyroiditis and thyroid adenocarcinoma in echidna and describe the use of IV contrast and CT as a diagnostic aid in this species.